Monday, September 14, 2009

Are Uyghurs a recent or ancient population?

They are basically seeing whether there are many private haplotypes in the Uyghur population compared to East Asian and European populations. They don't find this to be the case, thus suggesting that Uyghurs are the result of a recent admixture process.

Haplotype-Sharing Analysis Showing Uyghurs Are Unlikely Genetic Donors
Shuhua Xu, Wenfei Jin and Li Jin
Molecular Biology and Evolution 2009 26(10):2197-2206
Abstract: The Uyghur (UIG) are a group of people primarily residing in Xinjiang of China, which is geographically located in Central Asia, from where modern humans were presumably spread in all directions reaching Europe, east, and northeast Asia about 40 kya. A recent study suggested that the UIG are ancestry donors of the East Asian (EAS) gene pool. However, an alternative hypothesis, that is, the UIG is an admixture population with both EAS and EUR ancestries is also supported by our previous studies. To test the two competing hypotheses, here we conducted a haplotype-sharing analysis (HSA) based on empirical and simulated data of high-density single nucleotide polymorphisms. Our results showed that more than 95% of UIG haplotypes could be found in either EAS or EUR populations, which contradicts the expectation of the null models assuming that UIG are donors. Simulation studies further indicated that the proportion of UIG private haplotypes observed in empirical data is only expected in alternative models assuming that UIG is an admixture population. Interestingly, the estimated ancestry contribution of 44%:56% (EAS:EUR) based on HSA is consistent with our previous estimation with STRUCTURE analysis. Although the history of UIGs could be complex, our method is explicit and conservative in rejecting the null hypothesis. We concluded that the gene pool of modern UIGs is more likely a sole recipient with contribution from both EAS and EUR.

Tuesday, September 01, 2009

Human mutation rate estimate

via this story at Nature News, their estimate based on Y chromosome is 100-200 new mutations per genome per generation, or about one mutation in every 30 million bases, which is in agreement with previous indirect estimates. Apparently this is the first direct measurement of the human mutation rate.

Human Y Chromosome Base-Substitution Mutation Rate Measured by Direct Sequencing in a Deep-Rooting Pedigree
Yali Xue, Qiuju Wang, Quan Long, Bee Ling Ng, Harold Swerdlow, John Burton, Carl Skuce, Ruth Taylor, Zahra Abdellah, Yali Zhao, Asan, Daniel G. MacArthur, Michael A. Quail, Nigel P. Carter, Huanming Yang and Chris Tyler-Smith
Current Biology, 27 August 2009
Abstract: Understanding the key process of human mutation is important for many aspects of medical genetics and human evolution. In the past, estimates of mutation rates have generally been inferred from phenotypic observations or comparisons of homologous sequences among closely related species [1,2,3]. Here, we apply new sequencing technology to measure directly one mutation rate, that of base substitutions on the human Y chromosome. The Y chromosomes of two individuals separated by 13 generations were flow sorted and sequenced by Illumina (Solexa) paired-end sequencing to an average depth of 11 or 20, respectively [4]. Candidate mutations were further examined by capillary sequencing in cell-line and blood DNA from the donors and additional family members. Twelve mutations were confirmed in 10.15 Mb; eight of these had occurred invitro and four invivo. The latter could be placed in different positions on the pedigree and led to a mutation-rate measurement of 3.0x10-8 mutations/nucleotide/generation (95% CI: 8.9x10-9 7.0x10-8), consistent with estimates of 2.3x10-8 - 6.3x10-8 mutations/nucleotide/generation for the same Y-chromosomal region from published human-chimpanzee comparisons [5] depending on the generation and split times assumed.

Sunday, August 30, 2009

Got some performance enhancing polymorphisms?

First of all, gotta love the term "Performance Enhanding Polymorphisms" (PEPs). This is a review paper describing what we know about the genetics of athletic performance. They focus on ACE, ACTN3, MSTN, NOS3 and several other genes. In light of recent performances, and just out of curiosity, let's get some genotypes on these three people:
Usain Bolt
Lance Armstrong
Michael Phelps

Genetics of Athletic Performance
Elaine A. Ostrander, Heather J. Huson, and Gary K. Ostrander
Annual Review of Genomics and Human Genetics Vol. 10: 407-429
Abstract: Performance enhancing polymorphisms (PEPs) are examples of natural genetic variation that affect the outcome of athletic challenges. Elite athletes, and what separates them from the average competitor, have been the subjects of discussion and debate for decades. While training, diet, and mental fitness are all clearly important contributors to achieving athletic success, the fact that individuals reaching the pinnacle of their chosen sports often share both physical and physiological attributes suggests a role for genetics. That multiple members of a family often participate in highly competitive events, such as the Olympics, further supports this argument. In this review, we discuss what is known regarding the genes and gene families, including the mitochondrial genome, that are believed to play a role in human athletic performance. Where possible, we describe the physiological impact of the critical gene variants and consider predictions about other potentially important genes. Finally, we discuss the implications of these findings on the future for competitive athletics.

Friday, August 28, 2009

Geographical origin and dating of the lactase persistence allele

See Dienekes' blog post about a new paper that purports the origin of the lactase persistence allele to be somewhere in the Austria/Czech Republic area, around 7,500 years ago.
From the abstract:
Using data on −13,910*T allele frequency and farming arrival dates across Europe, and approximate Bayesian computation to estimate parameters of interest, we infer that the −13,910*T allele first underwent selection among dairying farmers around 7,500 years ago in a region between the central Balkans and central Europe, possibly in association with the dissemination of the Neolithic Linearbandkeramik culture over Central Europe. Furthermore, our results suggest that natural selection favouring a lactase persistence allele was not higher in northern latitudes through an increased requirement for dietary vitamin D.

Saturday, August 22, 2009

A set of AIMs that can distinguish within continents

An ancestry informative marker set for determining continental origin: validation and extension using human genome diversity panels.
Nassir R, Kosoy R, Tian C, White PA, Butler LM, Silva G, Kittles R, Alarcon-Riquelme ME, Gregersen PK, Belmont JW, De La Vega FM, Seldin MF.
BMC Genet. 2009 Jul 24;10(1):39.
ABSTRACT: BACKGROUND: Case-control genetic studies of complex human diseases can be confounded by population stratification. This issue can be addressed using panels of ancestry informative markers (AIMs) that can provide substantial population substructure information. Previously, we described a panel of 128 SNP AIMs that were designed as a tool for ascertaining the origins of subjects from Europe, Sub-Saharan Africa, Americas, and East Asia. RESULTS: In this study, genotypes from Human Genome Diversity Panel populations were used to further evaluate a 93 SNP AIM panel, a subset of the 128 AIMS set, for distinguishing continental origins. Using both model-based and relatively model-independent methods, we here confirm the ability of this AIM set to distinguish diverse population groups that were not previously evaluated. This study included multiple population groups from Oceana, South Asia, East Asia, Sub-Saharan Africa, North and South America, and Europe. In addition, the 93 AIM set provides population substructure information that can, for example, distinguish Arab and Ashkenazi from Northern European population groups and Pygmy from other Sub-Saharan African population groups. CONCLUSION: These data provide additional support for using the 93 AIM set to efficiently identify continental subject groups for genetic studies, to identify study population outliers, and to control for admixture in association studies.

Tuesday, August 04, 2009

Epigenetics and disease causality

Obviously this has a lot of important implications, one of which is the fact that we may be overestimating the heritability of traits like obesity and diabetes.

Epigenetic Inheritance and the Missing Heritability Problem
Montgomery Slatkin
Genetics July 2009; 182: 845
Abstract: Epigenetic phenomena, and in particularly heritable epigenetic changes, or transgenerational effects, are the subject of much discussion in the current literature. This paper presents a model of transgenerational epigenetic inheritance and explores the effect of epigenetic inheritance on the risk and recurrence risk of a complex disease. The model assumes that epigenetic modifications of the genome are gained and lost at specified rates and that each modification contributes multiplicatively to disease risk. The potentially high rate of loss of epigenetic modifications causes the probability of identity in state in close relatives to be smaller than is implied by their relatedness. As a consequence, the recurrence risk to close relatives is reduced. Although epigenetic modifications may contribute substantially to average risk, they will not contribute much to recurrence risk and heritability unless they persist on average for many generations. If they do persist for long times, they are equivalent to mutations and hence are likely to be in linkage disequilibrium with SNPs surveyed in genome-wide association studies. Thus epigenetic modifications are a potential solution to the problem of missing causality of complex diseases but not to the problem of missing heritability. The model highlights the need for empirical estimates of the persistence times of heritable epialleles.

Thursday, July 09, 2009

Human mtDNA subject to selection by climate?

Climate shaped the worldwide distribution of human mitochondrial DNA sequence variation
François Balloux, Lori-Jayne Lawson Handley, Thibaut Jombart, Hua Liu and Andrea Manica
Proceedings Royal Society B
Abstract: There is an ongoing discussion in the literature on whether human mitochondrial DNA (mtDNA) evolves neutrally. There have been previous claims for natural selection on human mtDNA based on an excess of non-synonymous mutations and higher evolutionary persistence of specific mitochondrial mutations in Arctic populations. However, these findings were not supported by the reanalysis of larger datasets. Using a geographical framework, we perform the first direct test of the relative extent to which climate and past demography have shaped the current spatial distribution of mtDNA sequences worldwide. We show that populations living in colder environments have lower mitochondrial diversity and that the genetic differentiation between pairs of populations correlates with difference in temperature. These associations were unique to mtDNA; we could not find a similar pattern in any other genetic marker. We were able to identify two correlated non-synonymous point mutations in the ND3 and ATP6 genes characterized by a clear association with temperature, which appear to be plausible targets of natural selection producing the association with climate. The same mutations have been previously shown to be associated with variation in mitochondrial pH and calcium dynamics. Our results indicate that natural selection mediated by climate has contributed to shape the current distribution of mtDNA sequences in humans.

Thursday, June 11, 2009

Social brain hypothesis shot down twice in two days?!

Why do some animals have bigger brains? There's several hypotheses out there, including the monogamy one which was tested in the first paper below, but the strongest one, in my mind, the social complexity (or Machiavellian intelligence) hypothesis, has been "shot down" twice in recent papers. These hypotheses are not necessarily mutually exclusive, and the ecological determinants of brain size are likely to differ by taxa/clade/lineage.

Sociality, ecology, and relative brain size in lemurs

Evan L. MacLean, Nancy L. Barrickman, Eric M. Johnson and Christine E. Wall
Journal of Human Evolution Volume 56, Issue 5, May 2009, Pages 471-478
Abstract: The social brain hypothesis proposes that haplorhine primates have evolved relatively large brains for their body size primarily as an adaptation for living in complex social groups. Studies that support this hypothesis have shown a strong relationship between relative brain size and group size in these taxa. Recent reports suggest that this pattern is unique to haplorhine primates; many nonprimate taxa do not show a relationship between group size and relative brain size. Rather, pairbonded social monogamy appears to be a better predictor of a large relative brain size in many nonprimate taxa. It has been suggested that haplorhine primates may have expanded the pairbonded relationship beyond simple dyads towards the evolution of complex social groups. We examined the relationship between group size, pairbonding, and relative brain size in a sample of 19 lemurs; strepsirrhine primates that last share a common ancestor with monkeys and apes approximately 75 Ma. First, we evaluated the social brain hypothesis, which predicts that species with larger social groups will have relatively larger brains. Secondly, we tested the pairbonded hypothesis, which predicts that species with a pairbonded social organization will have relatively larger brains than non-pairbonded species. We found no relationship between group size or pairbonding and relative brain size in lemurs. We conducted two further analyses to test for possible relationships between two nonsocial variables, activity pattern and diet, and relative brain size. Both diet and activity pattern are significantly associated with relative brain size in our sample. Specifically, frugivorous species have relatively larger brains than folivorous species, and cathemeral species have relatively larger brains than diurnal, but not nocturnal species. These findings highlight meaningful differences between Malagasy strepsirrhines and haplorhines, and between Malagasy strepsirrhines and nonprimate taxa, regarding the social and ecological factors associated with increases in relative brain size. The results suggest that factors such as foraging complexity and flexibility of activity patterns may have driven selection for increases in brain size in lemurs.

Brain-size evolution and sociality in Carnivora
John A. Finarelli, John J. Flynn
PNAS June 9, 2009 vol. 106 no. 23 9345-9349
Abstract: Increased encephalization, or larger brain volume relative to body mass, is a repeated theme in vertebrate evolution. Here we present an extensive sampling of relative brain sizes in fossil and extant taxa in the mammalian order Carnivora (cats, dogs, bears, weasels, and their relatives). By using Akaike Information Criterion model selection and endocranial volume and body mass data for 289 species (including 125 fossil taxa), we document clade-specific evolutionary transformations in encephalization allometries. These evolutionary transformations include multiple independent encephalization increases and decreases in addition to a remarkably static basal Carnivora allometry that characterizes much of the suborder Feliformia and some taxa in the suborder Caniformia across much of their evolutionary history, emphasizing that complex processes shaped the modern distribution of encephalization across Carnivora. This analysis also permits critical evaluation of the social brain hypothesis (SBH), which predicts a close association between sociality and increased encephalization. Previous analyses based on living species alone appeared to support the SBH with respect to Carnivora, but those results are entirely dependent on data from modern Canidae (dogs). Incorporation of fossil data further reveals that no association exists between sociality and encephalization across Carnivora and that support for sociality as a causal agent of encephalization increase disappears for this clade.

Monday, June 01, 2009

Hygiene hypothesis: exposure to parasites and autoimmune diseases

They use re-sequencing data on interleukin genes retrieved from Seattle SNPs.

Parasites represent a major selective force for interleukin genes and shape the genetic predisposition to autoimmune conditions
Matteo Fumagalli, Uberto Pozzoli, Rachele Cagliani, Giacomo P. Comi, Stefania Riva, Mario Clerici, Nereo Bresolin, and Manuela Sironi
The Journal of Experimental Medicine doi:10.1084/jem.20082779
Abstract: Many human genes have adapted to the constant threat of exposure to infectious agents; according to the "hygiene hypothesis," lack of exposure to parasites in modern settings results in immune imbalances, augmenting susceptibility to the development of autoimmune and allergic conditions. Here, by estimating the number of pathogen species/genera in a specific geographic location (pathogen richness) for 52 human populations and analyzing 91 interleukin (IL)/IL receptor genes (IL genes), we show that helminths have been a major selective force on a subset of these genes. A population genetics analysis revealed that five IL genes, including IL7R and IL18RAP, have been a target of balancing selection, a selection process that maintains genetic variability within a population. Previous identification of polymorphisms in some of these loci, and their association with autoimmune conditions, prompted us to investigate the relationship between adaptation and disease. By searching for variants in IL genes identified in genome-wide association studies, we verified that six risk alleles for inflammatory bowel (IBD) or celiac disease are significantly correlated with micropathogen richness. These data support the hygiene hypothesis for IBD and provide a large set of putative targets for susceptibility to helminth infections.

Wednesday, May 27, 2009

GWAS and fine mapping in Africans

Genome-wide and fine-resolution association analysis of malaria in West Africa.
Jallow M, Teo YY, Small KS et al.
Nat Genet. 2009 May 24. [Epub ahead of print]
Abstract: We report a genome-wide association (GWA) study of severe malaria in The Gambia. The initial GWA scan included 2,500 children genotyped on the Affymetrix 500K GeneChip, and a replication study included 3,400 children. We used this to examine the performance of GWA methods in Africa. We found considerable population stratification, and also that signals of association at known malaria resistance loci were greatly attenuated owing to weak linkage disequilibrium (LD). To investigate possible solutions to the problem of low LD, we focused on the HbS locus, sequencing this region of the genome in 62 Gambian individuals and then using these data to conduct multipoint imputation in the GWA samples. This increased the signal of association, from P = 4 x 10(-7) to P = 4 x 10(-14), with the peak of the signal located precisely at the HbS causal variant. Our findings provide proof of principle that fine-resolution multipoint imputation, based on population-specific sequencing data, can substantially boost authentic GWA signals and enable fine mapping of causal variants in African populations.

Tuesday, May 12, 2009

Geography and genetics of p53 in E. Asia

Winter Temperature and UV Are Tightly Linked to Genetic Changes in the p53 Tumor Suppressor Pathway in Eastern Asia
Hong Shi,Si-jie Tan,Hua Zhong,Wenwei Hu,Arnold Levine,Chun-jie Xiao,Yi Peng,Xue-bin Qi,Wei-hua Shou,Run-lin Z. Ma,Yi Li,Bing Su, andXin Lu
AJHG Volume 84, Issue 4, 534-541, 02 April 2009

Abstract: The tumor suppressor p53 is a master sensor of stress. Two human-specific polymorphisms, p53 codon 72 and MDM2 SNP309, influence the activities of p53. There is a tight association between cold winter temperature and p53 Arg72 and between low UV intensity and MDM2 SNP309 G/G in a cohort of 4029 individuals across Eastern Asia that suggests causative selection. Moreover, the two polymorphisms are not coselected. Haplotype-based selection analysis further suggests that this is a striking example of two functional polymorphisms being strongly selected for in human populations in response to environmental stresses.

Thursday, April 16, 2009

GWAS articles in NEJM

Razib's got the links.

Lumberjacks and obesity genes

Thought I'd post on this one, because the title caught my eye, and because I was keen on finding a good lumberjack picture. They look for association between variants in 18 genes and obesity related phenotypes. I'm a bit perplexed by the choice of genes...
Association study between candidate genes and obesity-related phenotypes using a sample of lumberjacks.
Chamberland A, Tremblay N, Audet M, Gilbert B, Pérusse L, Vohl MC, Laprise C.
Public Health Genomics. 2009;12(4):253-8. Epub 2009 Feb 16.
INTRODUCTION: Complex traits such as obesity are modulated by genetic and environmental factors and lead to varied clinical presentations.The aim of this study was to investigate associations between candidate genes and obesity-related phenotypes using a sample of 252 lumberjacks issued from a founder population and sharing a common and circumscribed environment. METHODS: Thirty-seven variants in 18 genes were genotyped. The restriction fragment length polymorphism method and the template-directed dye-terminator incorporation assay with fluorescence polarization detection were employed for the genotyping assays. Multivariate logistic regression models were built in order to calculate the relative odds of exhibiting obesity-related phenotypes associated with the presence of the studied polymorphism. Among them, 21 single nucleotide polymorphisms were tested for associations with obesity phenotypes. RESULTS: Significant associations were found between carriers of the minor alleles of APOE-epsilon2, FABP2-A54T, UCP1-L229M, LPL-HindIII, LPL-S447X and LPL-T1973C, patients bearing a combination of LPL-D9N, LPL-N291S and LPL-P207L and obesity-related phenotypes. CONCLUSION: The present results suggest that a particular population such as lumberjacks, sharing the same environment, could help target genes involved in complex traits.

Sunday, April 12, 2009

Answer to my question (sorta): heritability of melanoma

I wondered about the heritability of tanning response in a recent post about a GWAS for tanning response. Albeit not a direct answer, according to this paper below, there is a relatively high degree of heritability (~55%) for variation in development of melanoma:
A Population-Based Study of Australian Twins with Melanoma Suggests a Strong Genetic Contribution to Liability.
Shekar SN, Duffy DL, Youl P, Baxter AJ, Kvaskoff M, Whiteman DC, Green AC, Hughes MC, Hayward NK, Coates M, Martin NG.
J Invest Dermatol. 2009 Apr 9.
Abstract: Melanoma runs within families, but this may be due to either shared genetic or shared environmental influences within those families. The concordance between pairs of non-identical twins compared to that between identical twins can be used to determine whether familial aggregation is due to genetic or environmental factors. Mandatory reporting of melanoma cases in the state of Queensland yielded approximately 12,000 cases between 1982 and 1990. Twins in this study and from the adjacent state of New South Wales (125 pairs in total) were used to partition variation in liability to melanoma into genetic and environmental factors. Identical twins were more concordant for melanoma (4 of 27 pairs) than non-identical twins (3 of 98 pairs; P-value approximately 0.04). Identical co-twins of affected individuals were 9.8 times more likely to be affected than by chance. However, non-identical co-twins of affected individuals were only 1.8 times more likely to be affected than by chance. An MZ:DZ recurrence risk ratio of 5.6 suggests that some of the genetic influences on melanoma are due to epistatic (gene-gene) interactions. Using these data and population prevalences, it was estimated that 55% of the variation in liability to melanoma is due to genetic influences.

Friday, April 10, 2009

Evolutionary medicine gaining ground

Description of Evolutionary Medicine meeting in the current Science:
Apparently there is a growing trend of teaching medicine from an evolutionary perspective in medical schools. The figure here shows the increase in connections between ecology & evolution and Medicine as reflected in citations (look at connections between the two reddish dots).
Some interesting parts from Elizabeth Pennisi's article:
For instance, anthropologist Kathleen Barnes of Johns Hopkins University has evidence that for some asthmatics, this overly energetic inflammatory response may be a holdover from the body's successes in coping with parasitic disease.

...With genomic data in hand, "medical students are much more equipped to understand the connections between all organisms,"...

Barnes and her colleagues have found that asthma is associated with the defective Duffy gene in populations in Brazil, Columbia, and the Caribbean whose recent African ancestors lived where malaria was endemic.
Also, in this issue, from the same meeting, a piece about schizophrenia and autism by Constance Holden, arguing for the evolutionary connection between these two conditions (I think that an equally appropriate spectrum is: William's syndrome-Autism)
At the Sackler Colloquium on Evolution in Health and Medicine held last week at the National Academy of Sciences (NAS) in Washington, D.C., evolutionary geneticist Bernard Crespi of Simon Fraser University in Burnaby, Canada, threw some evolutionary firepower at the question. He proposes that both schizophrenia and autism are disorders of the "social brain"--but at opposite ends of the same spectrum.

A number of studies have shown some overlap in genomic "hot spots" for CNVs in schizophrenia and autism, with, in some cases, deletions in one condition just where there are duplications for the other

That would fit with their theory that psychotic disorders--including not only schizophrenia but also bipolar disorder and some major depression--result from "overdevelopment" of the social brain, and autism spectrum disorders reflect underdevelopment of that brain. Many scientists believe socialization is the main force behind the rapid expansion of human brains, said Crespi, pointing out that in primates the size of the cortex increases with size of social groups. The components of the social brain, according to Crespi, include language, self-awareness, "social emotions" such as pride and guilt, logical thinking, pursuit of goals, and awareness of the mental states of others.

Monday, April 06, 2009

Utility of GWAS prostate cancer risk alleles in other populations

Generalizability of Associations from Prostate Cancer Genome-Wide Association Studies in Multiple Populations.
Waters KM, Le Marchand L, Kolonel LN, Monroe KR, Stram DO, Henderson BE, Haiman CA.
Cancer Epidemiol Biomarkers Prev.
2009 Mar 24. [Epub ahead of print]
Abstract: Genome-wide association studies have identified multiple common alleles associated with prostate cancer risk in populations of European ancestry. Testing these variants in other populations is needed to assess the generalizability of the associations and may guide fine-mapping efforts. We examined 13 of these risk variants in a multiethnic sample of 2,768 incident prostate cancer cases and 2,359 controls from the Multiethnic Cohort (African Americans, European Americans, Latinos, Japanese Americans, and Native Hawaiians). We estimated ethnic-specific and pooled odds ratios and tested for ethnic heterogeneity of effects using logistic regression. In ethnic-pooled analyses, 12 of the 13 variants were positively associated with risk, with statistically significant associations (P less than 0.05) noted with six variants: JAZF1, rs10486567 [odds ratio (OR), 1.23; 95% confidence interval (95% CI, 1.12-1.35); Xp11.2, rs5945572 (OR, 1.31; 95% CI, 1.13-1.51); HNF1B, rs4430796 (OR, 1.15; 95% CI, 1.06-1.25); MSMB, rs10993994 (OR, 1.13; 95% CI, 1.04-1.23); 11q13.2, rs7931342 (OR, 1.13; 95% CI, 1.03-1.23); 3p12.1, rs2660753 (OR, 1.11; 95% CI, 1.01-1.21); SLC22A3, rs9364554 (OR, 1.10; 95% CI, 1.00-1.21); CTBP2, rs12769019 (OR, 1.11; 95% CI, 0.99-1.25); HNF1B, rs11649743 (OR, 1.10; 95% CI, 0.99-1.22); EHBP1, rs721048 (OR, 1.08; 95% CI, 0.94-1.25); KLK2/3, rs2735839 (OR, 1.06; 95% CI, 0.97-1.16); 17q24.3, rs1859962 (OR, 1.04; 95% CI, 0.96-1.13); and LMTK2, rs6465657 (OR, 0.99; 95% CI, 0.89-1.09). Significant ethnic heterogeneity of effects was noted for four variants (EHBP1, Phet = 3.9 x 10(-3); 11q13, Phet = 0.023; HNF1B (rs4430796), Phet = 0.026; and KLK2/3, Phet = 2.0 x 10(-3)). Although power was limited in some ethnic/racial groups due to variation in sample size and allele frequencies, these findings suggest that a large fraction of prostate cancer variants identified in populations of European ancestry are global markers of risk. For many of these regions, fine-mapping in non-European samples may help localize causal alleles and better determine their contribution to prostate cancer risk in the population.

Friday, April 03, 2009

GWAS deluge

I wonder how exactly they operationalize this phenotype...I don't have full text access, but I'll assume they control for constitutive skin color. I still think this is a bit weird. Is there a heritable component to skin burning that is really independent of constitutive skin color?

Genome-Wide Association Study of Tanning Phenotype in a Population of European Ancestry.

Nan H, Kraft P, Qureshi AA, Guo Q, Chen C, Hankinson SE, Hu FB, Thomas G, Hoover RN, Chanock S, Hunter DJ, Han J.
J Invest Dermatol. 2009 Apr 2
Abstract: We conducted a multistage genome-wide association study (GWAS) of tanning response after exposure to sunlight in over 9,000 men and women of European ancestry who live in the United States. An initial analysis of 528,173 single-nucleotide polymorphisms (SNPs) genotyped on 2,287 women identified LOC401937 (rs966321) on chromosome 1 as a novel locus highly associated with tanning ability, and we confirmed this association in 870 women controls from a skin cancer case-control study with joint P-value=1.6 x 10(-9). We further genotyped this SNP in two subsequent replication studies (one with 3,750 women and the other with 2,405 men). This association was not replicated in either of these two studies. We found that several SNPs reaching the genome-wide significance level are located in or adjacent to the loci previously known as pigmentation genes: MATP, IRF4, TYR, OCA2, and MC1R. Overall, these tanning ability-related loci are similar to the hair color-related loci previously reported in the GWAS of hair color.

Blue eyes follow-up

According to this paper, blue eyes in N. European humans is predicted very well by these variants:
HERC2 rs12913832
OCA2 rs1800407
SLC24A4 rs12896399
SLC45A2 rs16891982
TYR rs1393350
IRF4 rs12203592

It might be interesting to look at the five latter ones in the blue-eyed lemur... although, it looks like only one SNP (in HERC2, I assume) is sufficient to explain the majority of the variation in humans.

Thursday, April 02, 2009

Convergent evolution via different genetic mechanism: blue eyes in lemurs and humans

I guess we are to assume that the blue eye color is the same in humans and lemurs. From looking at some pictures, the color appears pretty similar. Haven't read the paper, but are all cases of blue eye color in humans explained by the same variation in HERC2? I might have a post on this somewhere.
I suppose we shouldn't be too surprised by this finding, given what we know about convergence of light skin color evolution in Europeans and E. Asians via different genes.

Brief communication: Blue eyes in lemurs and humans: Same phenotype, different genetic mechanism.
Bradley BJ, Pedersen A, Mundy NI.
Am J Phys Anthropol. 2009 Mar 10.
Almost all mammals have brown or darkly-pigmented eyes (irises), but among primates, there are some prominent blue-eyed exceptions. The blue eyes of some humans and lemurs are a striking example of convergent evolution of a rare phenotype on distant branches of the primate tree. Recent work on humans indicates that blue eye color is associated with, and likely caused by, a single nucleotide polymorphism (rs12913832) in an intron of the gene HERC2, which likely regulates expression of the neighboring pigmentation gene OCA2. This raises the immediate question of whether blue eyes in lemurs might have a similar genetic basis. We addressed this by sequencing the homologous genetic region in the blue-eyed black lemur (Eulemur macaco flavifrons; N = 4) and the closely-related black lemur (Eulemur macaco macaco; N = 4), which has brown eyes. We then compared a 166-bp segment corresponding to and flanking the human eye-color-associated region in these lemurs, as well as other primates (human, chimpanzee, orangutan, macaque, ring-tailed lemur, mouse lemur). Aligned sequences indicated that this region is strongly conserved in both Eulemur macaco subspecies as well as the other primates (except blue-eyed humans). Therefore, it is unlikely that this regulatory segment plays a major role in eye color differences among lemurs as it does in humans. Although convergent phenotypes can sometimes come about via the same or similar genetic changes occurring independently, this does not seem to be the case here, as we have shown that the genetic basis of blue eyes in lemurs differs from that of humans.

Saturday, March 07, 2009

Disentangling genetic signatures of natural selection and demography among Native Americans and W. Beringians

Haplotypic background of a private allele at high frequency in the Americas.
Schroeder KB, Jakobsson M, Crawford MH, Schurr TG, Boca SM, Conrad DF, Tito RY, Osipova LP, Tarskaia LA, Zhadanov SI, Wall JD, Pritchard JK, Malhi RS, Smith DG, Rosenberg NA.
Mol Biol Evol. 2009 Feb 17. [Epub ahead of print]
Abstract: Recently, the observation of a high-frequency private allele, the 9-repeat allele at microsatellite D9S1120, in all sampled Native American and Western Beringian populations has been interpreted as evidence that all modern Native Americans descend primarily from a single founding population. However, this inference assumed that all copies of the 9-repeat allele were identical by descent and that the geographic distribution of this allele had not been influenced by natural selection. To investigate whether these assumptions are satisfied, we genotyped 34 SNPs across approximately 500 kilobases (kb) around D9S1120 in 21 Native American and Western Beringian populations and 54 other worldwide populations. All chromosomes with the 9-repeat allele share the same haplotypic background in the vicinity of D9S1120, suggesting that all sampled copies of the 9-repeat allele are identical by descent. Ninety-one percent of these chromosomes share the same 76.26 kb haplotype, which we call the "American Modal Haplotype" (AMH). Three observations lead us to conclude that the high frequency and widespread distribution of the 9-repeat allele are unlikely to be the result of positive selection: 1) aside from its association with the 9-repeat allele, the AMH does not have a high frequency in the Americas, 2) the AMH is not unusually long for its frequency compared to other haplotypes in the Americas, and 3) in Latin American mestizo populations, the proportion of Native American ancestry at D9S1120 is not unusual compared to that observed at other genomewide microsatellites. Using a new method for estimating the time to the most recent common ancestor (MRCA) of all sampled copies of an allele on the basis of an estimate of the length of the genealogy descended from the MRCA, we calculate the mean time to the MRCA of the 9-repeat allele to be between 7,325 and 39,900 years, depending on the demographic model used. The results support the hypothesis that all modern Native Americans and Western Beringians trace a large portion of their ancestry to a single founding population which may have been isolated from other Asian populations prior to expanding into the Americas.

Discovery of rare variants in regions identified by GWASs

P-ter at GNXP and Dan at Genetic Future discuss what seems to be an interesting and important recently published paper in Science (abstract below) that finds four rare variants with larger-than-usual effects for Type-1 Diabetes. The last line of the abstract pretty much says it all. The evolutionary interpretation is of course very interesting, especially since the rare variants reduce risk. Dan talks about this.

Rare Variants of IFIH1, a Gene Implicated in Antiviral Responses, Protect Against Type 1 Diabetes
Sergey Nejentsev, Neil Walker , David Riches, Michael Egholm, John A. Todd
Science DOI: 10.1126/science.1167728
Abstract: Genome-wide association studies (GWAS) are widely used to map genomic regions contributing to common human diseases, but they often do not identify the precise causative genes and sequence variants. To identify causative type 1 diabetes (T1D) variants, we resequenced exons and splice sites of ten candidate genes in pools of DNA from 480 patients and 480 controls and tested their disease association in over 30,000 subjects. We discovered four rare variants that lowered T1D risk independently of each other (OR = 0.51 – 0.74; P = 1.3 x 10-3 – 2.1 x 10-16) in IFIH1, a gene located in a region previously associated with T1D by GWAS. These variants are predicted to alter the expression and structure of IFIH1 (MDA5), a cytoplasmic helicase that mediates induction of interferon response to viral RNA. This firmly establishes the role of IFIH1 in T1D and demonstrates that resequencing studies can pinpoint disease-causing genes in genomic regions initially identified by GWAS.

Friday, March 06, 2009

Introgression of black coat color into wolves from dogs

Molecular and Evolutionary History of Melanism in North American Gray Wolves
Tovi M. Anderson, Bridgett M. vonHoldt, Sophie I. Candille, Marco Musiani, Claudia Greco, Daniel R. Stahler, Douglas W. Smith, Badri Padhukasahasram, Ettore Randi, Jennifer A. Leonard, Carlos D. Bustamante, Elaine A. Ostrander, Hua Tang, Robert K. Wayne, Gregory S. Barsh
Science 6 March 2009: Vol. 323. no. 5919, pp. 1339 - 1343
Abstract: Morphological diversity within closely related species is an essential aspect of evolution and adaptation. Mutations in the Melanocortin 1 receptor (Mc1r) gene contribute to pigmentary diversity in natural populations of fish, birds, and many mammals. However, melanism in the gray wolf, Canis lupus, is caused by a different melanocortin pathway component, the K locus, that encodes a beta-defensin protein that acts as an alternative ligand for Mc1r. We show that the melanistic K locus mutation in North American wolves derives from past hybridization with domestic dogs, has risen to high frequency in forested habitats, and exhibits a molecular signature of positive selection. The same mutation also causes melanism in the coyote, Canis latrans, and in Italian gray wolves, and hence our results demonstrate how traits selected in domesticated species can influence the morphological diversity of their wild relatives.

Wednesday, February 25, 2009

Predictive ability of SNPs for prostate cancer

Utility of Incorporating Genetic Variants for the Early Detection of Prostate Cancer
Robert K. Nam, William W. Zhang, John Trachtenberg, Arun Seth, Laurence H. Klotz, Aleksandra Stanimirovic, Sanoj Punnen, Vasundara Venkateswaran, Ants Toi, D. Andrew Loblaw, Linda Sugar, Katherine A. Siminovitch, Steven A. Narod
Clinical Cancer Research, 10.1158/1078-0432
Abstract Purpose: Several single nucleotide polymorphisms (SNP) have been associated with the risk of prostate cancer. The clinical utility of using SNPs in the early detection of prostate cancer has not been evaluated.Experimental Design: We examined a panel of 25 SNPs from candidate genes and chromosomal regions in 3,004 unselected men who were screened for prostate cancer using serum prostate-specific antigen (PSA) and digital rectal examination. All underwent a prostate biopsy. We evaluated the ability of these SNPs to help predict the presence of prostate cancer at biopsy.Results: Of the 3,004 patients, 1,389 (46.2%) were found to have prostate cancer. Fifteen of the 25 SNPs studied were significantly associated with prostate cancer (P = 0.02-7 x 10-8). We selected a combination of 4 SNPs with the best predictive value for further study. After adjusting for other predictive factors, the odds ratio for patients with all four of the variant genotypes compared with men with no variant genotype was 5.1 (95% confidence interval, 1.6-16.5; P = 0.006). When incorporated into a nomogram, genotype status contributed more significantly than PSA, family history, ethnicity, urinary symptoms, and digital rectal examination (area under the curve = 0.74). The positive predictive value of the PSA test ranged from 42% to 94% depending on the number of variant genotypes carried (P = 1 x 10-15).Conclusions: SNP genotyping can be used in a clinical setting for the early detection of prostate cancer in a nomogram approach and by improving the positive predictive value of the PSA test.

Wednesday, February 18, 2009

Teaching material for evolution by natural selection

In honor of Darwin's recent 200th birthday, Nature came up with a list of 15 Evolutionary Gems. My major complaint, and a pretty major one at that, is that none of the examples are related to human evolution - something that students could probably relate to a bit more easily, and might increase the chance that they actually pay attention.
Basically, what I'm getting at: Why is lactase persistence not there? or the several other well-documented and clear cut examples from human evolution?

Elucidating the genetics of eye color

Getting closer to predicting not-so-complex "complex" phenotypes from genetic data?
Eye and skin color are great model phenotypes with which we can learn about the genetics of complex traits, as I've often argued. Here's another step in the process.

Interactions Between HERC2, OCA2 and MC1R May Influence Human Pigmentation Phenotype.
Branicki W, Brudnik U, Wojas-Pelc A.
Ann Hum Genet. 2009 Feb 4. [Epub ahead of print]
Abstract: Human pigmentation is a polygenic trait which may be shaped by different kinds of gene-gene interactions. Recent studies have revealed that interactive effects between HERC2 and OCA2 may be responsible for blue eye colour determination in humans. Here we performed a population association study, examining important polymorphisms within the HERC2 and OCA2 genes. Furthermore, pooling these results with genotyping data for MC1R, ASIP and SLC45A2 obtained for the same population sample we also analysed potential genetic interactions affecting variation in eye, hair and skin colour. Our results confirmed the association of HERC2 rs12913832 with eye colour and showed that this SNP is also significantly associated with skin and hair colouration. It is also concluded that OCA2 rs1800407 is independently associated with eye colour. Finally, using various approaches we were able to show that there is an interaction between MC1R and HERC2 in determination of skin and hair colour in the studied population sample.

Saturday, January 24, 2009

Genetics of lipid profiles in European and African Americans

Genetic Differences between the Determinants of Lipid Profile Phenotypes in African and European Americans: The Jackson Heart Study
Rahul C. Deo, David Reich, Arti Tandon, Ermeg Akylbekova, Nick Patterson, Alicja Waliszewska, Sekar Kathiresan, Daniel Sarpong, Herman A. Taylor, Jr., James G. Wilson
PLoS Genetics 5(1): e1000342.
Abstract: Genome-wide association analysis in populations of European descent has recently found more than a hundred genetic variants affecting risk for common disease. An open question, however, is how relevant the variants discovered in Europeans are to other populations. To address this problem for cardiovascular phenotypes, we studied a cohort of 4,464 African Americans from the Jackson Heart Study (JHS), in whom we genotyped both a panel of 12 recently discovered genetic variants known to predict lipid profile levels in Europeans and a panel of up to 1,447 ancestry informative markers allowing us to determine the African ancestry proportion of each individual at each position in the genome. Focusing on lipid profiles—HDL-cholesterol (HDL-C), LDL-cholesterol (LDL-C), and triglycerides (TG)—we identified the lipoprotein lipase (LPL) locus as harboring variants that account for interethnic variation in HDL-C and TG. In particular, we identified a novel common variant within LPL that is strongly associated with TG (p = 2.7×10−6) and explains nearly 1% of the variability in this phenotype, the most of any variant in African Americans to date. Strikingly, the extensively studied “gain-of-function” S447X mutation at LPL, which has been hypothesized to be the major determinant of the LPL-TG genetic association and is in trials for human gene therapy, has a significantly diminished strength of biological effect when it is found on a background of African rather than European ancestry. These results suggest that there are other, yet undiscovered variants at the locus that are truly causal (and are in linkage disequilibrium with S447X) or that work synergistically with S447X to modulate TG levels. Finally, we find systematically lower effect sizes for the 12 risk variants discovered in European populations on the African local ancestry background in JHS, highlighting the need for caution in the use of genetic variants for risk assessment across different populations.

Friday, January 09, 2009

Gene importance and evolutionary rate

Why Is the Correlation between Gene Importance and Gene Evolutionary Rate So Weak?
Zhi Wang, Jianzhi Zhang
PLoS Genetics 5(1): e1000329.
Abstract: One of the few commonly believed principles of molecular evolution is that functionally more important genes (or DNA sequences) evolve more slowly than less important ones. This principle is widely used by molecular biologists in daily practice. However, recent genomic analysis of a diverse array of organisms found only weak, negative correlations between the evolutionary rate of a gene and its functional importance, typically measured under a single benign lab condition. A frequently suggested cause of the above finding is that gene importance determined in the lab differs from that in an organism's natural environment. Here, we test this hypothesis in yeast using gene importance values experimentally determined in 418 lab conditions or computationally predicted for 10,000 nutritional conditions. In no single condition or combination of conditions did we find a much stronger negative correlation, which is explainable by our subsequent finding that always-essential (enzyme) genes do not evolve significantly more slowly than sometimes-essential or always-nonessential ones. Furthermore, we verified that functional density, approximated by the fraction of amino acid sites within protein domains, is uncorrelated with gene importance. Thus, neither the lab-nature mismatch nor a potentially biased among-gene distribution of functional density explains the observed weakness of the correlation between gene importance and evolutionary rate. We conclude that the weakness is factual, rather than artifactual. In addition to being weakened by population genetic reasons, the correlation is likely to have been further weakened by the presence of multiple nontrivial rate determinants that are independent from gene importance. These findings notwithstanding, we show that the principle of slower evolution of more important genes does have some predictive power when genes with vastly different evolutionary rates are compared, explaining why the principle can be practically useful despite the weakness of the correlation.

Friday, January 02, 2009

Genetic information doesn't add much for predicting diabetes

Predicting type 2 diabetes based on polymorphisms from genome-wide association studies: a population-based study.
van Hoek M, Dehghan A, Witteman JC, van Duijn CM, Uitterlinden AG, Oostra BA, Hofman A, Sijbrands EJ, Janssens AC.
Diabetes. 2008 Nov;57(11):3122-8.
OBJECTIVE: Prediction of type 2 diabetes based on genetic testing might improve identification of high-risk subjects. Genome-wide association (GWA) studies identified multiple new genetic variants that associate with type 2 diabetes. The predictive value of genetic testing for prediction of type 2 diabetes in the general population is unclear. RESEARCH DESIGN AND METHODS: We investigated 18 polymorphisms from recent GWA studies on type 2 diabetes in the Rotterdam Study, a prospective, population-based study among homogeneous Caucasian individuals of 55 years and older (genotyped subjects, n = 6,544; prevalent cases, n = 686; incident cases during follow-up, n = 601; mean follow-up 10.6 years). The predictive value of these polymorphisms was examined alone and in addition to clinical characteristics using logistic and Cox regression analyses. The discriminative accuracy of the prediction models was assessed by the area under the receiver operating characteristic curves (AUCs). RESULTS: Of the 18 polymorphisms, the ADAMTS9, CDKAL1, CDKN2A/B-rs1412829, FTO, IGF2BP2, JAZF1, SLC30A8, TCF7L2, and WFS1 variants were associated with type 2 diabetes risk in our population. The AUC was 0.60 (95% CI 0.57-0.63) for prediction based on the genetic polymorphisms; 0.66 (0.63-0.68) for age, sex, and BMI; and 0.68 (0.66-0.71) for the genetic polymorphisms and clinical characteristics combined. CONCLUSIONS: We showed that 9 of 18 well-established genetic risk variants were associated with type 2 diabetes in a population-based study. Combining genetic variants has low predictive value for future type 2 diabetes at a population-based level. The genetic polymorphisms only marginally improved the prediction of type 2 diabetes beyond clinical characteristics.
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