November 5, 2012

Causes of skin and hair color variance in Europeans remain undetermined

Portuguese & N. Irish
Our ability to predict pigmentation traits from genetic loci remains limited but this new paper adds some honest research on the matter:

Sophie I. Candille et al., Genome-Wide Association Studies of Quantitatively Measured Skin, Hair, and Eye Pigmentation in Four European Populations. PLoS ONE, 2012. Open access ··> LINK [doi:10.1371/journal.pone.0048294]

One of the findings is that women have darker skin shades than men in Europe (but not among peoples with dark skin from several continents, where men are darker). Another unstated but curiously counterintuitive finding is that Portuguese (from Porto) have on average the same skin tone as Polish (from Warsaw) do:

Table 1. Skin, hair, and eye pigmentation by sex and country.

However for hair and eye color, Polish have lighter shades, approaching the Irish (Dublin) extreme values, while Portuguese approach Italians (Rome) in hair color and show darker eyes on average than anybody else among the sampled populations.

Another curiosity of the survey is that Irish women show significantly lighter hair shades than Irish men, a phenomenon not appreciable elsewhere.

The authors found that, in general:

... in this European sample, pigmentation phenotypes are mainly stratified by country, whereas height is mainly stratified by sex.

They also found that:

Skin and eye pigmentation are correlated in Ireland. Hair and eye pigmentation are correlated in Portugal. Skin and hair pigmentation are correlated in Poland and Italy (Table S2).

What I find rather curious and suggestive of complex genetic influences affecting more than just one pigmentation trait at the same time. But which ones? And why do they seem to operate differently in different populations?

The GWAS analysis found these loci as significant:

Table 2. GWAS, replication, and combined association results for all signals with p-value<10−5 in the GWAS.

Apparently neither the SCIN nor the FLNB genes have been related with pigmentation before. Therefore the authors applied a strong test of reliability (replication in the table), correcting for geographical structure, which actually discarded all loci except the already known ones for eye color in relation to OCA2/HERC2, which were: rs1667394, rs8039195, rs1635168, rs16950987, and rs8028689.

However further analysis showed that rs1667394 is in linkage disequilibrium (LD) with rs12913832 (OCA2), which is the actual culprit of blue eyes (a well known SNP that explains some 45% of the eye color variance among Dutch).

In regard to the failure to detect markers of skin and hair color variance, they conclude that:

The fact that we did not detect reproducible associations with skin or hair color suggests that, unlike eye color, skin and hair pigmentation variation in Europe are not determined by major loci.

Furthermore, genes that have been shown to contribute to skin color variance in South Asians (rs1426654 SLC24A5, rs16891982 SLC45A2, and rs1042602 TYR) or in African-European admixed populations (rs1426654 in SLC24A5 again), fail to show any importance in intra-European variance for this trait. However rs1426654 is fixated in Northern Europeans (CEU), so it cannot show any variation.

Other SNPs (rs16891982 and r183671 in SLC45A2, which are in LD) may contribute to skin pigmentation, however the pattern mentioned (in which Italians and Portuguese are contrasted with Polish and Irish) rather reminds me of the variation for hair and eye color instead.

They also mention that rs885479 in MC1R has not provided any clear association in previous studies but that they did find some association with skin color, however they did not practice the replication test for this SNP.

In the end not much new other than some cold water but an straightforward study for the record.

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