How people get blue eyes

Genome-wide association studies can uncover links between genetic variants and phenotypes, even in the absence of any knowledge of how these links come about. All it takes is to make a statistical case linking genetic variation with the recorded phenotypic information.

This is somewhat unsatisfactory for a couple of reasons. First, we would like to know how cause and effect works, rather than simply observe that it does. Why do some people with certain genetic alleles have blue eyes?

Second, such functional studies allow us to predict phenotypes from genotypes. A great number of genetic mutations may cause particular phenotypes, and we are only able to discover associations between a subset of them that happens to exist in a population. Developing knowledge about function, rather than just statistical association, may help us in the future to infer the phenotypes of individuals from the deep past for which all non-osteological traces of phenotype have vanished, and may have been affected by genetic variants that are now extinct.

Many human traits are governed by a great number of genes, either through additive effects, or through complex interactions. Eye color is an example of a particular trait the genetic underpinnings of which in Caucasoids (other races have eyes that are uniformly brown) have been known for a while. Now a new study shows precisely how genetic mutations disrupt the formation of pigment in melanocytes, resulting in light-pigmented irides.


Genome Res doi:10.1101/gr.128652.111

HERC2 rs12913832 modulates human pigmentation by attenuating chromatin loop formation between a long-range enhancer and the OCA2 promoter

Mijke Visser et al.

Pigmentation of skin, eye and hair reflects some of the most evident common phenotypes in humans. Several candidate genes for human pigmentation are identified, and the SNP rs12913832 has strong statistical association with human pigmentation. It is located within an intron of the non-pigment gene HERC2, 21 kb upstream of the pigment gene OCA2, and the region surrounding rs12913832 is highly conserved among animal species. However, the exact functional role of HERC2 rs12913832 in human pigmentation is unknown. Here we demonstrate that the HERC2 rs12913832 region functions as an enhancer regulating OCA2 transcription. In darkly pigmented human melanocytes carrying the rs12913832 T-allele, we detected binding of the transcription factors HLTF, LEF1 and MITF to the HERC2 rs12913832 enhancer, and a long-range chromatin loop between this enhancer and the OCA2 promoter which leads to elevated OCA2 expression. In contrast, in lightly pigmented melanocytes carrying the rs12913832 C-allele, chromatin-loop formation, transcription factor recruitment and OCA2 expression are all reduced. Hence, we demonstrate that allelic variation of a common non-coding SNP located in a distal regulatory element not only disrupts the regulatory potential of this element but also affects its interaction with the relevant promoter. We provide the key mechanistic insight that allele-dependent differences in chromatin-loop formation (i.e. structural differences in the folding of gene loci) results in differences in allelic gene expression that affects common phenotypic traits. This concept is highly relevant for future studies aiming to unveil the functional basis of genetically-determined phenotypes including diseases.

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