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, 89 (2), 334-43

GWAS Findings for Human Iris Patterns: Associations With Variants in Genes That Influence Normal Neuronal Pattern Development

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GWAS Findings for Human Iris Patterns: Associations With Variants in Genes That Influence Normal Neuronal Pattern Development

Mats Larsson et al. Am J Hum Genet.

Abstract

Human iris patterns are highly variable. The origins of this variation are of interest in the study of iris-related eye diseases and forensics, as well as from an embryological developmental perspective, with regard to their possible relationship to fundamental processes of neurodevelopment. We have performed genome-wide association scans on four iris characteristics (crypt frequency, furrow contractions, presence of peripupillary pigmented ring, and number of nevi) in three Australian samples of European descent. Both the discovery (n = 2121) and replication (n = 499 and 73) samples showed evidence for association between (1) crypt frequency and variants in the axonal guidance gene SEMA3A (p = 6.6 × 10(-11)), (2) furrow contractions and variants within the cytoskeleton gene TRAF3IP1 (p = 2.3 × 10(-12)), and (3) the pigmented ring and variants in the well-known pigmentation gene SLC24A4 (p = 7.6 × 10(-21)). These replicated findings individually accounted for around 1.5%-3% of the variance for these iris characteristics. Because both SEMA3A and TRAFIP1 are implicated in pathways that control neurogenesis, neural migration, and synaptogenesis, we also examined the evidence of enhancement among such genes, finding enrichment for crypts and furrows. These findings suggest that genes involved in normal neuronal pattern development may also influence tissue structures in the human iris.

Figures

Figure 1
Figure 1
Crypt Frequency Crypt frequency measures the degree of hypoplasia in the two top cell layers in the iris; i.e., the anterior border layer and the underlying stroma. The arrows indicate examples of crypt markings of interest in the iris tissue. The categories above depict (A) only parallel and/or densely packed curly fibers, (B) spots of wavy fibers and 1–3 shallow crypts, (C) at least four shallow crypts, (D) at least five marked crypts and/or a tissue texture that is more permeable than in category 3, (E) at least four large crypts that extend at least 2/3 of the distance between the collarette and the periphery of the iris.
Figure 2
Figure 2
Extension and Evenness of the Pigmented Ring The pigmented ring measures the amount of melanin that surrounds the pupil and distinguishes different shades of green and hazel eye color (91% of people judged to have a green or hazel eye color by a distant observer in this sample had a pigmented ring). The categories above depict (A) the absence of a pigmented ring; (B) an uneven pigmented ring, covering 18–348 degrees; (C) an even pigmented ring, covering > 348 degrees.
Figure 3
Figure 3
Distinction and Extension of Furrow Contractions Furrows manifest as the result of a tendency of the iris to fold in exactly the same location each time the iris adapts to different light conditions. They relate to the overall thickness and density of the iris (thicker irises have more extended furrows). The arrows indicate example of furrows on the iris. The categories above depict (A) the absence of furrows, or short furrows extending < 90 degrees; (B) furrows extending between 90 and 288 degrees; (C) distinct furrows, extending at least 288 degrees; (D) more marked furrows than in category 3, extending at least 324 degrees; (E) at least two concentric furrows, with a primary furrow of > 324 degrees and a second furrow of > 288 degrees.
Figure 4
Figure 4
Frequency of Iris Nevi Iris nevi are melanin accumulations on the anterior border layer. The arrows point to examples of iris nevi. The categories above depict (A) the absence of iris nevi, (B) at least one iris nevus, (C) at least three iris nevi, (D) at least five iris nevi.
Figure 5
Figure 5
Q-Q Plots for the 24,812 SNPs Located within the 219 Genes in the Krasnova et al. Set55 See Table S9 as well.

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