Experimental long-distance haplotyping of OCA2-HERC2 variants

The regulatory HERC2 SNP, rs12913832, is strongly associated with blue and brown eye colour. However, eye colour in heterozygous rs12913832 individuals is observed to vary greatly. Missense mutations in OCA2, such as rs1800407 and rs74653330, are associated with lighter eye colour in some but not all heterozygous rs12913832 individuals. Determining the physical linkage of these variants might help to further explain eye colour variation. So far, experimental haplotyping of these variants has been challenging because the genomic distance between them (∼ 135 kb) exceeds the fragment lengths produced by commonly used DNA isolation kits. The aim for this study was to explore novel methods for long distance haplotyping to assess associations between OCA2-HERC2 haplotypes and eye colour. DNA was isolated from frozen blood samples collected from Norwegians that are known to be heterozygous for both HERC2 rs12913832 and OCA2 SNPs, either rs1800407 (n = 23) or rs74653330 (n = 17), using the newly commercially available Monarch® HMW (heigh molecular weight) DNA Extraction Kit (New England BioLabs_inc). We successfully isolated DNA fragments up to 210 kb, which were long enough to haplotype OCA2-HERC2 loci by droplet digital PCR (ddPCR). Three haplotypes were observed in the study population: rs12913832:A-rs1800407:T in 22/23 individuals, rs12913832:A-rs1800407:C in 1/23 individuals and rs12913832:A-rs74653330:T in 16/16 individuals. As expected, all individuals with the rs12913832:A-rs74653330:T haplotype had intermediate to blue eye colour. However, the rs12913832:A-rs1800407:T haplotype was observed in both blue and brown-eyed individuals, suggesting more research is needed.     

Analysis strategies to establish vWF intron 40 haplotypes

Sequencing of a 0.65 kb region in the intron 40 of the vWF gene demonstrated a complex variability. Five STRs named Pol K, F, P1, P2-a and P2-b and an indel polymorphisms (I) are present. We established a routine analysing method to puzzle out the Pol K/F/I/P haplotypes which does not require a sequencing procedure. To recognise the combined polymorphisms as haplotypes, we performed short and middle range PCRs in combination with Nde I and BsmA I restriction tests. Comparison of the amplicon and restriction fragment length reveals the most likely haplotypes of each person involved a kinship test. Furthermore, a SNP allele specific PCR was employed. Additional information can be achieved by typing Pol P2-a and P2-b. Establishing of intron 40 vWF haplotypes using the methods described here can greatly support the resolution of complex kinship cases. This statement is illustrated by demonstration of a family study.