Homozygosity mapping of achromatopsia to chromosome 2 using DNA pooling

Achromatopsia is an autosomal recessive disease of the retina,characterized clinically by an inability to distinguish colors, impairedvisual acuity, nystagmus and photophobia. A genome-wide search for linkagewas performed using an inbred Jewish kindred from Iran. To facilitate thegenome-wide search, we utilized a DNA pooling strategy which takesadvantage of the likelihood that the disease in this inbred kindred isinherited by all affected individuals from a common founder. Equal molaramounts of DNA from all affected individuals were pooled and used as thePCR template for short tandem repeat polymorphic markers (STRPs). PooledDNA from unaffected members of the kindred was used as a control. Areduction in the number of alleles in the affected versus control pool wasobserved at several loci. Upon genotyping of individual family members,significant linkage was established between the disease phenotype andmarkers localized on chromosome 2. The highest LOD score observed was 5.4(theta = 0). When four additional small unrelated families were genotyped,the combined peak LOD score was 8.2. Analysis of recombinant chromosomesrevealed that the disease gene lies within a 30 cM interval which spans thecentromere. Additional fine-mapping studies identified a region ofhomozygosity in all affected individuals, narrowing the region to 14 cM. Acandidate gene for achromatopsia was excluded from this disease interval byradiation hybrid mapping. Linkage of achromatopsia to chromosome 2 is anessential first step in the identification of the disease-causing gene.