Cas9-guided haplotyping of three truncation variants in autosomal recessive disease

An autosomal recessive disease is caused by biallelic loss-of-function mutations. However, when more than two disease-causing variants are found in a patient’s gene, it has been challenging to determine which two of the variants are responsible for the disease phenotype. To decipher the pathogenic variants by precise haplotyping, we applied nanopore Cas9-targeted sequencing (nCATS) to three truncation COL7A1 variants detected in a patient with recessive dystrophic epidermolysis bullosa (EB). The distance between the most 5’ and 3’ variants was around 19 kb at the level of genomic DNA. nCATS successfully delineated that the most 5’ and 3’ variants were located in one allele while the variant in between was in the other allele. Intriguingly, the proband’s mother, who was phenotypically intact, was heterozygous for the allele that harbored the two truncation variants, which could otherwise be misinterpreted as those of typical recessive dystrophic EB. Our study illuminates nCATS as a useful tool to determine haplotypes of complicated genetic cases. Haplotyping of multiple variants in a gene can tell which variant should be therapeutically targeted when nucleotide-specific gene therapy is applied. Genomic region of COL7A1 which is about 8 kb long was enriched by PCR or nCATS, then applied for MinION sequencing.