The importance of phase analysis in multiexon copy number variation detected by aCGH in autosomal recessive disorder loci.

Cohen Syndrome (CS) is a rare autosomal recessive disorder caused by homozygous or compound heterozygous pathogenic variants in VPS13B, also known as COH1. Over 100 pathogenic variants in VSP13B, primarily truncations, and copy number variants, have been found in patients with CS. Here, we present an 11-month-old girl with CS caused by two multi-exonic small deletions in VSP13B in trans. Array comparative genomic hybridization has revolutionized the field of genome copy number analysis down to the exonic level, however it has its limitations. It cannot detect balanced structural variation nor determine the phase of copy number variants. Heterozygous multi-exonic copy number variation in autosomal recessive genes should be interpreted in the context of a clinical phenotype, and, if warranted, phase analysis should be performed before sequence analysis for that gene is pursued. This patient emphasizes the need of obtaining clinical information and determining the phase in multi-exonic copy number variants for accurate diagnosis and risk counseling.