<p><i>Background</i>: Retinitis pigmentosa (RP) is the most common cause of inherited retinal degeneration and can occur in non-syndromic and syndromic forms. Syndromic RP is accompanied by other symptoms such as intellectual disability, hearing loss, or congenital abnormalities. Both forms are known to exhibit complex genetic interactions that can modulate the penetrance and expressivity of the phenotype.</p> <p><i>Materials and methods</i>: In an individual with atypical RP, hearing loss, ataxia and cerebellar atrophy, whole exome sequencing was performed. The candidate pathogenic variants were tested by developing an <i>in vivo</i> zebrafish model and assaying for retinal and cerebellar integrity.</p> <p><i>Results</i>: Exome sequencing revealed a complex heterozygous protein-truncating mutation in <i>RP1L1</i>, p.[(Lys111Glnfs*27; Gln2373*)], and a heterozygous nonsense mutation in <i>C2orf71</i>, p.(Ser512*). Mutations in both genes have previously been implicated in autosomal recessive non-syndromic RP, raising the possibility of a digenic model in this family. Functional testing in a zebrafish model for two key phenotypes of the affected person showed that the combinatorial suppression of <i>rp1l1</i> and <i>c2orf71l</i> induced discrete pathology in terms of reduction of eye size with concomitant loss of rhodopsin in the photoreceptors, and disorganization of the cerebellum.</p> <p><i>Conclusions</i>: We propose that the combination of heterozygous loss-of-function mutations in these genes drives syndromic retinal dystrophy, likely through the genetic interaction of at least two loci. Haploinsufficiency at each of these loci is insufficient to induce overt pathology.</p
