Non-coding variants are a rare cause of recessive developmental disordersin transwith coding variants

Purpose Identifying pathogenic non-coding variants in individuals with developmental disorders (DD) is challenging due to the large search space. It is common to find a single protein-altering variant in a recessive gene in DD patients, but the prevalence of pathogenic non-coding ;second hits; in trans with these is unknown. Methods In 4,073 genetically undiagnosed rare disease trio probands from the 100,000 Genomes project, we identified rare heterozygous loss-of-function (LoF) or ClinVar pathogenic variants in recessive DD-associated genes. Using stringent region-specific filtering, we identified rare non-coding variants on the other haplotype. Identified genes were clinically evaluated for phenotypic fit, and where possible, we performed functional testing using RNA-sequencing. Results We found 2,430 probands with one or more rare heterozygous pLoF or ClinVar pathogenic variants in recessive DD-associated genes, for a total of 3,761 proband-variant pairs. For 1,366 (36.3%) of these pairs, we identified at least one rare non-coding variant in trans . After stringent bioinformatic filtering and clinical review, five were determined to be a good clinical fit (in ALMS1, NPHP3, LAMA2, IGHMBP2 and GAA ). Conclusion We developed a pipeline to systematically identify and annotate compound heterozygous coding/non-coding genotypes. Using this approach we uncovered new diagnoses and conclude that this mechanism is a rare cause of DDs.