Identifying genetic mosaicism is important in establishing a diagnosis, assessing recurrence risk, and providing accurate genetic counseling. Next-generation sequencing has allowed for the identification of mosaicism at levels below those detectable by conventional Sanger sequencing or chromosomal microarray analysis. The CAUSES Clinic was a pediatric translational trio-based genome-wide (exome or genome) sequencing study of 500 families (531 children) with suspected genetic disease at BC Children's and Women's Hospitals. Here we present 12 cases of apparent mosaicism identified in the CAUSES cohort: 9 cases of parental mosaicism for a disease-causing variant found in a child and 3 cases of mosaicism in the proband for a de novo variant. In 6 of these cases, there was no evidence of mosaicism on Sanger sequencing; the variant was not detected on Sanger sequencing in 3 cases, and it appeared to be heterozygous in 3 others. These cases are examples of 6 clinical manifestations of mosaicism: a proband with classical clinical features of mosaicism (e.g., segmental abnormalities of skin pigmentation or asymmetrical growth of bilateral body parts), a proband with unusually mild manifestations of a disease, a mosaic proband who is clinically indistinguishable from the constitutive phenotype, a mosaic parent with no clinical features of the disease, a mosaic parent with mild manifestations of the disease and a family in which both parents are unaffected and two siblings have the same disease-causing constitutional mutation. Our data demonstrates the importance of considering the possibility of mosaicism whenever exome or genome sequencing is performed and that its detection via genome-wide sequencing can permit more accurate genetic counseling.

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