Germline mosaicism in a family with MBD5 haploinsufficiency

Haploinsufficiency of the methyl-CpG-binding domain protein 5 (MBD5) gene causes a neurodevelopmental disorder that includes intellectual disability, developmental delay, speech impairment, seizures, sleep disturbances, and behavioral difficulties. Microdeletion of 2q23.1 is the most common cause of haploinsufficiency, although MBD5 haploinsufficiency may also cause this genetic disorder. We report a family harboring a heterozygous loss-of-function variant in MBD5 (NM_018328.5:c.728delC; p.Pro243Hisfs*26), which includes three affected siblings with varying phenotypic features. Both parents were phenotypically normal but deep coverage sequencing of the parents showed germline mosaicism in the mother.