Case Series of Nizon‐Isidor Syndrome by Heterozygous Variants in MED12L With Further Evidence of Mitotic Instability in One Case With Diploid–Triploid Mosaicism

Nizon-Isidor syndrome is a rare disorder caused by heterozygous variants in MED12L, with only eight documented cases in the literature. Here, we present three additional cases of this syndrome. Proband 1 was a 7-year-old female who presented with developmental delay, right-leg hemihypertrophy, laryngeal cleft, esotropia, abnormal skin pigmentation, sectoral iris hypopigmentation, dysphagia, periventricular nodular heterotopia, seizures, morbid obesity, and a pelvic kidney. Genome sequencing (GS) revealed a MED12L variant, NM_053002.5:c.3559+2T>G. Both computational models and transcriptomic analysis confirmed that this variant induced splice loss of MED12L exon 25. Probands 2 and 3 presented with overlapping phenotypes of developmental delay; sequencing confirmed c.3441_3444dup; p.(G1149Nfs*13) and seq[GRCh37] del(3)(q25.1q25.1) chr3:g.?_151075120 variants affecting MED12L. Further investigation found diploid-triploid mosaicism in Proband 1, supporting the hypothesis that loss of MED12L function may increase risk for other cytogenetic abnormalities. Probands 2 and 3 did not harbor evidence of additional cytogenetic aberrations. In Proband 1, caloric restriction and semaglutide-pramlintide combination therapy were started at age eight and were effective in weight reduction. Overall, this report expands the phenotypic spectrum of Nizon-Isidor syndrome, highlights a potential link between MED12L and cytogenetic abnormalities, and demonstrates a case of weight loss through GLP-1 therapy in a child with a genetic obesity syndrome.