Skin hypopigmentation along Blaschko’s lines, also known as hypomelanosis of Ito (HI) is considered a non-specific manifestation of mosaicism.  Besides various mosaic chromosomal anomalies, only MTOR mutations in HI with hemimegalencephaly have been reported as an underlying single gene anomaly. Here we report on a novel mosaic neuroectodermal syndrome associated with postzygotic RHOA mutations. We studied six unrelated patients ascertained through our Mosaic Undiagnosed Skin Traits And Related Disorders (M.U.S.T.A.R.D.) project. All had a hitherto undescribed mosaic neuroectodermal syndrome, with a strikingly similar combination of linear hypopigmentation and hypotrichosis along Blaschko’s lines, facial dysmorphism, ocular and dental anomalies, short fingers and toes, and asymptomatic leukoencephalopathy on MRI. We initially performed whole-exome sequencing (WES) in affected skin from two patients, and identified the same postzygotic variant of RHOA in both probands, absent in their parents’ blood. This variant with a high pathogenicity score was also found in two unrelated individuals with a similar phenotype by targeted ultra-deep sequencing of RHOA. The fifth patient carried another postzygotic RHOA variant, detected by WES. In the sixth patient, no changes were detected. Transfection of NIH 3T3 cells with these mutant RhoA constructs resulted in reduced cell spreading, decreased stress fiber formation, and decreased phosphorylation of downstream effectors of RhoA, MYPT2 and MLC2, suggesting a dominant-negative effect of both variants. RHOA encodes RhoA, a RAS-related Rho GTPase, which controls morphogenesis, chemotaxis, axonal guidance, and cell cycle progression. RhoA is a highly conserved protein particularly intolerant to amino acid substitutions. Hence, RHOA mutations are likely to be embryonic lethal and may survive only by mosaicism. Our findings highlight the role of RHOA in human development and disease, and expand the clinical spectrum of mosaic neuroectodermal syndromes.

Postzygotic mutations of RHOA cause a mosaic neuroectodermal syndrome