Pedigree and Functional Analysis of Two Cryptic OTC Variants Causing Ornithine Transcarbamylase Deficiency in Two Unrelated Chinese Male Patients.

BACKGROUND: Ornithine transcarbamylase deficiency (OTCD, MIM#311250) is a rare X-linked urea cycle disorder causing hyperammonemia. While around 600 pathogenic OTC variants have been reported, cryptic changes like synonymous or in-frame variants remain poorly characterized and are easily overlooked in routine screening. METHODS: We analyzed the clinical and genetic profiles of two unrelated Chinese male patients clinically diagnosed with OTCD. Whole-exome sequencing (WES) was performed to identify the genetic etiology. A minigene assay was then used to assess the splicing effect of the detected synonymous variant. RESULTS: WES revealed two rare OTC variants: a synonymous variant (NM_000531.6: c.663G>A, p.Lys221Lys) and a novel de novo inframe variant (c.756_761dupAGCAGC, p.Ala253_Ala254dup). Minigene assay confirmed that the c.663G>A variant causes exon 6 skipping, leading to the deletion of 41 amino acids (c.541_663del, p.Glu181_Lys221del). Both variants were classified as clinically pathogenic, and the genotype-phenotype relationships were potentially established. CONCLUSION: Our study expands the mutation spectrum of OTC and emphasizes the importance of cryptic variants interpretation and the need for additional functional studies to verify the pathogenicity of these variants.