Expanding the MYCN Variant Spectrum in Feingold Syndrome Type 1: A Novel N-Terminal Missense Variant Segregating in an Affected Family.

This study reports a previously unreported heterozygous MYCN missense variant, c.454G>A (p.Ala152Thr), identified in a child and two affected relatives, with clinical findings consistent with Feingold syndrome type 1, an autosomal dominant developmental disorder most commonly caused by loss-of-function variants in MYCN. The proband presented with a cleft palate, craniofacial dysmorphism, feeding difficulties, hypotonia, and characteristic digital anomalies. Similar features were observed in the father and sibling. Clinical exome sequencing revealed the novel MYCN variant, which was confirmed by Sanger sequencing and demonstrated co-segregation with the phenotype. Although most pathogenic MYCN variants leading to FS1 truncate the protein, this missense change lies within the N-terminal transactivation domain, a region involved in transcriptional regulation and protein stability. The physicochemical alteration introduced at residue Ala152 may plausibly affect MYCN function, consistent with haploinsufficiency as the established disease mechanism. According to the 2024 ACGS Best Practice Guidelines, the variant was classified as a variant of uncertain significance leaning toward pathogenicity. This report expands the mutational spectrum of MYCN, supports the potential clinical relevance of N-terminal missense variation in MYCN, and highlights intrafamilial phenotypic variability in FS1.