Wiedemann-Steiner Syndrome (WSS): A Neonatal Case Report Expanding the Phenotypic Spectrum of a Previously Reported Missense Variant.

We present a neonatal case of Wiedemann-Steiner syndrome (WSS) with a de novo, previously reported KMT2A missense variant (c.3464G>A; p.Cys1155Tyr; NM_001197104.2), and provide a focused literature review of this specific variant. WSS (OMIM#605130) is a rare neurodevelopmental disorder caused by heterozygous variants in the KMT2A gene, which encodes a histone H3 lysine K4 (H3K4) methyltransferase involved in transcriptional regulation. Clinically, the syndrome is characterized by developmental delay, distinctive facial features, short stature, hypertrichosis, and neurological manifestations such as hypotonia and seizures. In this single-patient report, we describe additional clinical findings, including interstitial lung disease and hypertrophic pyloric stenosis requiring surgical intervention. These may represent rare manifestations of WSS that require confirmation in further reports before a formal expansion of the phenotypic spectrum can be established. Most pathogenic KMT2A variants arise de novo and are typically nonsense or frameshift; however, missense variants have also been reported and may have complex functional consequences. Haploinsufficiency is considered the primary pathogenic mechanism, leading to the disruption of chromatin modification and transcriptional regulation. While emerging genotype-phenotype correlations are being identified, considerable variability remains. Given the single-patient nature of this study, these observations should be considered hypothesis-generating and require confirmation in additional cases.