Phenotypic Divergence of JAG1- and NOTCH2-Associated Alagille Syndrome & Disease-Specific NOTCH2 Variant Classification Guidelines

Abstract

Background & Aims Alagille syndrome (ALGS) is a rare, autosomal dominant disorder with high phenotypic heterogeneity. Disease-causing variants are primarily identified in Jagged1 (JAG1), with fewer reported in NOTCH2. JAG1 variants cause disease through a mechanism of haploinsufficiency, but the mechanism for NOTCH2 variants is not completely understood, making classification of variants more challenging. Using a large, international patient cohort acquired through the Global ALagille Alliance (GALA) study, we sought to improve classification of NOTCH2 variants and study phenotypic differences between NOTCH2- and JAG1-related disease. Methods Clinical and molecular data from 952 individuals with ALGS in GALA were analysed and disease features compared between those with JAG1 (n = 902) and NOTCH2 (n = 34) variants. Previously reported and newly identified NOTCH2 variants were reinterpreted based on disease-specific modifications to the American College of Medical Genetics and Genomics (ACMG) guidelines. The Kaplan–Meier method was utilised to assess native liver survival (NLS) and overall survival (OS) and gene comparisons were made with the log-rank test. Results Thirty NOTCH2 variants, including 18 novel variants, were identified and classified in our GALA cohort. Phenotypic analyses revealed a significantly lower incidence of characteristic facies, posterior embryotoxon, cardiac involvement and butterfly vertebrae in individuals with NOTCH2 variants compared to those with JAG1 variants (p < 0.001). No differences were identified in NLS or OS. Review of 61 previously reported NOTCH2 variants resulted in the re-classification of 19 likely pathogenic or pathogenic to VOUS (31.1%) with less than half retaining their originally published classification (34.4%; n = 21). Conclusions We report on a large global study on NOTCH2 genetics and phenotype, which increases the number of reported NOTCH2 variants by 30%. All variants were reclassified using current guidelines, and comparison of the JAG1 and NOTCH2 cohorts demonstrates clear phenotypic divergence between these groups. These data suggest that reliance on classical clinical phenotyping may miss patients with NOTCH2-related disease and supports an inclusive approach to genetic testing.