Impairment of different protein domains causes variable clinical presentation within Pitt-Hopkins syndrome and suggests intragenic molecular syndromology of TCF4

An intragenic phenotypic map of loss-of-function variants in TCF4 is suggested here for the first time: variants within exons 1-4 and exons 4-6 give rise to a recurrent phenotype with mild ID not in the spectrum of Pitt-Hopkins syndrome (biallelic preservation of both the NLS and bHLH domains); variants within exons 7-8 cause a severe phenotype resembling PTHS but in absence of the typical facial dysmorphism (impairment limited to the NLS domain); variants within exons 9-19 cause typical Pitt-Hopkins syndrome (impairment of at least the bHLH domain). Understanding the TCF4 molecular syndromology can allow for proper nosology in the current era of whole genomic investigations.