Gonadal Mosaicism for an <i>ASH1L</i> Intragenic Deletion Makes a Bridge Between <scp>MRD52</scp> and 1q22 Microdeletion

ASH1L gene encodes a histone lysine methyltransferase, highly expressed in both embryonic and adult human brain. De novo loss-of-function variants in ASH1L are described in an ultrarare monogenic neurodevelopmental disorder, previously called mental retardation type 52 (MRD52). At the same time, a few cases are reported in the literature and DECIPHER with 1q22 microdeletions spanning ASH1L. We report three siblings presenting non-syndromic intellectual disability (ID) and an ASH1L intragenic deletion extending from exons 2 to 12 detected at SNP-array. Both parents resulted noncarrier suggesting gonadal/gonosomal mosaicism in one of the parents. This observation restricted the smallest region of overlap of the 1q22 microdeletion to ASH1L, and allowed to consider MRD52 and 1q22 microdeletion the same ASH1L-related neurodevelopmental disorder. We also reported the first example of gonadal/gonosomal mosaicism for an ASH1L deleterious variant, a fact that should generate the suspicion of recurrence also in sporadic cases of ASH1L-related neurodevelopmental disorder.