Striking phenotypic overlap between Nicolaides-Baraitser and Coffin-Siris syndromes in monozygotic twins with ARID1B intragenic deletion.

The chromatin remodeling AT-Rich interaction domain containing 1B protein (ARID1B) also known as BAF-associated factor, 250-KD, B (BAF250B) codified by the ARID1B gene (MIM#614556), is a small subunit of the mammalian SWI/SNF or BAF complex, an ATP-dependent protein machinery which is able to activate or repress gene transcription, allowing protein access to histones through DNA relaxed conformation. ARID1B gene mutations have been associated with two hereditary syndromic conditions, namely Coffin-Siris (CSS, MIM#135900) and Nicolaides-Baraitser syndromes (NCBRS, MIM#601358), characterized by neurodevelopment delay, craniofacial dysmorphisms and skeletal anomalies. Furthermore, intellectual impairment and central nervous system (CNS) alterations, comprising abnormal corpus callosum, have been associated with mutations in this gene. Moreover, ARID1B anomalies resulted to be involved in neoplastic events and Hirschprung disease. Here we report on two monozygotic male twins, displaying clinical appearance strikingly resembling NCBRS and CSS phenotype, who resulted carriers of a novel 6q25.3 microdeletion, encompassing only part of the ARID1B gene. The deleted segment was not inherited from the only parent tested and afflicted the first exons of the gene, coding for protein disordered region. We also provide, for the first time, a review of previously published ARID1B mutated patients with NCBRS and CSS phenotype and a computer-assisted dysmorphology analysis of NCBRS and ARID1B related CSS individuals, through the Face2Gene suite, confirming the existence of highly overlapping facial gestalt of both conditions. The present findings indicate that ARID1B could be considered a contributing gene not only in CSS but also in NCBRS phenotype, although the main gene related to this latter condition is the SMARCA2 gene (MIM#600014), another component of the BAF complex. So, ARID1B study should be considered in such individuals.

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