Structural basis for PHD(V)C5HCH(NSD1)–C2HR(Nizp1) interaction: implications for Sotos syndrome

Sotos syndrome is an overgrowth syndrome caused by mutations within the functional domains of NSD1 gene coding for NSD1, a multidomain protein regulating chromatin structure and gene expression. In particular, PHD(V)C5HCH(NSD1) tandem domain, composed by a classical (PHD(V)) and an atypical (C5HCH) plant homeo-domain (PHD) finger, is target of several pathological missense-mutations. PHD(V)C5HCH(NSD1) is also crucial for NSD1-dependent transcriptional regulation and interacts with the C2HR domain of transcriptional repressor Nizp1 (C2HR(Nizp1)) in vitro. To get molecular insights into the mechanisms dictating the patho-physiological relevance of the PHD finger tandem domain, we solved its solution structure and provided a structural rationale for the effects of seven Sotos syndrome point-mutations. To investigate PHD(V)C5HCH(NSD1) role as structural platform for multiple interactions, we characterized its binding to histone H3 peptides and to C2HR(Nizp1) by ITC and NMR. We observed only very weak electrostatic interactions with histone H3 N-terminal tails, conversely we proved specific binding to C2HR(Nizp1). We solved C2HR(Nizp1) solution structure and generated a 3D model of the complex, corroborated by site-directed mutagenesis. We suggest a mechanistic scenario where NSD1 interactions with cofactors such as Nizp1 are impaired by PHD(V)C5HCH(NSD1) pathological mutations, thus impacting on the repression of growth-promoting genes, leading to overgrowth conditions.