Isolated NIBPL missense mutations that cause Cornelia de Lange syndrome alter MAU2 interaction

Abstract

Cornelia de Lange syndrome (CdLS; or Brachmann-de Lange syndrome) is a dominantly inherited congenital malformation disorder with features that include characteristic facies, cognitive delays, growth retardation and limb anomalies. Mutations in nearly 60% of CdLS patients have been identified in NIPBL, which encodes a regulator of the sister chromatid cohesion complex. NIPBL, also known as delangin, is a homolog of yeast and amphibian Scc2 and C. elegans PQN-85. Although the exact mechanism of NIPBL function in sister chromatid cohesion is unclear, in vivo yeast and C. elegans experiments and in vitro vertebrate cell experiments have demonstrated that NIPBL/Scc2 functionally interacts with the MAU2/Scc4 protein to initiate loading of cohesin onto chromatin. To test the significance of this model in the clinical setting of CdLS, we fine-mapped the NIBPL-MAU2 interaction domain and tested the functional significance of missense mutations and variants in NIPBL and MAU2 identified in these minimal domains in a cohort of patients with CdLS. We demonstrate that specific novel mutations at the N-terminus of the MAU2-interacting domain of NIBPL result in markedly reduced MAU2 binding, although we appreciate no consistent clinical difference in the small group of patients with these mutations. These data suggest that factors in addition to MAU2 are essential in determining the clinical features and severity of CdLS.

Figure 1

Localization of NIPBL–MAU2 interaction. (a) NIPBL fragments used in confirmation and localization of NIPBL binding to MAU2 are demonstrated in gray to the left. Amino-acid residues included are indicated. The right panel demonstrates yeast two-hybrid colony assays, indicating interaction-dependent growth on tryptophan-, leucine- and histidine-deficient media (−T/L/H) of the positive control p53 with SV40TAg (T) in the uppermost row. Interaction-independent growth on tryptophan- and leucine-deficient plates is indicated to the right. NIBPL clones were tested with empty (-) and MAU2 containing AD fusion vectors. (b) The left panel depicts the NIPBL (aa 1–300) and MAU2 deletion constructs used for liquid β-galactosidase assay. The right panel indicates the interaction of the NIPBL/delangin fragment (1–300) and the different MAU2 protein fragments by liquid β-galactosidase assay.

Figure 2

The effect of NIPBL mutations on NIPBL/delangin–MAU2 interaction. (a) NIPBL/delangin motifs include a glutamine-rich domain (GLN), nuclear localization signal (NLS) and HEAT-repeat motifs. The minimal MAU-2 interaction fragment used (aa 1–38) and relative positions of mutations tested are demonstrated. (b) Constructs tested are indicated in the right panel and interaction data from mammalian two-hybrid assays are indicated in relative luciferase units for each CdLS mutation tested at the left. Significance of difference in activity of the G15R and P29Q versus wild-type (WT) are indicated. (c) Western blotting of NIPBL-BD 1–300 fragments with missense mutations from two-hybrid assays, using anti-GAL4 (DBD) antibody (Santa Cruz). All proteins are expressed at similar levels.

Figure 3

Clinical features of patients. Frontal and profile face photos and photos of hands are shown for the patients described. Images for an individual patient are outlined by a black box, with the patient's mutation indicated.