Background Cleidocranial dysplasia (CCD) is an autosomal dominant skeletal disorder characterised by hypoplastic or absent clavicles, increased head circumference, large fontanels, dental anomalies and short stature. Although CCD is usually caused by mutations leading to haploinsufficiency of RUNX2, the underlying genetic cause remains unresolved in about 25% of cases.

Methods Array comparative genomic hybridisation was performed to detect copy number variations (CNVs). Identified CNVs were characterised by quantitative PCR and sequencing analyses. The effect of candidate genes on mineralisation was evaluated using viral overexpression in chicken cells.

Results In 2 out of 16 cases, the authors identified microduplications upstream of MSX2 on chromosome 5q35.2. One of the unrelated affected individuals presented with a phenocopy of CCD. In addition to a classical CCD phenotype, the other subject had a complex synpolydactyly of the hands and postaxial polydactyly of the feet which have so far never been reported in association with CCD or CNVs on 5q35.2. The duplications overlap in an ∼219 kb region that contains several highly conserved non-coding elements which are likely to be involved in MSX2 gene regulation. Functional analyses demonstrated that the inhibitory effect of Msx2 overexpression on mineralisation cannot be ameliorated by forced Runx2 expression.

Conclusions These results indicate that CNVs in non-coding regions can cause developmental defects, and that the resulting phenotype can be distinct from those caused by point mutations within the corresponding gene. Taken together, these findings reveal an additional mechanism for the pathogenesis of CCD, particularly with regard to the regulation of MSX2.