Curation CCD RUNX2

A Japanese sporadic CCD patient had a heterozygous RUNX2 c.181_189dupGCGGCGGCT variant, expanding the polyalanine tract from 17A to 20A (+3Ala), with severe short stature, cranial suture delay, dental abnormalities, mild brachydactyly, and mild clavicular hypoplasia.

PMID:40585427 reports RUNX2 GCN/polyalanine expansions as secondary findings detected during reanalysis of WES/WGS data from neurologic probands. Using a pathogenic threshold of 20, the authors identified 3 probands with polyalanine expansions in RUNX2 with the lengths of 23, 21, and 20 repeats. The 21-repeat expansion was also verified in the proband's healthy mother. None of the probands or the healthy mother had reported CCD-related phenotype. The authors suggest incomplete penetrance within the low pathogenic range or the possibility that the pathogenic range may be longer than 23 repeats.

Family CCD-B had an in-frame 30-bp RUNX2/CBFA1 polyalanine duplication expanding the tract from 17A to 27A, segregating with affected family members who had brachydactyly and minor clinical findings of CCD.

RUNX2 contains a Q/A repeat domain with a wild-type 23Q/17A tract; the familial CCD variant expanded the glutamine tract to 27Q/17A (p.Q71_E72insQQQQ), consistent with repeat-slippage susceptibility of this locus.

RUNX2 is a RUNT-family transcription factor controlling bone morphogenesis; the RUNX2 repeat-containing IDR drove phase separation in optoDroplet assays and formed liquid-like droplets.

RUNX2 polyQ/polyA repeat peptides formed alpha-helical coiled-coil structures in vitro, with polyalanine length and coiled-coil stability promoting higher-order assembly.

The CCD-associated RUNX2 +10A alanine expansion enhanced homotypic phase separation, unblended from MED1 co-condensates, and significantly reduced IDR-driven luciferase reporter activity.

In HEK293T and U2OS non-patient cell systems, the RUNX2 +10A IDR showed altered condensate/aggregate behavior and reduced MED1 recruitment compared with wild type.

In HEK293 cells, polyA-expanded RUNX2(+12A) and coiled-coil-stabilized RUNX2 mutants mislocalized and aggregated; coiled-coil-disrupting mutations reduced aggregation and mislocalization.