Differential disruption of autoinhibition and defect in assembly of cytoskeleton during cell division decide the fate of humanDIAPH1-related cytoskeletopathy

Background Diaphanous-related formin 1 (DIA1), which assembles the unbranched actin microfilament and microtubule cytoskeleton, is encoded by DIAPH1 . Constitutive activation by the disruption of autoinhibitory interactions between the N-terminal diaphanous inhibitory domain (DID) and C-terminal diaphanous autoregulatory domain (DAD) dysregulates DIA1, resulting in both hearing loss and blood cell abnormalities. Methods and results Here, we report the first constitutively active mutant in the DID (p.A265S) of humans with only hearing loss and not blood cell abnormality through whole exome sequencing. The previously reported DAD mutants and our DID mutant (p.A265S) shared the finding of diminished autoinhibitory interaction, abnormally upregulated actin polymerisation activity and increased localisations at the plasma membrane. However, the obvious defect in the DIA1-driven assembly of cytoskeleton ‘during cell division’ was only from the DAD mutants, not from p.A265S, which did not show any blood cell abnormality. We also evaluated the five DID mutants in the hydrophobic pocket since four of these five additional mutants were predicted to critically disrupt interaction between the DID and DAD. These additional pathogenic DID mutants revealed varying degrees of defect in the DIA1-driven cytoskeleton assembly, including nearly normal phenotype during cell division as well as obvious impaired autoinhibition, again coinciding with our key observation in DIA1 mutant (p.A265S) in the DID. Conclusion Here, we report the first mutant in the DID of humans with only hearing loss. The differential cell biological phenotypes of DIA1 during cell division appear to be potential determinants of the clinical severity of DIAPH1- related cytoskeletopathy in humans.