Investigating the role of DIAPH1 in double strand break repair as the underlying cause of a novel DNA repair syndrome in humans

Woodward, Beth Louise ORCID: 0000-0003-3246-913X (2025). Investigating the role of DIAPH1 in double strand break repair as the underlying cause of a novel DNA repair syndrome in humans. University of Birmingham. Ph.D.

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Abstract

DNA double-strand break (DSB) repair is crucial for maintaining genome stability and preventing disease. The cell cycle phase and chromatin context largely determine which DSB repair pathway is used to restore damaged DNA. Several human diseases are associated with defective DSB repair and often arise from inherited variations in genes with critical roles in DSB repair pathways. One of the most well characterised human DSB repair deficiency disorders is Nijmegen Breakage Syndrome (NBS), which is caused by biallelic mutations in the NBN/NBS1 gene. Typically, NBS patients display a range of neurodevelopmental defects that include microcephaly, short stature, dysmorphic facial features, intellectual disability and immunodeficiency. Moreover, affected patients also have an increased risk of developing tumours, particularly those of lymphoid origin.
Recently, it has been shown that nuclear actin filaments play an important role in clustering DSBs to facilitate repair by homologous recombination (HR). However, the specific mechanism with which nuclear actin and various actin nucleating factors regulate HR remains unclear. Interestingly, patients with biallelic mutations in the actin nucleating factor DIAPH1 exhibit clinical features that significantly overlap with those typically associated with NBS, such as microcephaly, short stature, intellectual disability, immunodeficiency and tumour predisposition. This indicates that DIAPH1 may play a role in regulating HR, and that some of the clinical deficits associated with DIAPH1 mutations could stem from a DSB repair defect.
Consistent with this clinical similarity, this study demonstrates that cells derived from patients with biallelic mutations in DIAPH1 have a HR repair defect comparable to loss of Nbs1. Notably, this DSB repair defect is observed in a subset of patients with Baraitser-Winter Cerebrofrontofacial (BWCFF) Syndrome caused by de novo mutations in ACTG1 (-actin) but not ACTB (β-actin). Lastly, this study shows that DIAPH1 and -actin support HR-dependent repair by facilitating the relocalisation of the MRE11/RAD50/NBS1 (MRN) complex to sites of DNA damage to initiate DNA-end resection. Consequently, loss of DIAPH1 protein compromises the entire HR pathway, which inevitably gives rise to genome instability. These findings offer a novel mechanistic explanation for the overlapping clinical symptoms observed in patients with biallelic DIAPH1 mutations, BWCFF Syndrome, and NBS, and provides further insight into the role of actin and actin-nucleating factors in DSB repair.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):