Characterising formaldehyde genotoxicity in cockayne syndrome, and its influence on cell fate
Cockayne syndrome (CS) is a rare genetic disorder caused by autosomal recessive loss-of-function mutations in the CSB or CSA proteins. These proteins are involved in transcription-coupled nucleotide excision repair (TC-NER), a DNA repair mechanism triggered by lesions which stall the transcriptional machinery. This stalling activates recruitment of CSB, which recruits proteins that backtrack the transcriptional machinery from the lesion, making it accessible for excision by endonucleases XPA, XPG and XPF-ERCC1. Loss of CSB function prevents lesion repair, resulting in a severe phenotype including cachexia, progeria, growth retardation, photosensitivity, neurodegeneration and kidney failure, with patients dying in the first or second decade of life. Treatments for this disease are currently limited, consisting mainly of symptomatic and supportive measures. The source of the DNA damage which stalls transcription is unclear, but recent study by the Patel group has revealed formaldehyde as an endogenous metabolite which forms transcription-stalling DNA lesions. Knockout of Adh5, the primary enzyme responsible for formaldehyde detoxification, precipitates the cockayne syndrome phenotype in CSBm/m mice, which previously only subtly mimicked the disease. My project aims to utilise this mouse model to interrogate the location, cause, and consequences of formaldehyde DNA damage in cockayne syndrome.
Curiously, CS patients do not have an increased risk of cancer, a feature which is distinct from other DNA repair deficiency syndromes including xeroderma pigmentosum (XP) - a disease caused by deficiency in XP-A, XP-G, XP-D, XP-E, XP-F, XP-B, XP-V or XP-C proteins. These XP proteins aid in both transcription coupled nucleotide repair (TC-NER), and global nucleotide excision repair (GG-NER) - a mechanism which operates genome-wide to sense and repair DNA helix-distorting lesions. Unlike in CS, XP patients have an extreme predisposition to cancer, with a 10,000 fold increased risk of skin cancer. The dissimilarity of these two syndromes indicates that defects in GG-NER, but not TC-NER, predispose to carcinogenesis. Alternatively, XP proteins may be involved in tumour suppressive functions distinct from CSB. Regardless, CSB deficiency appears to protect against malignant transformation, with Adh5-/-/CSBm/m mice showing no signs of cancer, compared to Adh5-/- mice which accumulate liver and haematopoietic malignancies. The reason for this seemingly paradoxical observation is unclear, as is the exact cellular response to transcriptional stress in CSB-/- cells. Thus, my project also aims to elucidate the cellular consequences of CSB deficiency in cells, and their mechanism of avoiding malignancy.
Overall, this project aims to provide a deeper understanding of the pathophysiology underlying cockayne syndrome, opening avenues for novel treatments and providing insight on the wider mechanisms of cancer development.