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Nour Fayyad

Deciphering the role of xeroderma pigmentosum C (XPC) protein in base excision repair (BER) and oxidative stress

Published on 30 June 2021
Thesis presented June 30, 2021

Xeroderma pigmentosum C (XPC) protein initiates global genome-nucleotide excision repair (GG-NER) to remove UV-induced helix-distorting DNA lesions such as pyrimidine (6-4) pyrimidone photoproducts [(6-4) PPs] and cyclobutane pyrimidine dimers (CPDs). XPC deficient (XP-C) patients show a persistence of such lesions triggering high skin cancer incidences. They also suffer from internal cancers that could be due to the accumulation of oxidative DNA damage. Such oxidative DNA damage, including 8-oxoguanine (8-oxoGua), is usually repaired by the base excision repair (BER) pathway. Despite growing evidence about how XPC enhances the activity of several BER DNA glycosylases, the effect of XPC mutations on other BER factors and their activities is still elusive. Herein, we seek to answer this open question by characterizing normal and XP-C fibroblasts derived from patients, optimizing the conditions, and dividing our project into two parts.
In part one, we showed a global downregulation of BER’s genes in XP-C cells post-UVB compared to normal ones. Furthermore, the major proteins linked to oxidative DNA damage repair (OGG1, MYH, and APE1) were downregulated. This led to an ineffectiveness of BER in excising UVB-induced oxidized purines. In part two, we investigated whether treating XP-C cells with different drugs balancing the redox state could boost BER’s activity in XP-C cells. We showed that nicotinamide (NIC) and N-acetyl cysteine (NAC) pretreatments increase glutathione level, decrease ROS level, and enhance BER’s gene expression and activity post-UVB. Meanwhile, buthionine sulfoximine/dimethylfumuate (BSO/DMF) pretreatment depletes glutathione level, increases ROS level, and impairs BER’s gene expression and activity post-UVB.
Based on these results, we could propose that pretreatment with drugs that could enhance glutathione’s level may protect XP-C cells from an imbalanced redox state that affects DNA repair. This could pave the way for therapeutic strategies for XP-patients and other DNA repair-deficient patients.
Future work is required to check the efficiency of such treatments on 3D reconstructed skin and in vivo models. Additionally, studying the interactome linking XPC and glutathione signaling could be interesting.

UVB, DNA repair, Xeroderma Pigmentosum C, Drug treatment, oxidative stress, Skin cancer

On-line thesis.