Alain Sarasin

The maintenance of mitochondrial proteins homeostasis, which is essential for proper cell function, is affected in pathophysiological ageing, yet several underlying mechanisms remain unexplored. We show that in normal and accelerated ageing cells, POLG1, the enzyme responsible for mitochondrial DNA replication, is degraded by the protease cathepsin B, which is overexpressed, escapes from lysosomes, and is stabilized by the chaperone activity of another protease, HTRA3. This degradation is in part counteracted by RAC1, a small GTPase also stabilized by HTRA3. POLG1 depletion, that occurs in progeroid Cockayne syndrome and senescent cells, is linked respectively to impairment or downregulation of the CSB protein, which promote cellular senescence. Our experiments in engineered cells, demonstrate that senescence itself, and not the absence of CSB, triggers the accumulation of cathepsin B and HTRA3, leading to POLG1 degradation. In summary, we uncover a complex, multi-step process that controls the degradation of POLG1 in mitochondria, a process that is activated by cell senescence and becomes more pronounced in Cockayne syndrome cells, providing new insight in the regulation of mitochondrial proteostasis in ageing and progeroid disorders.

Cockayne syndrome (CS) and UV‐sensitive syndrome (UVSS) are rare genetic disorders caused by mutation of the DNA repair and multifunctional CSA or CSB protein, but only CS patients display a progeroid and neurodegenerative phenotype, providing a unique conceptual and experimental paradigm. As DNA methylation (DNAm) remodelling is a major ageing marker, we performed genome‐wide analysis of DNAm of fibroblasts from healthy, UVSS and CS individuals. Differential analysis highlighted a CS‐specific epigenomic signature (progeroid‐related; not present in UVSS) enriched in three categories: developmental transcription factors, ion/neurotransmitter membrane transporters and synaptic neuro‐developmental genes. A large fraction of CS‐specific DNAm changes were associated with expression changes in CS samples, including in previously reported post‐mortem cerebella. The progeroid phenotype of CS was further supported by epigenomic hallmarks of ageing: the prediction of DNAm of repetitive elements suggested an hypomethylation of Alu sequences in CS, and the epigenetic clock returned a marked increase in CS biological age respect to healthy and UVSS cells. The epigenomic remodelling of accelerated ageing in CS displayed both commonalities and differences with other progeroid diseases and regular ageing. CS shared DNAm changes with normal ageing more than other progeroid diseases do, and included genes functionally validated for regular ageing. Collectively, our results support the existence of an epigenomic basis of accelerated ageing in CS and unveil new genes and pathways that are potentially associated with the progeroid/degenerative phenotype.

Background: Xeroderma pigmentosum (XP) is a rare genetic disorder characterized by a high incidence of skin cancers. These patients are deficient in nucleotide excision repair caused by mutations in one of the 7 XP genes. Methods: We diagnosed 181 XP patients using UV-induced DNA repair measurements and/or DNA sequencing from 1982 to 2022 in France. Results: As all XP patients, the French ones are very sensitive to UV exposure but since they are usually very well protected, they develop relatively few skin cancers. A majority of French XP patients originate from North Africa and bear a founder mutation on the XPC gene. The striking discovery is that these patients are at a very high risk to develop aggressive and lethal internal tumors such as hematological malignancies (more than a 100-fold risk compared to the general population for myelodysplasia/leukemia) with a median age of death of 25 years, and brain, gynecological, and thyroid tumors with even lower median ages of death. The high mutation rates found in XP-C internal tumors allow us to think that these XP patients could be successfully treated by immunotherapies. A full analysis of the molecular origins of these DNA repair-deficient tumors is discussed. Several explanations for this high predisposition risk are proposed. Conclusions: As the age of the XP population is increasing due to better photo-protection, the risk of lethal internal tumors is a new Damocles sword that hangs over XP-C patients. This review of the French cohort is of particular importance for alerting physicians and families to the prevention and early detection of aggressive internal tumors in XP patients.

Xeroderma Pigmentosum variant (XP-V) is an autosomal recessive disease with an increased risk to develop cutaneous neoplasms in sunlight exposed regions. These cells are deficient in the translesion synthesis DNA polymerase eta. Eleven skin tumors from a genetic cluster of XP-V patients had their exome sequenced. Mutational signatures identified for most tumors were related to ultraviolet exposure, such as C>T transitions targeted to pyrimidine dimers. However, four samples carry different mutational signatures, with C>A mutations associated with tobacco usage. Basal cell carcinomas showed a distinct C>A mutation spectra reflecting a novel mutational signature. Higher levels for retroposon insertions were detected in the XP-V tumors, compared to non-XP skin tumors. The results reveal other possible causes for XP-V tumors and the involvement of polymerase eta in suppressing retrotransposition. The expected high mutation burden, found in most of these tumors, renders these XP patients good candidates for immunotherapy with checkpoint blockers.

Background: Xeroderma pigmentosum (XP) is a rare, autosomal, recessive DNA repair-deficiency disorder with a frequency of 1–3 per million livebirths in Europe and USA but with higher frequencies in isolated islands or in countries with a high level of consanguinity. XP is characterized by high incidence of skin cancers on sun-exposed sites. Recent improvement in life expectancy of XP patients suggests an increased risk of frequently aggressive and lethal internal tumors. Our purpose was to quantify relative risks of internal tumor development for XP patients by tumor type, XP-subtype, patients’ ages and ethnicity through comparison with the US general population.Methods: We analyzed four independent international well-characterized XP cohorts (from USA, UK, France and Brazil) with a total of 434 patients, where 11.3% developed internal tumors and compared them to the American general population. We also compiled, through PubMed/Medline, a dataset of 89 internal tumors in XP patients published between 1958 and 2020.Results: In the combined 4-XP cohort, relative risk of internal tumors was 34 (95% confidence interval (CI) 25–47) times higher than in the general population (p-value = 1.0E−47) and tumor arose 50 years earlier. The XP-C group was at the highest risk for the 0–20 years old-patients (OR = 665; 95% CI 368–1200;p-value = 4.3E−30). The highest risks were observed for tumors of central nervous system (OR = 331; 95% CI 171–641;p-value = 2.4E−20), hematological malignancies (OR = 120; 95% CI 77–186;p-value = 3.7E−36), thyroid (OR = 74; 95% CI 31–179;p-value = 1.2E−8) and gynecological tumors (OR = 91; 95% CI 42–193;p-value = 3.5E−12). The type of mutation on theXPCgene is associated with different classes of internal tumors. The majority of French XP-C patients (80%) are originated from North Africa and carried theXPCdelTG founder mutation specific from the South Mediterranean area. The OR is extremely high for young (0–20 years) patients with more than 1300-fold increase for the French XPs carrying the founder mutation.Conclusion: Because the age of XP population is increasing due to better sun-protection and knowledge of the disease, these results are of particular importance for the physicians to help in early prevention and detection of internal tumors in their XP patients. Few preventive blood analyses or simple medical imaging may help to better detect early cancer appearance in this population.

Background Xeroderma pigmentosum (XP) is a rare, autosomal, recessive DNA repair-deficiency disorder with a frequency of 1-3 per million livebirths in Europe and USA but with higher frequencies in isolated islands or in countries with a high level of consanguinity. XP is characterized by high incidence of skin cancers on sun-exposed sites. Recent improvement in life expectancy of XP patients suggests an increased risk of frequently aggressive and lethal internal tumors. Our purpose was to quantify relative risks of internal tumor development for XP patients by tumor type, XP-subtype, patients’ ages and ethnicity through comparison with the US general population. Methods We analyzed four independent international well-characterized XP cohorts (from USA, UK, France and Brazil) with a total of 434 patients, where 11.3% developed internal tumors and compared them to the American general population. We also compiled, through PubMed/Medline, a dataset of 89 internal tumors in XP patients published between 1958 and 2020. Results In the combined 4-XP cohort, relative risk of internal tumors was 34 (95% confidence interval (CI) 25-47) times higher than in the general population (p-value=1.0E-47) and tumor arose 50 years earlier. The XP-C group was at the highest risk for the 0-20 years old-patients (OR=665; 95% CI 368-1200; p-value=4.3E-30). The highest risks were observed for tumors of central nervous system (OR=331; 95% CI 171-641; p-value=2.4E-20), hematological malignancies (OR=120; 95% CI 77-186; p-value=3.7E-36), thyroid (OR=74; 95% CI 31-179; p-value=1.2E-8) and gynecological tumors (OR=91; 95% CI 42-193; p-value=3.5E-12). The type of mutation on the XPC gene is associated with different classes of internal tumors. The majority of French XP-C patients (80%) are originated from North Africa and carried the XPC delTG founder mutation specific from the South Mediterranean area. The OR is extremely high for young (0-20 years) patients with more than 1,300-fold increase for the French XPs carrying the founder mutation. Conclusion Because the age of XP population is increasing due to better sun-protection and knowledge of the disease, these results are of particular importance for the physicians to help in early prevention and detection of internal tumors in their XP patients. Few preventive blood analyses or simple medical imaging may help to better detect early cancer appearance in this population.

Cockayne syndrome (CS) and UV-sensitivity syndrome (UVSS) are rare genetic disorders caused by mutation of the DNA repair and chromatin remodelling proteins CSA or CSB, but only CS patients display a progeroid and neurodegenerative phenotype. As epigenetic modifications constitute a well-established hallmark of ageing, we characterized genome-wide DNA methylation (DNAm) of fibroblasts from CS versus UVSS patients and healthy donors. The analysis of differentially methylated positions and regions revealed a CS-specific epigenetic signature, enriched in developmental transcription factors, transmembrane transporters, and cell adhesion factors. The CS-specific signature compared to DNAm changes in other progeroid diseases and regular ageing, identifyied commonalities and differences in epigenetic remodelling. CS shares DNAm changes with normal ageing more than other progeroid diseases do, and according to the methylation clock CS samples show up to 13-fold accelerated ageing. Thus, CS is characterized by a specific epigenomic signature that partially overlaps with and exacerbates DNAm changes occurring in physiological aging. Our results unveil new genes and pathways that are potentially relevant for the progeroid/degenerative CS phenotype.

Xeroderma pigmentosum (XP) is a rare, genetic, autosomal nucleotide excision repair-deficient disease characterized by sun-sensitivity and early appearance of skin and ocular tumors. Thirty-two black-skinned XP from Comoros, located in the Indian Ocean, were counted, rendering this area the highest world prevalence of XP. These patients exhibited a new homozygous XPC mutation at the 3'-end of the intron12 (IVS 12-1G>C) leading to the absence of XPC protein. This mutation, characteristic of the consanguineous Comorian families, is associated with a founder effect with an estimated age of about 800 years. Analysis of mt-DNA and Y-chromosome identified the haplogroups of patients, who are derived from the Bantu people. Although the four Comorian islands were populated by the same individuals during the 7-10th centuries, XP was found now only in the Comorian island of Anjouan. To avoid the slavery process caused by the arrival of the Arabs around the 11-13th centuries, inhabitants of Anjouan, including XP-heterozygotes, hid inland of the island protected by volcanoes. This population lived with an endogamic style, without connection with the other islands. XP patients still live in the same isolated villages as their ancestries. Local history and geography may, thus, explain the high incidence of XP located exclusively in one island.