Anita Lombardi, Lavinia Arseni, Roberta Carriero, Emmanuel Compe, Elena Botta, Debora Ferri, Martina Uggè, Giuseppe Biamonti, Fiorenzo A. Peverali, Silvia Bione, and Donata Orioli

PNAS June 29, 2021 118 (26) e2024502118

Significance

Xeroderma pigmentosum (XP) and trichothiodystrophy (TTD), which may arise from mutations in the same genes, are distinct clinical entities with opposite skin cancer predisposition. Whereas XP is characterized by cutaneous photosensitivity and cancer proneness frequently associated with neurodegeneration, TTD shows hair anomalies, physical and mental retardation, and, in 50% of cases, cutaneous photosensitivity but no skin cancer despite the accumulation of unrepaired ultraviolet-induced DNA lesions. This study identifies a TTD-specific transcription deregulation of PTGIS (prostaglandin I₂ synthase) that results in reduced levels of prostaglandin I₂. Reduced PTGIS is found in all TTD but not in XP patients, thus representing a biomarker for this disorder.

Abstract

The cancer-free photosensitive trichothiodystrophy (PS-TTD) and the cancer-prone xeroderma pigmentosum (XP) are rare monogenic disorders that can arise from mutations in the same genes, namely ERCC2/XPD or ERCC3/XPB. Both XPD and XPB proteins belong to the 10-subunit complex transcription factor IIH (TFIIH) that plays a key role in transcription and nucleotide excision repair, the DNA repair pathway devoted to the removal of ultraviolet-induced DNA lesions. Compelling evidence suggests that mutations affecting the DNA repair activity of TFIIH are responsible for the pathological features of XP, whereas those also impairing transcription give rise to TTD. By adopting a relatives-based whole transcriptome sequencing approach followed by specific gene expression profiling in primary fibroblasts from a large cohort of TTD or XP cases with mutations in ERCC2/XPD gene, we identify the expression alterations specific for TTD primary dermal fibroblasts. While most of these transcription deregulations do not impact on the protein level, very low amounts of prostaglandin I₂ synthase (PTGIS) are found in TTD cells. PTGIS catalyzes the last step of prostaglandin I₂ synthesis, a potent vasodilator and inhibitor of platelet aggregation. Its reduction characterizes all TTD cases so far investigated, both the PS-TTD with mutations in TFIIH coding genes as well as the nonphotosensitive (NPS)-TTD. A severe impairment of TFIIH and RNA polymerase II recruitment on the PTGIS promoter is found in TTD but not in XP cells. Thus, PTGIS represents a biomarker that combines all PS- and NPS-TTD cases and distinguishes them from XP.

See: https://www.pnas.org/content/118/26/e2024502118

Figure 1: Schematic representation of the genes differentially expressed in TTD7PV skin fibroblasts compared to those from the healthy TTD7PVmother. (A and B) In total, 718 and 730 genes are differentially expressed in TTD7PV fibroblasts cultured under basal condition (A) or 2 h after 10 J/m² UV irradiation (B), respectively. Red and blue bars indicate the up- and down-regulated genes, respectively. (C) Venn diagram and schematic representation of the transcript coding genes whose expression is modified following UV irradiation in TTD7PVmother (green, 332 genes) or TTD7PV (yellow, 502 genes) fibroblasts. The expression of 250 genes is commonly modified in TTD and control cells in response to UV light, whereas the transcriptional alteration of 82 and 252 genes occurs specifically in the control and patient fibroblasts, respectively. Red and blue bars indicate the up- and down-regulated genes, respectively. (D) Scatter plots in logarithmic scale representing the comparison of the 174 messenger RNA levels [log₂(−ΔCt)] found in the TTD patient RNA pool versus healthy parent RNA pool from fibroblasts cultured in basal condition (Left) and upon UV irradiation (Right). Only values with a log₂ fold change higher than ±|2| are indicated (gene names and fold change values, SI Appendix, Table S12). The previously identified deregulated gene (MMP1) in TTD cells and the most down-regulated gene found in this study (WISP2) have been pinpointed. The black line indicates fold changes of 1 (=no deregulation), whereas the pink lines correspond to log₂ fold changes ±|2|.