A disease-associated XPA allele interferes with TFIIH binding and primarily affects transcription-coupled nucleotide excision repair

Protein-protein interactions in the core nucleotide excision repair pathway

Nucleotide excision repair (NER) clears genomes of DNA adducts formed by UV light, environmental agents, and antitumor drugs. Gene mutations that lead to defects in the core NER reaction cause the skin cancer-prone disease xeroderma pigmentosum. In NER, DNA lesions are excised within an oligonucleotide of 25-30 residues via a complex, multi-step reaction that is regulated by protein-protein interactions. These interactions were first characterized in the 1990s using pull-down, co-IP and yeast two-hybrid assays. More recently, high-resolution structures and detailed functional studies have started to yield detailed pictures of the progression along the NER reaction coordinate. In this review, we highlight how the study of interactions among proteins by structural and/or functional studies have provided insights into the mechanisms by which the NER machinery recognizes and excises DNA lesions. Furthermore, we identify reported, but poorly characterized or unsubstantiated interactions in need of further validation.

Case report: Xeroderma pigmentosum Group A with erythropoietic protoporphyria in a young Chinese patient

Xeroderma pigmentosum is a rare autosomal recessive genodermatoses characterized by a deficiency in nucleotide excision repair. Erythropoietic protoporphyria is a rare inherited metabolic disease caused by the perturbation of heme. Xeroderma pigmentosum-erythropoietic protoporphyria is exceedingly rare. Hereby, we firstly report a young Chinese patient of xeroderma pigmentosum Group A with erythropoietic protoporphyria carrying an XPA Met214AsnfsTer7 frameshift mutation and a homozygous splicing mutation, c.315-48T>C, in the proband's intron3 of FECH.

STK19 facilitates the clearance of lesion-stalled RNAPII during transcription-coupled DNA repair

Transcription-coupled DNA repair (TCR) removes bulky DNA lesions impeding RNA polymerase II (RNAPII) transcription. Recent studies have outlined the stepwise assembly of TCR factors CSB, CSA, UVSSA, and TFIIH around lesion-stalled RNAPII. However, the mechanism and factors required for the transition to downstream repair steps, including RNAPII removal to provide repair proteins access to the DNA lesion, remain unclear. Here, we identify STK19 as a new TCR factor facilitating this transition. Loss of STK19 does not impact initial TCR complex assembly or RNAPII ubiquitylation but delays lesion-stalled RNAPII clearance, thereby interfering with the downstream repair reaction. Cryo-EM and mutational analysis reveal that STK19 associates with the TCR complex, positioning itself between RNAPII, UVSSA, and CSA. The structural insights and molecular modeling suggest that STK19 positions the ATPase subunits of TFIIH onto DNA in front of RNAPII. Together, these findings provide new insights into the factors and mechanisms required for TCR.

A Practical Site-specific Method for the Detection of Bulky DNA Damages

Helix-distorting DNA damages block RNA and DNA polymerase, compromising cell function and fate. In human cells, these damages are removed primarily by nucleotide excision repair (NER). Here, we describe damage-sensing PCR (dsPCR), a PCR-based method for the detection of these DNA damages. Exposure to DNA damaging agents results in lower PCR signal in comparison to non-damaged DNA, and repair is measured as the restoration of PCR signal over time. We show that the method successfully detects damages induced by ultraviolet (UV) radiation, by the carcinogenic component of cigarette smoke benzo[a]pyrene diol epoxide (BPDE) and by the chemotherapeutic drug cisplatin. Damage removal measured by dsPCR in a heterochromatic region is less efficient than in a transcribed and accessible region. Furthermore, lower repair is measured in repair-deficient knock-out cells. This straight-forward method could be applied by non-DNA repair experts to study the involvement of their gene-of-interest in repair. Furthermore, this method is fully amenable for high-throughput screening of DNA repair activity.

Cross-species investigation into the requirement of XPA for nucleotide excision repair

After reconstitution of nucleotide excision repair (excision repair) with XPA, RPA, XPC, TFIIH, XPF-ERCC1 and XPG, it was concluded that these six factors are the minimal essential components of the excision repair machinery. All six factors are highly conserved across diverse organisms spanning yeast to humans, yet no identifiable homolog of the XPA gene exists in many eukaryotes including green plants. Nevertheless, excision repair is reported to be robust in the XPA-lacking organism, Arabidopsis thaliana, which raises a fundamental question of whether excision repair could occur without XPA in other organisms. Here, we performed a phylogenetic analysis of XPA across all species with annotated genomes and then quantitatively measured excision repair in the absence of XPA using the sensitive whole-genome qXR-Seq method in human cell lines and two model organisms, Caenorhabditis elegans and Drosophila melanogaster. We find that although the absence of XPA results in inefficient excision repair and UV-sensitivity in humans, flies, and worms, excision repair of UV-induced DNA damage is detectable over background. These studies have yielded a significant discovery regarding the evolution of XPA protein and its mechanistic role in nucleotide excision repair.

Methods for Assessment of Nucleotide Excision Repair Efficiency

Nucleotide excision repair (NER) is responsible for removing a wide variety of bulky adducts from DNA, thus contributing to the maintenance of genome stability. The efficiency with which proteins of the NER system recognize and remove bulky adducts depends on many factors and is of great clinical and diagnostic significance. The review examines current concepts of the NER system molecular basis in eukaryotic cells and analyzes methods for the assessment of the NER-mediated DNA repair efficiency both in vitro and ex vivo.

Nucleotide excision repair in Human cell lines lacking both XPC and CSB proteins

Nucleotide excision repair removes UV-induced DNA damage through two distinct sub-pathways, global repair and transcription-coupled repair (TCR). Numerous studies have shown that in human and other mammalian cell lines that the XPC protein is required for repair of DNA damage from nontranscribed DNA via global repair and the CSB protein is required for repair of lesions from transcribed DNA via TCR. Therefore, it is generally assumed that abrogating both sub-pathways with an XPC-/-/CSB-/- double mutant would eliminate all nucleotide excision repair. Here we describe the construction of three different XPC-/-/CSB-/- human cell lines that, contrary to expectations, perform TCR. The XPC and CSB genes were mutated in cell lines derived from Xeroderma Pigmentosum patients as well as from normal human fibroblasts and repair was analyzed at the whole genome level using the very sensitive XR-seq method. As predicted, XPC-/- cells exhibited only TCR and CSB-/- cells exhibited only global repair. However, the XPC-/-/CSB-/- double mutant cell lines, although having greatly reduced repair, exhibited TCR. Mutating the CSA gene to generate a triple mutant XPC-/-/CSB-/-/CSA-/- cell line eliminated all residual TCR activity. Together, these findings provide new insights into the mechanistic features of mammalian nucleotide excision repair.