Xeroderma pigmentosum: diagnostic procedures, interdisciplinary patient care, and novel therapeutic approaches

Case report: Identification of a novel heterozygous germline ERCC2 mutation in a patient with dermatofibrosarcoma protuberans

Dermatofibrosarcoma protuberans (DFSP) is a kind of soft tissue sarcoma, mostly occurs in the trunk, followed by proximal extremities and head and neck. Surgical resection is the most important treatment for DFSP, but the local recurrence rate of DFSP is high. Except reported specific chromosomal tran7slocations occurred in DFSP, the association between DNA repair gene mutations and DFSP still unknown. In this report we found a 19-year-old boy with DFSP carries a novel heterozygous germline ERCC2 mutation, which belongs to the nucleotide excision repair (NER) pathway and genetic defects in ERCC2 may contribute to the cancer susceptibility xeroderma pigmentosum (XP), Cocaine syndrome (CS), and trichothiodystrophy (TTD). Different mutations of the ERCC2 gene can lead to diverse diseases, but there are no targeted therapies. In summary, our results enlarged the mutation spectrum of the DFSP patients. It also provides new insights into genetic counseling and targeted therapeutic strategies for patients with DFSP.

Xeroderma Pigmentosum: Gene Variants and Splice Variants

The nucleotide excision repair (NER) is essential for the repair of ultraviolet (UV)-induced DNA damage, such as cyclobutane pyrimidine dimers (CPDs) and 6,4-pyrimidine-pyrimidone dimers (6,4-PPs). Alterations in genes of the NER can lead to DNA damage repair disorders such as Xeroderma pigmentosum (XP). XP is a rare autosomal recessive genetic disorder associated with UV-sensitivity and early onset of skin cancer. Recently, extensive research has been conducted on the functional relevance of splice variants and their relation to cancer. Here, we focus on the functional relevance of alternative splice variants of XP genes.

Genetisch bedingte Hauterkrankungen – Xeroderma pigmentosum und das CEDNIK-Syndrom

Die Rostocker Hautklinik ist Europäisches Referenznetzwerkzentrum für seltene Hauterkrankungen mit den besonderen Schwerpunkten Xeroderma pigmentosum und Ichthyosen. Diese Themen vertreten wir auch in der medizinischen Grundlagenforschung.

Xeroderma pigmentosum (XP) ist eine seltene, autosomal-rezessive Erkrankung, die entsprechend der Gendefekte in 7 Komplementationsgruppen – XP-A bis XP-G sowie die sog. XP-Variante (XP-V) – eingeteilt wird. XP ist ein Nukleotid-Exzisions-Reparatur-Defektsyndrom und äußert sich v. a. durch vorzeitige Hautalterung und frühzeitige Entwicklung von Hauttumoren.

Das seltene, neurokutane CEDNIK-Syndrom ist eine autosomal-rezessive Erkrankung, der eine Loss-of-Function-Mutation in SNAP29 zugrunde liegt. SNAP29 ist ein SNARE-Protein und an intrazellulären Membranfusionen beteiligt. CEDNIK ist ein Akronym für den mit dem Syndrom assoziierten Symptomkomplex aus zerebraler Dysgenese, Neuropathie, Ichthyose und Palmoplantarkeratosen. CEDNIK-Patienten weisen neben der Ichthyose zudem Gedeihstörungen, eine psychomotorische Retardierung und faziale Dysmorphien auf.

Context-Dependent Strategies for Enhanced Genome Editing of Genodermatoses

The skin provides direct protection to the human body from assault by the harsh external environment. The crucial function of this organ is significantly disrupted in genodermatoses patients. Genodermatoses comprise a heterogeneous group of largely monogenetic skin disorders, typically involving mutations in genes encoding structural proteins. Therapeutic options for this debilitating group of diseases, including epidermolysis bullosa, primarily consist of wound management. Genome editing approaches co-opt double-strand break repair pathways to introduce desired sequence alterations at specific loci. Rapid advances in genome editing technologies have the potential to propel novel genetic therapies into the clinic. However, the associated phenotypes of many mutations may be treated via several genome editing strategies. Therefore, for potential clinical applications, implementation of efficient approaches based upon mutation, gene and disease context is necessary. Here, we describe current genome editing approaches for the treatment of genodermatoses, along with a discussion of the optimal strategy for each genetic context, in order to achieve enhanced genome editing approaches.

Splice variants of the endonucleases XPF and XPG contain residual DNA repair capabilities and could be a valuable tool for personalized medicine

The two endonucleases XPF and XPG are essentially involved in nucleotide excision repair (NER) and interstrand crosslink (ICL) repair. Defects in these two proteins result in severe diseases like xeroderma pigmentosum (XP). We applied our newly CRISPR/Cas9 generated human XPF knockout cell line with complete loss of XPF and primary fibroblasts from an XP-G patient (XP20BE) to analyze until now uncharacterized spontaneous mRNA splice variants of these two endonucleases. Functional analyses of these variants were performed using luciferase-based reporter gene assays. Two XPF and XPG splice variants with residual repair capabilities in NER, as well as ICL repair could be identified. Almost all variants are severely C-terminally truncated and lack important protein-protein interaction domains. Interestingly, XPF-202, differing to XPF-003 in the first 12 amino acids only, had no repair capability at all, suggesting an important role of this region during DNA repair, potentially concerning protein-protein interaction. We also identified splice variants of XPF and XPG exerting inhibitory effects on NER. Moreover, we showed that the XPF and XPG splice variants presented with different inter-individual expression patterns in healthy donors, as well as in various tissues. With regard to their residual repair capability and dominant-negative effects, functionally relevant spontaneous XPF and XPG splice variants present promising prognostic marker candidates for individual cancer risk, disease outcome, or therapeutic success. This merits further investigations, large association studies, and translational research within clinical trials in the future.

XPF knockout via CRISPR/Cas9 reveals that ERCC1 is retained in the cytoplasm without its heterodimer partner XPF

The XPF/ERCC1 heterodimeric complex is essentially involved in nucleotide excision repair (NER), interstrand crosslink (ICL), and double-strand break repair. Defects in XPF lead to severe diseases like xeroderma pigmentosum (XP). Up until now, XP-F patient cells have been utilized for functional analyses. Due to the multiple roles of the XPF/ERCC1 complex, these patient cells retain at least one full-length allele and residual repair capabilities. Despite the essential function of the XPF/ERCC1 complex for the human organism, we successfully generated a viable immortalised human XPF knockout cell line with complete loss of XPF using the CRISPR/Cas9 technique in fetal lung fibroblasts (MRC5Vi cells). These cells showed a markedly increased sensitivity to UVC, cisplatin, and psoralen activated by UVA as well as reduced repair capabilities for NER and ICL repair as assessed by reporter gene assays. Using the newly generated knockout cells, we could show that human XPF is markedly involved in homologous recombination repair (HRR) but dispensable for non-homologous end-joining (NHEJ). Notably, ERCC1 was not detectable in the nucleus of the XPF knockout cells indicating the necessity of a functional XPF/ERCC1 heterodimer to allow ERCC1 to enter the nucleus. Overexpression of wild-type XPF could reverse this effect as well as the repair deficiencies.

RNA-based therapies for genodermatoses

Genetic disorders affecting the skin, genodermatoses, constitute a large and heterogeneous group of diseases, for which treatment is generally limited to management of symptoms. RNA-based therapies are emerging as a powerful tool to treat genodermatoses. In this review, we discuss in detail RNA splicing modulation by antisense oligonucleotides and RNA trans-splicing, transcript replacement and genome editing by in vitro-transcribed mRNAs, and gene knockdown by small interfering RNA and antisense oligonucleotides. We present the current state of these therapeutic approaches and critically discuss their opportunities, limitations and the challenges that remain to be solved. The aim of this review was to set the stage for the development of new and better therapies to improve the lives of patients and families affected by a genodermatosis.

Identification and Functional Testing of ERCC2 Mutations in a Multi-national Cohort of Patients with Familial Breast- and Ovarian Cancer

The increasing application of gene panels for familial cancer susceptibility disorders will probably lead to an increased proposal of susceptibility gene candidates. Using ERCC2 DNA repair gene as an example, we show that proof of a possible role in cancer susceptibility requires a detailed dissection and characterization of the underlying mutations for genes with diverse cellular functions (in this case mainly DNA repair and basic cellular transcription). In case of ERCC2, panel sequencing of 1345 index cases from 587 German, 405 Lithuanian and 353 Czech families with breast and ovarian cancer (BC/OC) predisposition revealed 25 mutations (3 frameshift, 2 splice-affecting, 20 missense), all absent or very rare in the ExAC database. While 16 mutations were unique, 9 mutations showed up repeatedly with population-specific appearance. Ten out of eleven mutations that were tested exemplarily in cell-based functional assays exert diminished excision repair efficiency and/or decreased transcriptional activation capability. In order to provide evidence for BC/OC predisposition, we performed familial segregation analyses and screened ethnically matching controls. However, unlike the recently published RECQL example, none of our recurrent ERCC2 mutations showed convincing co-segregation with BC/OC or significant overrepresentation in the BC/OC cohort. Interestingly, we detected that some deleterious founder mutations had an unexpectedly high frequency of > 1% in the corresponding populations, suggesting that either homozygous carriers are not clinically recognized or homozygosity for these mutations is embryonically lethal. In conclusion, we provide a useful resource on the mutational landscape of ERCC2 mutations in hereditary BC/OC patients and, as our key finding, we demonstrate the complexity of correct interpretation for the discovery of “bonafide” breast cancer susceptibility genes.

Approximately 5–10% of breast/ovarian cancer (BC/OC) cases have inherited an increased risk of developing this malignancy. However, mutations in the two major breast cancer susceptibility genes BRCA1 and BRCA2 explain only 15–20% of all familial BC/OC cases. With the emergence of the high throughput NGS-technology, the number of proposed novel candidate genes for breast cancer predisposition continuously increases. However, a “bonafide” proof of cancer susceptibility requires a detailed characterization of candidate mutations, which we addressed in the current study. Using the DNA repair gene ERCC2 as an example, we performed a comprehensive multi-center approach, analyzing ERCC2 mutations in 1000+ patients with hereditary BC/OC. We identified 25 potential candidate mutations for cancer breast cancer susceptibility, some of them affecting ERCC2 functional activity in appropriate cell-culture based assays. However, a more dissected analysis showed no convincing co-segregation with BC/OC and there was no longer a significant overrepresentation in BC/OC when compared to regionally matched controls instead of the global ExAc variant data base, pointing to the relevance of founder-mutations. In conclusion, we provide a useful resource on the mutational landscape of ERCC2 mutations in hereditary BC/OC patients and, as our key finding, we highlight the complexity of correct interpretation for the discovery of “bonafide” breast cancer susceptibility genes.

[Practical aspects of molecular diagnostics in genodermatoses]

Genodermatoses are rare genetic disorders with a broad spectrum of cutaneous and extracutaneous manifestations that have a genetic background. A thorough clinical examination, laboratory workup and morphological analyses of the skin remain crucial for the diagnosis in the era of next generation sequencing (NGS). The diagnostic algorithm depends on the clinical and molecular heterogeneity and should be adapted for each group of genodermatoses. In cases with uncharacteristic phenotypes which cannot be classified, NGS-based testing accelerates the time to diagnosis and leads to the identification of new disorders and new disease-associated genes. The new knowledge on genotype-phenotype correlations should enable revision of the classification of genodermatoses on a molecular basis.