Diagnostic and severity scores for Cockayne syndrome

CS proteins and ubiquitination: orchestrating DNA repair with transcription and cell division

To face genotoxic stress, eukaryotic cells evolved extremely refined mechanisms. Defects in counteracting the threat imposed by DNA damage underlie the rare disease Cockayne syndrome (CS), which arises from mutations in the CSA and CSB genes. Although initially defined as DNA repair proteins, recent work shows that CSA and CSB act instead as master regulators of the integrated response to genomic stress by coordinating DNA repair with transcription and cell division. CSA and CSB exert this function through the ubiquitination of target proteins, which are effectors/regulators of these processes. This review describes how the ubiquitination of target substrates is a common denominator by which CSA and CSB participate in different aspects of cellular life and how their mutation gives rise to the complex disease CS.

Whole exome sequencing identifies a novel variant causing cockayne syndrome type I in a consanguineous Pakistani family

BACKGROUND

Cockayne syndrome (CS) is a rare neurodegenerative disorder characterized by impaired neurological functions, cachectic dwarfism, microcephaly and photosensitivity. Complementation assays identify two groups of this disorder, CS type I (CSA) and CS type II (CSB), caused by mutations in ERCC8 and ERCC6, respectively.

OBJECTIVES

This study aimed to investigate the genetic basis of a consanguineous Pakistani family with three affected individuals presenting with typical clinical symptoms of CS.

METHODS

We employed whole exome sequencing of the proband and then Sanger sequenced all the family members to confirm its segregation in the family. Different bioinformatics tools were used to predict pathogenicity of this variant.

RESULTS

Variants were filtered according to the pedigree structure. We identified a novel homozygous variant (c.202A>T; p.Ile68Phe) in ERCC8 gene in the proband. The variant was found to segregate in the family.

CONCLUSIONS

These findings add to the genetic heterogeneity of ERCC8 and expands the mutation spectrum. Also, identification of this variant can facilitate prenatal diagnosis/genetic counselling set ups in Pakistan where this disease largely remains undiagnosed.

Spectrum of ERCC6-Related Cockayne Syndrome (Type B): From Mild to Severe Forms

(1) Background: Cockayne syndrome (CS) is an ultra-rare multisystem disorder, classically subdivided into three forms and characterized by a clinical spectrum without a clear genotype-phenotype correlation for both the two causative genes ERCC6 (CS type B) and ERCC8 (CS type A). We assessed this, presenting a series of patients with genetically confirmed CSB. (2) Materials and Methods: We retrospectively collected demographic, clinical, genetic, neuroimaging, and serum neurofilament light-chain (sNFL) data about CSB patients; diagnostic and severity scores were also determined. (3) Results: Data of eight ERCC6/CSB patients are presented. Four patients had CS I, three patients CS II, and one patient CS III. Various degrees of ataxia and spasticity were cardinal neurologic features, with variably combined systemic characteristics. Mean age at diagnosis was lower in the type II form, in which classic CS signs were more evident. Interestingly, sNFL determination appeared to reflect clinical classification. Two novel premature stop codon and one novel missense variants were identified. All CS I subjects harbored the p.Arg735Ter variant; the milder CS III subject carried the p.Leu764Ser missense change. (4) Conclusion: Our work confirms clinical variability also in the ERCC6/CSB type, where manifestations may range from severe involvement with prenatal or neonatal onset to normal psychomotor development followed by progressive ataxia. We propose, for the first time in CS, sNFL as a useful peripheral biomarker, with increased levels compared to currently available reference values and with the potential ability to reflect disease severity.

A compound heterozygous mutation of ERCC8 is responsible for a family with Cockayne syndrome

BACKGROUND

Cockayne syndrome is an inherited heterogeneous defect in transcription-coupled DNA repair (TCR) cause severe clinical syndromes, which may affect the nervous system development of infants and even lead to premature death in some cases. ERCC8 diverse critical roles in the nucleotide excision repair (NER) complex, which is one of the disease-causing genes of Cockayne syndrome.

METHODS AND RESULTS

The mutation of ERCC8 in the patient was identified and validated using WES and Sanger sequencing. Specifically, a compound heterozygous mutation (c.454_460dupGTCTCCA p. T154Sfs*13 and c.755_759delGTTTT p.C252Yfs*3) of ERCC8 (CSA) was found, which could potentially be the genetic cause of Cockayne syndrome in the proband.

CONCLUSION

In this study, we identified a novel heterozygous mutation of ERCC8 in a Chinese family with Cockayne syndrome, which enlarging the genetic spectrum of the disease.

The Spectrum of MORC2-Related Disorders: A Potential Link to Cockayne Syndrome

BACKGROUND

Cockayne syndrome (CS) is a DNA repair disorder primarily associated with pathogenic variants in ERCC6 and ERCC8. As in other Mendelian disorders, there are a number of genetically unsolved CS cases.

METHODS

We ascertained five individuals with monoallelic pathogenic variants in MORC2, previously associated with three dominantly inherited phenotypes: an axonal form of Charcot-Marie-Tooth disease type 2Z; a syndrome of developmental delay, impaired growth, dysmorphic facies, and axonal neuropathy; and a rare form of spinal muscular atrophy.

RESULTS

One of these individuals bore a strong phenotypic resemblance to CS. We then identified monoallelic pathogenic MORC2 variants in three of five genetically unsolved individuals with a clinical diagnosis of CS. In total, we identified eight individuals with MORC2-related disorder, four of whom had clinical features strongly suggestive of CS.

CONCLUSIONS

Our findings indicate that some forms of MORC2-related disorder have phenotypic similarities to CS, including features of accelerated aging. Unlike classic DNA repair disorders, MORC2-related disorder does not appear to be associated with a defect in transcription-coupled nucleotide excision repair and follows a dominant pattern of inheritance with variants typically arising de novo. Such de novo pathogenic variants present particular challenges with regard to both initial gene discovery and diagnostic evaluations. MORC2 should be included in diagnostic genetic test panels targeting the evaluation of microcephaly and/or suspected DNA repair disorders. Future studies of MORC2 and its protein product, coupled with further phenotypic characterization, will help to optimize the diagnosis, understanding, and therapy of the associated disorders.

Cockayne syndrome without UV-sensitivity in Vietnamese siblings with novel ERCC8 variants

Cockayne syndrome (CS) is a rare progeroid disorder characterized by growth failure, microcephaly, photosensitivity, and premature aging, mainly arising from biallelic ERCC8 (CS-A) or ERCC6 (CS-B) variants. In this study we describe siblings suffering from classical Cockayne syndrome but without photosensitivity, which delayed a clinical diagnosis for 16 years. By whole-exome sequencing we identified the two novel compound heterozygous ERCC8 variants c.370_371del (p.L124Efs*15) and c.484G>C (p.G162R). The causality of the ERCC8 variants, of which one results in a frameshift and the other affects the WD3 domain, was tested and confirmed by a rescue experiment investigating DNA repair in H₂O₂ treated patient fibroblasts. Structural modeling of the p.G162R variant indicates effects on protein-protein interaction. This case shows the importance to test for ERCC6 and ERCC8 variants even if patients do not present with a complete CS phenotype.

Heterogeneous clinical features in Cockayne syndrome patients and siblings carrying the same CSA mutations

Background

Cockayne syndrome (CS) is a rare autosomal recessive disorder caused by mutations in ERCC6/CSB or ERCC8/CSA that participate in the transcription-coupled nucleotide excision repair (TC-NER) of UV-induced DNA damage. CS patients display a large heterogeneity of clinical symptoms and severities, the reason of which is not fully understood, and that cannot be anticipated in the diagnostic phase. In addition, little data is available for affected siblings, and this disease is largely undiagnosed in North Africa.

Methods

We report here the clinical description as well as genetic and functional characterization of eight Tunisian CS patients, including siblings. These patients, who belonged to six unrelated families, underwent complete clinical examination and biochemical analyses. Sanger sequencing was performed for the recurrent mutation in five families, and targeted gene sequencing was done for one patient of the sixth family. We also performed Recovery RNA Synthesis (RRS) to confirm the functional impairment of DNA repair in patient-derived fibroblasts.

Results

Six out of eight patients carried a homozygous indel mutation (c.598_600delinsAA) in exon 7 of ERCC8, and displayed a variable clinical spectrum including between siblings sharing the same mutation. The other two patients were siblings who carried a homozygous splice-site variant in ERCC8 (c.843+1G>C). This last pair presented more severe clinical manifestations, which are rarely associated with CSA mutations, leading to gastrostomy and hepatic damage. Impaired TC-NER was confirmed by RRS in six tested patients.

Conclusions

This study provides the first deep characterization of case series of CS patients carrying CSA mutations in North Africa. These mutations have been described only in this region and in the Middle-East. We also provide the largest characterization of multiple unrelated patients, as well as siblings, carrying the same mutation, providing a framework for dissecting elusive genotype–phenotype correlations in CS.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02257-1.

Statistical Approach of the Role of the Conserved CSB-PiggyBac Transposase Fusion Protein (CSB-PGBD3) in Genotype-Phenotype Correlation in Cockayne Syndrome Type B

Cockayne syndrome is a rare condition that encompasses a very wide spectrum of clinical severity. Mutations upstream of a transposon called PiggyBac Transposable Element Derived 3 in intron 5 of the CSB/ERCC6 gene could bring about less severe forms than mutations located downstream of that transposon insertion. Our aim was to study genotype-phenotype correlation by determining whether the position of each mutation of the CSB/ERCC6 gene has an impact on the phenotype. A hundred and forty-seven Cockayne patients, who had two pathogenic mutations in the CSB/ERCC6 gene and for whom clinical data was available, were retrospectively selected and included in the study. Data analysis was performed under the Bayesian paradigm. Analysis of the proportion of the different subtypes of Cockayne syndrome according to the position of the mutations was done using an ordinal logistic regression model. Using a vague prior, the risk of developing a more severe subtype when exposed to 2 mutations downstream compared to 2 mutations upstream was 2.0 [0.9–4.5]. Estimations varied through the sensitivity analysis. We could reasonably conclude that a relationship between the number of downstream mutations and the Cockayne syndrome clinical expression exists but it is still difficult to give a precise estimate of this relationship. The real effect could be more complex that the one described in the initial model and other genetic factors might be taken into consideration together with the mutation site to better explain clinical variability.

Cockayne Syndrome-Associated CSA and CSB Mutations Impair Ribosome Biogenesis, Ribosomal Protein Stability, and Global Protein Folding

Cockayne syndrome (CS) is a developmental disorder with symptoms that are typical for the aging body, including subcutaneous fat loss, alopecia, and cataracts. Here, we show that in the cells of CS patients, RNA polymerase I transcription and the processing of the pre-rRNA are disturbed, leading to an accumulation of the 18S-E intermediate. The mature 18S rRNA level is reduced, and isolated ribosomes lack specific ribosomal proteins of the small 40S subunit. Ribosomal proteins are susceptible to unfolding and the CS cell proteome is heat-sensitive, indicating misfolded proteins and an error-prone translation process in CS cells. Pharmaceutical chaperones restored impaired cellular proliferation. Therefore, we provide evidence for severe protein synthesis malfunction, which together with a loss of proteostasis constitutes the underlying pathophysiology in CS.