Functional and clinical relevance of novel mutations in a large cohort of patients with Cockayne syndrome

BACKGROUND

Cockayne syndrome (CS) is a rare, autosomal recessive multisystem disorder characterised by prenatal or postnatal growth failure, progressive neurological dysfunction, ocular and skeletal abnormalities and premature ageing. About half of the patients with symptoms diagnostic for CS show cutaneous photosensitivity and an abnormal cellular response to UV light due to mutations in either the ERCC8/CSA or ERCC6/CSB gene. Studies performed thus far have failed to delineate clear genotype-phenotype relationships. We have carried out a four-centre clinical, molecular and cellular analysis of 124 patients with CS.

METHODS AND RESULTS

We assigned 39 patients to the ERCC8/CSA and 85 to the ERCC6/CSB genes. Most of the genetic variants were truncations. The missense variants were distributed non-randomly with concentrations in relatively short regions of the respective proteins. Our analyses revealed several hotspots and founder mutations in ERCC6/CSB. Although no unequivocal genotype-phenotype relationships could be made, patients were more likely to have severe clinical features if the mutation was downstream of the PiggyBac insertion in intron 5 of ERCC6/CSB than if it was upstream. Also a higher proportion of severely affected patients was found with mutations in ERCC6/CSB than in ERCC8/CSA.

CONCLUSION

By identifying >70 novel homozygous or compound heterozygous genetic variants in 124 patients with CS with different disease severity and ethnic backgrounds, we considerably broaden the CSA and CSB mutation spectrum responsible for CS. Besides providing information relevant for diagnosis of and genetic counselling for this devastating disorder, this study improves the definition of the puzzling genotype-phenotype relationships in patients with CS.

A novel ERCC6 splicing variant associated with a mild Cockayne syndrome phenotype

BACKGROUND

Cockayne syndrome is an autosomal recessive, heterogeneous syndrome with classical features, including short stature, microcephaly, developmental delay, neuropathy, and photosensitivity. New genomic approaches offer improved molecular diagnostic potential.

METHODS

Whole-exome sequencing was employed to study a consanguineous extended family with severe short stature and variable presentations of peripheral neuropathy, lipoatrophy, photosensitivity, webbed neck, and hirsutism.

RESULTS

We identified a novel homozygous ERCC6 variant at the donor splice site of intron 9 (c.1992 + 3A>G), which was predicted to only slightly perturb splicing efficiencies. Assessment of primary fibroblast-derived mRNAs, however, revealed a dominant splicing species that utilized an unsuspected putative donor splice site within exon 9, resulting in predicted early protein termination (p.Arg637Serfs*34).

CONCLUSIONS

We describe a new splicing ERCC6 defect causal of Cockayne syndrome. The application of exome sequence analysis was integral to diagnosis, given the complexity of phenotypic presentation in the affected family members. The novel splicing defect, furthermore, illustrates how a seemingly minor change in the relative strength of a splice site can have significant biological consequences.