Novel Biallelic CTSD Gene Variants Cause Late‐Onset Ataxia and Retinitis Pigmentosa
2020-05-18

A 45-year-old male patient presented with a 2-year history of progressive gait impairment. On neurological examination, he exhibited a cerebellar syndrome with saccadic ocular pursuit, intention tremor, dysarthria, and gait ataxia. MRI revealed cerebellar atrophy (Fig. 1A and Supporting Information Fig. S1A). The patient’s visual function had rapidly deteriorated from the age of 32. Opthalmological examination showed a retinitis pigmentosa-like phenotype with central involvement (Fig. 1B and Supporting Information Fig. S1B). Based upon the ataxia/retinitis pigmentosa complex, exome sequencing prioritized two previously unreported compound heterozygous variants in CTSD: The c.57_63del variant predicts a frameshift and premature stop codon in exon 1 (p.A20Sfs*25; class 4 according to American College of Medical Genetics and Genomics guidelines). The c.1064C > T missense variant leads to a p.T355M amino acid change (class 3). Autosomal-recessive mutations in CTSD cause neuronal ceroid lipofuscinosis type 10 (NCL10).1 Whereas congenital NCL10 causes severe neurological deficits and early death, late infantile and juvenile NCL10 manifest with ataxia, retinitis pigmentosa, and cognitive decline, with the latest onset reported at 15 years of age.1, 2 In light of the adult-onset phenotype, we aimed to characterize the functional impact of the missense variant in vitro. In patient-derived fibroblasts, Cathepsin D enzymatic activity was significantly reduced to 29% of the lower regular limit. This is notably higher than in congenital or juvenile NCL10, where enzymatic activity is either severely diminished or completely lost.3 Both immunocytochemistry and western blot showed severely reduced levels of active Cathepsin D, whereas levels of the pre-proenzyme were unchanged (Fig. 1D,E). Given that Cathepsin D activity is crucial for lysosomal degradation, we analyzed autophagic functions. Patient fibroblasts showed higher basal levels of the autophagy receptor protein, p62, as compared to control cells (Fig. 1F,G). Furthermore, accumulation of the autophagosome membrane protein, light chain 3/phosphatidylethanolamine conjugate (LC3-II), in response to the lysosomal degradation inhibitor, Bafilomycin A1, was attenuated in patient cells as compared to control fibroblasts, suggesting that autophagic flux is compromised. We here report and validate two novel pathogenic CTSD variants in a patient with an exceptionally late onset of NCL10 presenting with retinitis pigmentosa at 32 years, followed by ataxia at 43 years, however without cognitive impairment during follow-up until age 47. Whereas the c.57_63del variant leads to a premature stop codon and thus slightly reduced enzymatic activity of Cathepsin D even in the unaffected mother (data not shown), the additional presence of the c.1064C > T missense variant may explain the intermediate phenotype of the index patient. Our biochemical analyses indicate that mutant Cathepsin D is produced, but not efficiently processed into the mature form, linked to autophagic dysfunction. An increased frequency of CTSD variants was observed in Parkinson’s and Alzheimer’s diseases.4, 5 The identification of the mildly pathogenic missense variant supports the notion that minor changes in Cathepsin D activity may predispose for neurodegenerative diseases in late adulthood. The late onset of this distinct phenotype caused by the variant reported herein prompts its analysis in cases of adult-onset ataxia and/or retinitis pigmentosa. Our findings implicate that enzymatic activity of Cathepsin D is inversely correlated to age at onset in NCL10. The authors thank the patient and his family for consent to participate in this study. This study was supported by the Deutsche Forschungsgemeinschaft (DFG; German Research Foundation; 270949263/GRK2162 to M.R. and J.W.; 418081722 to T.B.H.), by the German Bundesministerium für Bildung und Forschung (BMBF) through the treatHSP consortium (01GM1905B to M.R.) and the Juniorverbund in der Systemmedizin “mitOmics” (FKZ-01ZX1405C to T.B.H.), by the intramural Fortüne Program (#2435-0-0 to T.B.H.), and by the Lower Saxony Ministry of Science and Culture (Göttingen College for Translational Medicine to M.K.). (1) Research Project: A. Conception, B. Organization, C. Execution; (2) Statistical Analysis: A. Design, B. Execution, C. Review and Critique; (3) Manuscript Preparation: A. Writing of the First Draft, B. Review and Critique; (4) Other: (A) Clinical Care, (B) In-Vitro Analyses, (C) Enzymatic Activity Measurements, (D) Ophthalmological Examinations, (E), Neuroradiological Examination, (F), Genetic Analyses. M.R.: 1A, 1B, 1C, 2A, 2B, 2C, 3A, 3B, 4A G.M.: 2A, 2B, 2C, 3B, 4B M.K.: 3B, 4C C.H.: 3B, 4D F.E.: 3B, 4E T.B.H.: 3B, 4F Z.K.: 3B, 4A J.W.: 1A, 3B, 4A Jürgen Winkler reports personal fees outside of the submitted work from Desitin Arzneimittel GmbH, AbbVie GmbH & Co. KG, and Biogen GmbH. Appendix S1. Supporting information Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.