Dyskeratosis congenita (DC) is a rare inherited bone marrow failure syndrome, caused by genetic mutations that principally affect telomere biology. Approximately 35% of cases remain uncharacterised at the genetic level. To explore the genetic landscape, we conducted genetic studies on a large collection of clinically diagnosed cases of DC as well as cases exhibiting features resembling DC, referred to as 'DC-like' (DCL). This led us to identify several novel pathogenic variants within known genetic loci and in the novel X-linked gene, POLA1. In addition, we have also identified several novel variants in POT1 and ZCCHC8 in multiple cases from different families expanding the allelic series of DC and DCL phenotypes. Functional characterisation of novel POLA1 and POT1 variants, revealed pathogenic effects on protein-protein interactions with primase, CTC1-STN1-TEN1 (CST) and shelterin subunit complexes, that are critical for telomere maintenance. ZCCHC8 variants demonstrated ZCCHC8 deficiency and signs of pervasive transcription, triggering inflammation in patients' blood. In conclusion, our studies expand the current genetic architecture and broaden our understanding of disease mechanisms underlying DC and DCL disorders.

Barts Health NHS Trust London UK

Cambridge University Hospitals Cambridge Biomedical Campus Cambridge CB2 0QQ UK

Centre for Genomics and Child Health Blizard Institute Faculty of Medicine and Dentistry Queen Mary University of London Newark Street London E12AT UK

Clinical Genetics Birmingham Women's and Children's NHS Foundation Trust Birmingham B15 2TG UK

Clinical Haematology Queen Elizabeth Hospital Edgbaston Birmingham B15 2TH UK

Department of Analytical Environmental and Forensic Sciences Kings College London Franklin Wilkins Building Stamford Street London SE1 9NH UK

Department of Haematology Royal Bournemouth Hospital NHS Foundation Trust Bournemouth BH7 7DW UK

Department of Haematology Royal Devon and Exeter Hospital Exeter EX2 5DW UK

Department of Paediatrics and Inherited Metabolic Disorders 1st Faculty of Medicine Charles University and General University Hospital Prague Ke Karlovu 2 128 08 Praha 2 Prague Czech Republic

Department of Pediatric Hematology Oncology Aghia Sophia Children's Hospital Athens Greece

Department of Pediatrics' University General Hospital of Alexandroupolis Democritus University of Thrace Faculty of Medicine 6th Kilometer Alexandroupolis Makris 68 100 Alexandroupolis Thrace Greece

Institute of Ophthalmology Faculty of Brain Sciences University College London Gower St London WC1E 6BT UK

University Medical Center Utrecht 3508 GA Utrecht The Netherlands

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