Case Series: Clinical Significance of Heterozygous Pathogenic <scp> <i>RTEL1</i> </scp> Variants Identified via Routine Clinical Genetic Diagnostics

Whilst biallelic variants in RTEL1 are an established cause of telomere biology disorder (TBD), the significance of heterozygous variants has been more challenging to establish. In this nationwide analysis, we describe 18 individuals with heterozygous pathogenic RTEL1 variants from seven families. All were identified during routine clinical genetic investigation for a variety of indications. Each family carried a different variant in RTEL1. Eight individuals had been diagnosed with a TBD-related disease (five with a hematological disorder, four with pulmonary fibrosis, overlap of one). No cases of clinically significant liver fibrosis had been detected. Cutaneous features of dyskeratosis congenita (abnormal skin pigmentation, oral leukoplakia, nail dysplasia and/or prematurely gray hair) were observed in four individuals. Telomere length (lymphocyte) was measured in nine individuals from six families and was below the 1st percentile in eight individuals. These cases illustrate the wide spectrum of disease and reduced penetrance associated with pathogenic RTEL1 variants, providing a real-world perspective on previous findings from research cohorts. We discuss the challenges surrounding incidental findings, clinical surveillance, and reproductive counseling in this context.