Germline mutations of regulator of telomere elongation helicase 1, RTEL1, in Dyskeratosis congenita

Abstract

Dyskeratosis congenita (DC) is an inherited bone marrow failure and cancer predisposition syndrome caused by aberrant telomere biology. The classic triad of dysplastic nails, abnormal skin pigmentation, and oral leukoplakia is diagnostic of DC, but substantial clinical heterogeneity exists; the clinically severe variant Hoyeraal Hreidarsson syndrome (HH) also includes cerebellar hypoplasia, severe immunodeficiency, enteropathy, and intrauterine growth retardation. Germline mutations in telomere biology genes account for approximately one-half of known DC families. Using exome sequencing, we identified mutations in RTEL1, a helicase with critical telomeric functions, in two families with HH. In the first family, two siblings with HH and very short telomeres inherited a premature stop codon from their mother who has short telomeres. The proband from the second family has HH and inherited a premature stop codon in RTEL1 from his father and a missense mutation from his mother, who also has short telomeres. In addition, inheritance of only the missense mutation led to very short telomeres in the proband's brother. Targeted sequencing identified a different RTEL1 missense mutation in one additional DC proband who has bone marrow failure and short telomeres. Both missense mutations affect the helicase domain of RTEL1, and three in silico prediction algorithms suggest that they are likely deleterious. The nonsense mutations both cause truncation of the RTEL1 protein, resulting in loss of the PIP box; this may abrogate an important protein-protein interaction. These findings implicate a new telomere biology gene, RTEL1, in the etiology of DC.

Figure 1. Lymphocyte telomere lengths in families with RTEL1 mutations

Lymphocyte telomere lengths for DC or DC-like patients and unaffected relatives were measured by flow cytometry with fluorescent in situ hybridization (Alter et al. 2012). In family NCI-238, telomere lengths for the two siblings are shown; however, genotype data was unavailable for the sister and parents. In both the pedigrees and the telomere length graphs, the proband is indicated with an arrow.

Figure 2. Schematic of RTEL1 genomic structure and conserved domains

(a) RTEL1 is comprised of 35 exons spanning nearly 40,000 bases of genomic sequence on chromosome 20q13.33. Exons 20 through 35 have been expanded (blue boxes). In (a) and (b), the positions of the mutations in DC patients are labeled relative to transcript NP_116575. (b) Comparison of amino acid conservation of RTEL1 homologs (Materials and Methods). Higher percent identity at a given amino acid position is indicated by a deeper purple color.