Telomere phenotypes in females with heterozygous mutations in the dyskeratosis congenita 1 (DKC1) gene

Abstract

Dyskeratosis congenita (DC) is a telomere-mediated syndrome defined by mucocutaneous features. The X-linked mode of inheritance accounts for half the cases, and is thought to predominantly manifest in childhood as bone marrow failure. We identified two male probands who presented in the fifth decade with idiopathic pulmonary fibrosis and cancer. Their pedigrees displayed consecutively affected generations. Five of six females (83%) manifested mucocutaneous features of DC, and two had wound-healing complications. No mutations in autosomal dominant telomere genes were present, but exome sequencing revealed novel variants in the X-chromosome DKC1 gene that predicted missense mutations in conserved residues, p.Thr49Ser and p.Pro409Arg. Variants segregated with the telomere phenotype, and affected females were heterozygotes, showing skewed X-inactivation. Telomerase RNA levels were compromised in cells from DKC1 mutation carriers, consistent with their pathogenic role. These findings indicate that females with heterozygous DKC1 mutations may be at increased risk for developing penetrant telomere phenotypes that, at times, may be associated with clinical morbidity.

Keywords: DKC1; Dyskeratosis congenita; myelodysplastic syndrome; pulmonary fibrosis; telomerase.

Figure 1. Clinical features of male probands with telomere-mediated disease and their families

A&B. Clinical history of Family 1 and plot showing the adjacent age-adjusted telomere length of the proband and his daughter, respectively. C. Pedigree and history of Family 2. Shaded squares and circles represent males and females who have features of telomere-mediated disease, respectively. DKC1 genotypes at the variant nucleotide are included in parentheses in the pedigrees.

Figure 2. Molecular characterization of DKC1 mutations identified in two pedigrees

A&B. Integrative Genome Viewer images showing read sequences that diverge from reference in DKC1 for families 1 and 2 respectively (highlighted inside the rectangle). There are significantly fewer reads in panel A compared to panel B. C. Schema showing evolutionary conservation of mutated residues in the dyskerin protein. The S. cerevisiae alignment refers to the dyskerin homolog, NAP57. D&E. Quantitative real time PCR of telomerase RNA (TR) levels in male-derived cells from probands and controls. RNA was extracted from early passage lymphoblasts (D) and fibroblasts (E). The mean for controls represents independent cells with the number noted below. Replicate runs from the same individual are plotted for mutation carriers. Error bars represent standard error of the mean. F. Exome read coverage of coding and exon-flanking sequences of telomere syndrome genes in the two probands.