The biology and management of dyskeratosis congenita and related disorders of telomeres

BACKGROUND

Dyskeratosis congenita (DC) is a multisystem syndrome characterized by mucocutaneous abnormalities, bone marrow failure, and predisposition to cancer. Studies over the last 25 years have led to the identification of 18 disease genes. These have a principal role in telomere maintenance, and patients usually have very short/abnormal telomeres. The advances have also led to the unification of DC with a number of other diseases, now collectively referred to as the telomeropathies or telomere biology disorders.

WHAT IS COVERED

Clinical features, genetics, and biology of the different subtypes. Expert view on diagnosis, treatment of the hematological complications and future.

EXPERT VIEW

As these are very pleotropic disorders affecting multiple organs, a high index of suspicion is necessary to make the diagnosis. Telomere length measurement and genetic analysis of the disease genes have become useful diagnostic tools. Although hematological defects can respond to danazol/oxymetholone, the only current curative treatment for these is hematopoietic stem cell transplantation (HSCT) using fludarabine-based conditioning protocols. New therapies are needed where danazol/oxymetholone is ineffective and HSCT is not feasible.

Resonance assignment and secondary structure of the tandem harmonin homology domains of human RTEL1

Regulator of telomere elongation helicase 1 (RTEL1) is an Fe-S cluster containing DNA helicase that plays important roles in telomere DNA maintenance, DNA repair, and genomic stability. It is a modular protein comprising an N-terminal helicase domain, two tandem harmonin homology domains 1 & 2 (HHD1 and HHD2), and a C-terminal C4C4 type RING domain. The N-terminal helicase domain disassembles the telomere t/D-loop and unwinds the G-quadruplex via its helicase activity. The C-terminal RING domain interacts with telomere DNA binding protein TRF2 and helps RTEL1 recruitment to the telomere. The tandem HHD1 and HHD2 are characterized as a putative protein-protein interaction domain and have recently been shown to interact with a DNA repair protein SLX4. Several mutations associated with Hoyeraal-Hreidarsson syndrome and pulmonary fibrosis have been found in HHD1 and HHD2 of RTEL1. However, these domains have not been characterized for their structures. We have expressed and purified HHD1 and HHD2 of human RTEL1 for their characterization using solution NMR spectroscopy. Here, we report near complete backbone and sidechain ¹H, ¹³C and ¹⁵N chemical shift assignments and secondary structure of the HHD1 and HHD2 domains of human RTEL1.

Complications for a Hoyeraal-Hreidarsson Syndrome Patient with a Germline DKC1 A353V Variant Undergoing Unrelated Peripheral Blood Stem Cell Transplantation

Hoyeraal-Hreidarsson syndrome (HHS), caused by several different germline mutations resulting in severe telomeropathy, presents with early-onset growth anomalies and neurologic/developmental disorders including characteristic cerebellar hypoplasia. Early mortalities may arise from immunodeficiency and bone marrow failure if not successfully salvaged by allogeneic hematopoietic stem cell transplantation (HSCT). Few reports have characterized the persistent somatic progression of HHS after successful HSCT. We present an HHS patient with an X-linked recessive DKC1 c.1058C > T; Ala353Val mutation who successfully underwent unrelated HSCT at 5 years of age. After months of early infections and organ toxicities immediately post-transplant, he had more than two years of excellent quality of life with correction of bone marrow failure and immunodeficiency. However, episodic massive variceal bleeding and progressive respiratory insufficiency, which were secondary to non-cirrhotic portal hypertension and pulmonary arteriovenous shunts, respectively, developed over 2 years after HSCT and resulted in his death from respiratory failure 4 years after HSCT. This outcome suggests that while HSCT can correct bone marrow failure and immunodeficiency, it may fail to prevent or even aggravate other fatal processes, such as portal hypertension and pulmonary arteriovenous shunting.

RTEL1: functions of a disease-associated helicase

DNA secondary structures that arise during DNA replication, repair, and recombination (3R) must be processed correctly to prevent genetic instability. Regulator of telomere length 1 (RTEL1) is an essential DNA helicase that disassembles a variety of DNA secondary structures to facilitate 3R processes and to maintain telomere integrity. The past few years have witnessed the emergence of RTEL1 variants that confer increased susceptibility to high-grade glioma, astrocytomas, and glioblastomas. Mutations in RTEL1 have also been implicated in Hoyeraal-Hreidarsson syndrome, a severe form of the bone-marrow failure and cancer predisposition disorder, dyskeratosis congenita. We review these recent findings and highlight its crucial link between DNA secondary-structure metabolism and human disease.

Hoyeraal-Hreidarsson syndrome with a DKC1 mutation identified by whole-exome sequencing

BACKGROUND

Hoyeraal-Hreidarsson syndrome is a severe multisystem disorder that is characterized by bone-marrow failure, intrauterine growth retardation, microcephaly, immunodeficiency, and cerebellar atrophy. This rare disease shares clinical features with dyskeratosis congenita and, together, they are recognized as a group of disorders caused by telomere dysfunction. As the genetic background of dyskeratosis congenita or Hoyeraal-Hreidarsson syndrome has expanded rapidly, multiple causative genes and inheritance patterns pose a great challenge to their genetic diagnosis.

CASE PRESENTATION

A 3-month-old boy was referred for head titubation and tremulous movements of the trunk. Multiple petechiae also developed on his face and trunk at the age of 5 months. Extensive evaluation, including brain magnetic resonance imaging, hematologic tests, and bone-marrow evaluation, revealed cerebellar atrophy and aplastic anemia. His elder brother exhibited a similar clinical presentation and died from sepsis after hematopoietic stem cell transplantation. Although skin pigmentation or nail dystrophy was not evident, Hoyeraal-Hreidarsson syndrome was suggested as a differential diagnosis. Instead of the conventional gene-specific approach with Sanger sequencing, we used whole-exome sequencing for the genetic diagnosis of this patient with possible Hoyeraal-Hreidarsson syndrome and successfully identified a missense mutation (c.146C>T, p.Thr49Me) in DKC1.

CONCLUSION

This case suggests that whole-exome sequencing is particularly useful for the genetic diagnosis of extremely rare diseases with genetic heterogeneity, although there are many limitations, including cost and uneven or suboptimal coverage, to the application of this method as a routine genetic diagnosis.

Human telomeres and telomere biology disorders

Telomeres consist of long nucleotide repeats and a protein complex at chromosome ends essential for chromosome stability. Telomeres shorten with each cell division and thus are markers of cellular age. Dyskeratosis congenita (DC) is a cancer-prone inherited bone marrow failure syndrome caused by germ-line mutations in key telomere biology genes that result in extremely short telomeres. The triad of nail dysplasia, abnormal skin pigmentation, and oral leukoplakia is diagnostic of DC but highly variable. Patients with DC may also have but numerous other medical problems, including pulmonary fibrosis, liver abnormalities, avascular necrosis of the hips, and stenosis of the esophagus, lacrimal ducts, and/or urethra. All modes of inheritance have been reported in DC and de novo mutations are common. Broad phenotypic heterogeneity occurs within DC. Clinically severe variants of DC are Hoyeraal-Hreidarsson syndrome and Revesz syndrome. Coats plus syndrome joined the spectrum of DC with the discovery that it is caused by mutations in a telomere-capping gene. Less clinically severe variants, such as subsets of apparently isolated aplastic anemia or pulmonary fibrosis, have also been recognized. These patients may not have the DC-associated mucocutaneous triad or complicated medical features, but they do have the same underlying genetic etiology. This has led to the use of the descriptive term telomere biology disorder (TBD). This chapter will review the connection between telomere biology and human disease through the examples of DC and its related TBDs.