The Clinical and Genetic Features of Dyskeratosis Congenita, Cryptic Dyskeratosis Congenita, and Hoyeraal-Hreidarsson Syndrome in Japan

Background: Dyskeratosis congenita (DKC) is an inherited bone marrow failure syndrome typified by reticulated skin pigmentation, nails dystrophy, and mucosal leukoplakia. Hoyeraal-Hreidarsson syndrome (HHS) is considered to be a severe form of DKC. Unconventional forms of DKC, which develop slowly in adulthood without physical anomalies characteristic to DKC, have been reported. Clinical and genetic features of DKC have been investigated in Caucasian, Black, and Hispanic populations, but never in Asian populations. Therefore, the present study aimed to determine the clinical and genetic features of DKC, HHS, and cryptic DKC among Japanese patients.

Methods: We analyzed 16 patients diagnosed with DKC, 3 patients with HHS, and 21 patients with cryptic DKC between 2003 and 2014 in Japan. Telomere length was measured by Southern blot and/or flow-fluorescence in situ hybridization methods. Mutation analyses were performed using direct sequencing for DKC1, TERC, TERT, NOP10, NHP2, and TINF2. In some patients, we also analyzed the exon sequence and genome copy number using a next-generation sequencer.

Results: Age at diagnosis was significantly older in the following order: HHS, DKC, and cryptic DKC (p<0.001). Twenty-five percent of DKC and HHS patients, and 33% of cryptic DKC patients, were women. Two DKC patients and six cryptic DKC patients had a family history. Characteristic findings of DKC included nail dystrophy (93.75%), reticulated skin pigmentation (87.5%), and lingual leukoplakia (81.3%), with 11/15 (68.8%) patients showing all three physical abnormalities. Characteristic findings of HHS were reticulated skin pigmentation 100%), nail dystrophy (66.7%), and lingual leukoplakia (33.3%); none of the patients had all three abnormalities. Regarding peripheral blood anomalies in DKC patients, peripheral blood count results at diagnosis revealed a marked reduction in platelet count among the three types of blood cells assessed: 7/16 (43.8%) patients had a platelet count ≤20000/µl, whereas only 1/16 (6.3%) patient had a neutrophil count ≤1000/µl or Hb ≤7g/dl. Telomere length analysis revealed that telomere length was shortened in 6/7 (85.7%) DKC patients, and all HHS and cryptic DKC patients. Mutations of telomere regulated genes were found in 11/16 (68.7%) DKC patients (DKC1 mutations in 5 patients, TINF2 mutations in 3 patients, TERT mutations in 2 patients, and TERC mutations in 1 patient). Among these, those harboring the homozygous TERT c.1002_1004del mutation showed a large deletion in the region encoding the TERT gene in one allele on chromosome number 5 by SNP array analysis. This is the first report of a large deletion in the TERT gene. With respect to HHS patients, no causative gene mutation could be identified for any of the patients. With respect to cryptic DKC patients, 11/21 (52.4%) patients had gene mutations (TERT mutations in 5 patients, TINF2 mutations in 3 patients, RTEL1 mutations in 2 patients (1 family), and TERC mutations in 1 patient). Those with RTEL1 mutations had mutations of both alleles, whereas those with the other mutations had heterozygous mutations. While the RTEL1 mutation is often discovered in HHS patients in the form of autosomal recessive inheritance, these two patients did not have apparent physical abnormalities characteristic to DKC, and thus represent the first case of cryptic DKC involving RTEL1 mutations. Immunosuppressive agents such as cyclosporine and steroids were administered to five patients, but no apparent efficacy was observed. Anabolic steroid hormones were also administered to five patients, and mild improvement in anemia was observed in one DKC patient, and mild improvement in reduced platelet count in one HHS patient. Hematopoietic stem cell transplantation was performed in eight patients, resulting in long-term survival in six of these patients (post-transplantation 10-year survival rate, 58.3%).

Conclusions: The present study is the first to address DKC, HHS, and cryptic DKC in Japanese people, an Asian race. We found marked reductions in platelet counts in DKC patients in blood tests at diagnosis, a high prevalence of TINF2 mutations as the causative genetic mutation, and the existence of DKC patients with large deletions in the TERT gene and cryptic DKC patients with RTEL1 mutations on both alleles.