Homozygous and Compound Heterozygous Mutations in the Telomerase Reverse Transcriptase Gene in Two Families with Spontaneous Dyskeratosis Congenita and Idiopathic Aplastic Anemia.
Authors: Du Hong-Yan , Pumbo Elena , Manley Peter , Wilson David B. , Mason Philip , Bessler Monica 
Published: April 2020

Abstract
Dyskeratosis congenita (DC) is a rare inherited bone marrow failure syndrome. Classically, DC presents with progressive bone marrow failure, abnormal skin pigmentation, nail dystrophy, and mucosal leukoplakia. The pattern of inheritance in families with DC suggests an X-linked recessive, an autosomal dominant, and an autosomal recessive form of DC. However, in the majority of patients the occurrence of the disease is sporadic or the family history is unknown. Mutations in four different genes have been associated with DC so far. Mutations in DKC1 have been shown to account for the X-linked form of DC and DKC1 de novo mutations account for about one third of male patients with sporadic disease. Mutations in the telomerase RNA TERC and in the catalytic subunit of telomerase, TERT, have been shown to be responsible for the autosomal dominant form of DC. Interestingly, patients with heterozygous mutations in TERC and TERT often show a milder form of disease and a later age of onset and often lack the classic mucocutaeous features, thus are classified as atypical DC. Very recently homozygosity for a mutation in NOP10 has been identified in one family with autosomal recessive disease. The products of the genes mutated in DC are all components of the telomerase complex, suggesting that disease in patients with DC is caused by a defect in telomere maintenance. Here we investigated two patients, one UPN # 199.001 presenting with the classic manifestations of DC and the other UPN# 284.001 presenting with progressive bone marrow failure but no other clinical features suggestive of DC. In both patients the telomeres measured in peripheral blood mononuclear cells were very short, being defined as being below the 1st percentile. Mutation analysis in the genes associated with DC revealed that patient 199.001 was homozygous for a novel TERT (C2110T) gene mutation, causing an amino acid change (P704S) within the RT domain of TERT. Both parents were heterozygous for the C to T transition. Interestingly however, the father was in addition heterozygote for a second mutation in TERT (C1234T; H412Y) a mutation which has previously been described and has been shown to reduce telomerase activity by 50%. Investigations of the family revealed that the parent’s were distantly related, explaining the same TERT sequence alteration in both parents. Both arms of the family contained members with pulmonary fibrosis. In the second patient 284.001 we identified two different novel TERT gene mutations. One A2537G causes the amino acid change Y846C in the RT domain of TERT whereas the other C2628G causes H876Q also in the RT domain. One of the mutations was inherited from each parent and the parent with the A2537G mutation also had very short telomeres. These two families illustrate that the pattern of inheritance in patients with DC may be complex and show for the first time that homozygous or compound heterozygous TERT gene mutation may be associated with DC. Co-dominance of the three different TERT gene mutations and the inheritance of short telomeres have possibly contributed to development of disease in these patients who were thought to have sporadic DC and idiopathic aplastic anemia.