PF342 COMPENSATORY MUTATIONS IN PROMOTERS OF TELOMERASE GENES ARE RARE IN TELOMERE BIOLOGY DISORDERS WITH BONE MARROW FAILURE
Authors: R. Voss, S. Sahoo, V.B. Pastor Loyola, S. Krüger, F. Beier, B. Strahm, C. Niemeyer, M. Wlodarski
Journal: HemaSphere (2019)

Background: Dyskeratosis congenita, also referred to as telomere biology disorder (TBD), causes premature telomere shortening and manifests with progressive bone marrow failure (BMF). In addition to BMF, classical features include mucocutaneous triad (leukoplakia, nail dystrophy and skin dyspigmentation) and pulmonary fibrosis. To date, constitutional loss‐of‐function (LOF) mutations of 13 genes ( DKC1, TERT, TERC, TINF2, TCAB2, RTEL1, NHP2, NOP10, CTC1, USB1, PARN, ACD, NAF1 ) involved in telomere maintenance and elongation have been associated with TBD. Human cancers can reactivate telomerase activity by acquiring gain‐of‐function (GOF) mutations in TERT gene promoter leading to increased telomerase activity (Vinagre, Nat Commun 2013). Moreover, GOF mutations at TERT promoter hotspots c.‐124C&gt;T and c.‐146C&gt;T were found in 5% of patients with idiopathic lung fibrosis who had an underlying germline LOF mutation in TERT or TERC (Maryoung, JCI 2017). These mutations counterbalanced the LOF germline variants and the patients had normal blood counts. Aims: We hypothesized that GOF mutations in promoters of telomerase genes can be acquired in pediatric TBD and might explain the heterogeneity of hematologic manifestations. Methods: For this purpose, we investigated a total of 64 TBD patients (52 with classical DC phenotype and 12 DC‐like with BMF and short telomeres) and 9 mutation carriers (family members), all referred for diagnostics within the framework of the EWOG‐MDS and the Aachen Telomeropathy Registries. Two amplicon‐based deep sequencing panels were designed in order to screen for I) constitutional variants consisting of the 13 TBD genes in addition to SAMD9 and SAMD9L , and II) somatic mutations in the promoter regions of TERT, TERC, DKC1, RTEL1, TCAB1 and TINF2 . Results: Constitutional pathogenic mutations were found in TERT (20.9%), TERC (17.7%), DKC1 (16.1%), TINF2 (12.9%), RTEL1 (10%), TCAB1 (6.4%) and SAMD9 / SAMD9L (3.6%), while no mutations were found in 8 patients (14.8%). Previously reported hotspot mutations in the TERT and TERC gene promoters were not detected in our patients. With a median depth of 13283 reads even the presence of very small clones could be excluded. Conversely, germline variants of uncertain significance (VUS) were identified in the 5́UTR/promoter regions of TCAB1 at c.‐499dupG in three patients (P36, P16, P11; variant allele frequency (VAF) 43%, minor allele frequency in gnomAD healthy database (MAF) 0.003%) and in DKC1 at c.‐519C&gt;T (P15: VAF94%, MAF 0.004%). Another variant, a common polymorphism in the TERT promoter region at c.‐245T&gt;C, which was previously shown disrupt a preexisting ETS2 binding site, was observed in 8 patients (VAF50–100%, MAF 23%). Summary/Conclusion: In summary, activating mutations in promoters of telomerase genes previously found in cancers and idiopathic pulmonary fibrosis are absent in DC with BMF. However, we identified TCAB1 promoter variant in 3 patients, and DKC1 promoter in 1 patient. These variants might potentially act as phenotype modifiers and warrant functional studies.