Bone Marrow Failure Associated with Short Telomeres and Digenic Variants of Uncertain Significance in Telomere Biology Genes

DISCUSSION Telomere Biology Disorders (TBD) are a group of heritable disorders characterized by abnormally short telomeres. There are at least 15 genes associated with TBD including ACD, CTC1, DKC1, NHP2, NOP10, PARN, POT1, RTEL1, STN1, TERC, TERT, TINF2, WRAP53 NAF1 and ZCCHC8, which can be inherited in autosomal dominant (AD), autosomal recessive (AR), or X-linked recessive patterns. Genetic anticipation has been described in some families with more severe symptoms appearing in each successive generation. We report two patients that presented with bone marrow failure, who were subsequently identified to have short telomeres and variants of uncertain significance (VUS) in two different telomere genes inherited from their parents. These cases suggest a potential for digenic inheritance of TBD. CASE 1: At age 6, patient was diagnosed with stage 4 neuroblastoma. He was treated with chemotherapy, surgery, immunotherapy, and autologous stem cell rescue. At age 10, he developed frequent and severe epistaxis and was found to have pancytopenia. He underwent a bone marrow aspirate and biopsy which revealed a hypocellular marrow and der (1;7), which is associated with myelodysplastic syndrome (MDS). Genetic evaluation revealed one pathogenic variant in DNAJC21 (from father) and VUS in NAF1 (c.1375C>T) (from father) and RTEL1 (c.533T>C) (from mother). Because NAF1 and RTEL1 are related to AD or AR TBD, telomere length testing was performed. Patient was found to have low telomere length, < 1st percentile, in granulocytes, lymphocytes, memory T cells and NK cells (Figure 1). The patient's mother was found to have borderline low telomere length >=1 and <10 th percentiles in her lymphocytes, naïve T cells, memory T cells, and B cells, and low telomere length in her granulocytes. Father had borderline low telomere length >=1 and <10 th percentiles in granulocytes but normal telomere length in all other subsets. Neither parent reported any symptoms that would be consistent with TBD. We interpreted these findings as consistent with the patient having TBD and the patient's mother either also having TBD or being a carrier, and the potential that genetic anticipation may be involved. The NAF1 variant appears likely to be benign as father's telomeres were normal in length except for low telomere length in his granulocyte compartment. It is possible that the patient's prior treatment for neuroblastoma may have contributed to the shortened telomeres. Over the course of a year, patient developed worsening bone marrow failure. He then underwent reduced intensity conditioning using busulfan, fludarabine and anti-thymocyte globulin for a 4/6 matched cord stem cell transplant. He engrafted on day +10. CASE 2: A 15-year-old male presented with a depressed skull fracture and scalp hematoma following a sports-related head injury. He was incidentally found to have a platelet count of 69 x 10 3/microliter, mild anemia with macrocytosis, and mild leukopenia. Bone marrow biopsy revealed a hypocellular marrow with multilineage maturation, mild fibrosis, and no cytogenetic abnormalities. The patient's telomere lengths were low, below the 1 st percentile, in all 6 white blood cell subsets (Figure 2). He underwent clinical trio exome sequencing along with his parents. He was found to have VUSs in TERT (c.3158-80G>A) (from mother), TINF2 (c.814T>C) (from mother) and SRP72 (c.1928C>T) (from father). Mother was also found to have low telomere length (<1 st percentile) in all 6 subsets. Both parents have normal blood counts while patient continues to have pancytopenia and is transfusion independent. It is presently unclear which of the VUSs are contributing to the patient's phenotype as the TERT and TINF2 variants are both associated with bone marrow failure and heterozygous germline variants in SRP72 is associated with familial MDS. Genes that cause TBD can be inherited through various modes of inheritance. These two patients who presented with bone marrow failure, identified to have VUSs in more than one TBD-associated genes and functional evidence of shortened telomeres, highlight the potential for a digenic mode of inheritance underlying TBD. Synergy between two variants contributes to a penetrant phenotype and can result in earlier or more severe onset of disease. Our patients' parents were asymptomatic or mildly affected, compared to our patients. Larger cohort studies are needed to determine the prevalence of this mode of inheritance.