Pathogenic telomerase variants in childhood AML/MDS

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As a pediatric oncologist, one of the questions I hear most often from parents is “Why did this happen to my child?” For the vast majority of childhood cancer diagnoses, we do not know the answer to this question. Both acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) may be presenting features of dyskeratosis congenita (DC), a telomere biology disorder characterized by very short telomeres. Although AML/MDS are part of the DC clinical spectrum, children diagnosed with AML and MDS are not routinely screened for cancer predisposition. The objective of this work was to determine if a proportion of pediatric AML/MDS patients have underlying defects in telomere maintenance. To answer this question, we sequenced telomerase genes in children diagnosed with AML/MDS, and assessed the rare variants we found for clinical association with DC-like features and functional impact on telomere maintenance.  

Our laboratory at Baylor College of Medicine explores genetic and molecular factors underlying risk for childhood cancer and cancer therapy-related toxicities. This paper describes work conducted as part of my PhD thesis, during a time when I was transitioning from a junior faculty trainee in Dr. Alison Bertuch’s lab to an independent investigator. When we started this project, little was known about the relationship between telomerase variants and risk for childhood cancer, though there had been some provocative and highly relevant findings in adults with hematologic malignancies.

The direct primer extension assay was a key experiment applied for this study. In this assay, the telomerase RNA component and telomerase reverse transcriptase protein are generated and immunopurified together as telomerase, and then incubated with the ingredients required for telomere extension. The resulting telomere extension products are purified and run on a large, sequencing-size polyacrylamide gel (Fig. 1) to separate the DNA fragments by size. The results are captured on a phosphorimager screen, permitting quantification of telomerase activity and processivity. Dr. Bertuch’s lab is an active participant in an enthusiastic and supportive international scientific community of investigators who engage in telomere and telomerase research. For the direct primer extension assay and related experiments outlined in this paper, we collaborated with Dr. Tracy Bryan at the Children’s Medical Research Institute in NSW, Australia. Dr. Chris Tomlinson, then a post-doctoral student in the Bryan lab, performed these experiments and his work provided substantial mechanistic insight to the variants we described.  

Our paper supports rigorous functional testing of novel and rare variants found in association with a clinical phenotype. Follow up studies are under way to investigate the cases that had evidence of telomere shortening but normal telomerase activity and processivity, which may relate to the limitations of the in vitro assay. The takeaway message for the clinical oncologist is to obtain a complete family history in a child newly diagnosed with AML/MDS, in addition to doing a thorough physical exam with attention to mucocutaneous features before and during treatment. Genetic screening for underlying cancer predisposition should be considered if any red flags emerge at diagnosis, or if the patient experiences extreme chemotherapy-related toxicities during treatment.

Maria Monica Gramatges, MD, PhD

Associate Professor of Pediatrics, BAYLOR COLLEGE OF MEDICINE