Discordant neoplasms in monozygotic twins with a germline RECQL5 variant.

RECQL5 is a member of the RecQ helicase family involved in DNA replication, homologous recombination, and maintenance of genomic stability. While germline pathogenic variants in other RecQ helicases cause established cancer predisposition syndromes, the role of RECQL5 in human cancer susceptibility remains uncertain. We report monozygotic adolescent twins with distinct tumors: dysembryoplastic neuroepithelial tumor in one twin and Burkitt lymphoma in the other. Clinical genome sequencing was initially nondiagnostic, but reanalysis identified a rare heterozygous nonsense variant in RECQL5 (NM_004259.7:c.2698C>T, p.(Gln900Ter)), present in both twins and their unaffected mother. The variant is predicted to undergo nonsense-mediated mRNA decay or produce a truncated protein lacking the C-terminal SRI (Set2-Rpb1 interacting) domain, which mediates interaction with RNA polymerase II. However, tumor sequencing data were not available to evaluate loss of heterozygosity or second somatic events. Given the unaffected carrier parent, lack of tumor molecular confirmation, and the biological heterogeneity of the tumors, a causal relationship for this variant cannot be established. This case highlights the challenges of interpreting rare germline variants in genes with emerging but incompletely characterized disease associations. Although the available evidence is insufficient to establish a definitive causal relationship, the identification of a shared loss-of-function RECQL5 variant in monozygotic twins with distinct tumors is noteworthy and adds to the limited clinical evidence suggesting a potential role for RECQL5 in cancer susceptibility. Additional functional studies, tumor-based analyses and the accumulation of well-characterized clinical cases will be essential to determine whether RECQL5 contributes to hereditary cancer predisposition.