Non-Shelterin Telomere Genes Deregulation in Plasma Cell Disorders

Telomeres are essential structures that protect the ends of linear chromosomes. When telomeres become dysfunctional, critically short ends are sensed by the DNA repair machinery triggering senescence or apoptosis as well as creating interchromosomal fusions. Telomere structure and functions depend on the telomerase enzyme (hTERT, hTERC and DKC1) for elongation, on the sheterin complex that regulates telomere length (TL) and protects them against degradation and fusion, and on the non-shelterin complex. The latter is constituted by a set of multifunctional factors, including RPA1 and the MRE11-RAD50-NBS1 (MRN) complex, that facilitate telomerase-based telomere elongation. Particularly, RPA1 might be a crucial link between telomere homeostasis and the replication of chromosome ends as it regulate the telomerase action during the cell cycle. The role of MRN complex in telomere maintenance parallels its function in DNA repair. In telomerase-positive cells, a reduction of the MRN complex results in G-overhang shortening, suggesting MRN complex involvement in the recruitment or action of telomerase. Previous data from our group have provided the first evidence of modifications in the mRNA expression of shelterin members in plasma cell disorders (Panero et al, Mol Med 2010; 16: 471-8; Blood Cells Mol Dis 2014; 52: 134-9). The aim of the present study was to examine the expression profile of the non-shelterin complex: RPA1, RAD50, MRE11, NBS1 and DKC1 in patients with multiple myeloma (MM) and monoclonal gammopathy of undetermined significance (MGUS). Results were correlated with hTERT (telomerase catalytic subunit) mRNA levels, TL, and clinico-pathological characteristics of patients. Bone marrow samples from 70 cases: 35 MM (21 females; mean age: 67.9 years; range: 30-87 years; 35.3% stage III) and 35 MGUS (21 females; mean age: 68.7 years; range: 39-88 years) were studied. All patients gave informed consent and the study was approved by the Ethics Committee of our Institution. Gene expression was quantified by Real-time Quantitative PCR, using Taq-Man methodology, and TL measurements were performed by Terminal Restriction Fragments. A significant increase in the mean mRNA levels of all non-shelterin genes in patients with MM respect to cases with MGUS was observed (Table 1). In order to evaluate whether the expression of these factors was associated to telomerase activity, hTERT transcripts were also quantified. An upregulation of telomerase expression, with significant differences between entities was found (p=0.03). In MM, higher DKC1 mRNA levels (0.10±0.02) in patients who over-expressed telomerase compared to patients with low hTERT expression (0.04±0.004) (p=0.008) was observed. No association between gene expression and TL or clinical parameters was found. To the best of our knowledge, our findings show for the first time a global modification in the expression of non-shelterin genes in MM and MGUS, suggesting that their upregulation in MM may occur as a result of the increasing need for DNA repair and telomerase recruitment, contributing to the progression of the disease. In addition, DKC1 was related to hTERTlevels in MM patients, supporting the role that DKC1 plays in telomerase complex stabilization and telomere maintenance.