PB1954: COMPARING STR PROFILES OF DNA ISOLATED FROM DIVERSE TUMOR SITES IN MULTIPLE MYELOMA

Abstract

Background: Multiple myeloma (MM) course is often complicated by extramedullary lesions - plasmacytomas, which pathogenesis is still not completely understood. Genome instability underlies dissemination in various neoplasias, including MM. An important manifestation of genetic instability is the loss of heterozygosity, i.e., the loss of the second allele in the regions of the genome featuring inherited polymorphisms. STR profiling is a method that allows simultaneously assessing the degree of DNA degradation and giving an integral assessment of the stability of the tumor genome. To assess the heterogeneity of MM, not only the tumor substrate in the bone marrow and plasmacytoma but also in circulating cell-free DNA (cfDNA) present in the peripheral blood plasma could be effectively analyzed. Aims: To compare STR profiles of DNA isolated from various anatomical locations (plasma, bone marrow, plasmacytoma) in MM patients. Methods: The prospective study included 21 patients with newly diagnosed MM (11 women, 10 men) aged 39 to 82 years (median 53 years). MM was diagnosed according to the IMWG-2014 criteria. In 15 patients, MM was complicated by plasmacytoma, in 6 of them a tumor biopsy was performed and tumor DNA was isolated. In all patients, plasma circulating cfDNA and control DNA were isolated from buccal epithelium using a commercial kit (Qiagen, Germany) according to the manufacturer’s protocol. In 11 patients, positive immunomagnetic selection of CD138+ bone marrow cells was performed using a monoclonal antibody to CD138 (Miltenyi Biotec, Germany). STR-profiling using COrDIS Plus panel (Gordiz LLC, Russia) of isolated DNA samples (21 plasma circulating cfDNA samples, 11 CD138+ DNA samples of bone marrow cells, 6 plasmacytoma DNA samples) was performed. STR profiles were analyzed using GeneMapper v.4 software (Applied Biosystems, USA). Results: In 10 out of 11 patients, analysis of STR profiles in CD138+ bone marrow cells revealed loss of heterozygosity (LOH) variants with different allelic load, suggesting either a deletion/copy number neutral LOH, or a duplication of one of the two alleles affecting from one to six STR loci for every patient. In all patients for whom plasmacytoma DNA was isolated (n=6), various variants of LOH were also identified (from 1 to 6 loci) (Table 1). Of the 21 plasma circulating cfDNA samples, LOH variants were detected in ten cases (48%) (from 1 to 4 loci), in the other 10 cases, no tumor DNA was detected, the samples matched the control DNA from the buccal epithelium; only in one case, the result turned out to be non-informative due to a significant degree of degradation of the cfDNA. STR profiles of DNA isolated from different tumor sites of the same patient in most cases were identical. In one case aberrant STR loci in plasma circulating cfDNA and bone marrow CD138+ cells were different (patient No. 20). In this patient, MM was complicated by plasmacytomas of various anatomical locations (Th7, 8th rib). One can speculate that the discrepancy between plasma circulating cfDNA and bone marrow CD138+ STR profiles indicates the presence of various MM clones in different tumor loci. Image:Summary/Conclusion: Evaluation of plasma circulating cfDNA might be an effective approach for molecular diagnosis in MM. In 48% of MM patients, it is possible to detect tumor cfDNA in the plasma at the onset of the disease. Comparison of DNA STR profiles from different lesions might reveal the diversity of tumor clones in MM.