Reliability of Myeloma Model Systems: KP-6 Is a Novel Hyperdiploid Cell Line

Reliable model systems are critical for investigating the genetic and molecular drivers of disease pathogenesis. Although the number of cell line model systems for multiple myeloma is extensive, recent studies showed patient tumors and cell lines clustering separately based on gene expression profiles. Furthermore, the hyperdiploid karyotype, present in approximately 50% of myeloma patients, is also extremely underrepresented in established human myeloma cell lines (HMCLs): only one of 59 HMCLs contains this phenotype. These data suggests that the average HMCL does not accurately represent the average myeloma patient and questions the validity of using HMCLs as model systems. However, most HMCLs were established from patients with pleural effusions or plasma cell leukemia, which are highly progressed and rare forms of myeloma. To assess how cell lines correlate with patient material, we performed a copy number abnormality (CNA) comparison between four myeloma primary tumors and their corresponding HMCLs. Here, we show that established HMCLs maintain the majority of the CNAs, contain a ten-fold greater frequency of 7q gains compared to patients, and that KP-6 is the second hyperdiploid HMCL known to date.

CNAs were identified using array comparative genomic hybridization (Agilent 244k) with data analyzed at a resolution of 26kb. On average 59% (46.7-77.3) of the CNAs were shared between the tumors and their cell lines, 17% (8.5-25.9) were unique to the patient samples, and 24% (13.6-36.6) were only detected in the HMCLs. To put these differences in context we also compared the diagnostic material from which JMW-1 was established with a progression sample collected 9 months later after 3 cycles of VAD followed by an autologous peripheral blood stem cell transplant. The patient diagnostic and relapse tumors showed 58% similarity based on CNAs while the diagnostic and relapse tumors contained 8% and 34% unique events respectively. These results indicate that established HMCLs are no more divergent from their primary cells as is the same patient's relapse material. Overall the HMCLs studied appear highly representative of the patient from which they were derived, with most differences attributed to an enrichment of subclonal populations present in the primary tumor admixture.

Of particular interest, ubiquitous 7q gains were an unexpected unifying event across all patient tumors and their HMCLs. We then analyzed the 729 baseline myeloma tumors with copy number data from the MMRF CoMMpass study against our panel of 59 similarly characterized HMCLs. We observed that HMCLs contained a 10-fold increase in the occurrence of 7q gains with respect to non-hyperdiploid patients with 7q gains (35/59 in HMCLs and 19/344 in patients). Because 97% of HMCLs are non-hyperdiploid and 7q gains are extremely rare in this population, over-representation of 7q gains in cell lines suggests that this event may promote extra-medullary growth and the establishment of cell lines. Profiling of patient tumors for gains on 7q may therefore increase the probability of establishing a new HMCL and help expand our model systems for understanding this disease. Further studies are required to understand the potential biological significance of this event in vitro .

We also identified the KP-6 cell line as a novel hyperdiploid HMCL established from an extramedullary site of an advanced hyperdiploid patient. The presence of trisomies for at least four of the odd-numbered chromosomes is characteristic of a hyperdiploid myeloma patient. From the patient's tumor, clear trisomies were observed on chromosomes 3, 7, 11, 15, 19, and 21 and the HMCL established from this hyperdiploid patient maintains full trisomies of chromosomes 15, 19, and 21, with partial p-arm trisomies on chromosomes 7, 11, and 17. Since this is only the second bona fied hyperdiploid myeloma cell line to date, KP-6 promises to play a major role in our ability to understand the pathogenesis and progression of this prominent patient population.

Overall, our research indicates that, based on CNAs, HMCLs are as representative of the primary tumor as is the same patient's tumor at a later time point. Additionally, HMCLs contain a 10-fold overabundance of 7q gains. Finally, we showed that KP-6 is the second hyperdiploid HMCL known to date. Taken together, these results suggest that HMCLs are reliable model systems for investigating the underlying biology of multiple myeloma.