Intraclonal Heterogeneity Associates with Clonal Stability in Multiple Myeloma

Multiple myeloma (MM) pathogenesis has been explained for many years by the cancer biology dogma introduced by Peter Nowell: first, a single plasma cell would be immortalized by an error in the immunoglobulin genes rearrangement process; then, a progressive stepwise acquisition of somatic cell mutations would induce a sequential selection and domination by the fittest clone. In line with this idea of “myeloma stability”, SNP arrays studies in diagnostic-relapse paired samples have revealed the presence of common clonal characteristics. Biologically, the M-protein remains usually constant across MM evolution and further, the variable domain of the rearranged immunoglobulin heavy chain genes (or CDR3 region) has been used as a patient-specific myeloma fingerprint in minimal residual disease (MRD) studies. However, massive genome studies with Next Generation Sequencing (NGS) have challenged this concept, showing a significant intraclonal heterogeneity at diagnosis with the possible presence of several clonal progenitors or tumor-initiating cells.

In this study, we have characterized and compared the CDR3 region in 52-paired samples from 26 MM patients aiming: 1) to assess mono-clonality in MM evolution through the analysis of the CDR3 sequence and, 2) to validate ASO RQ-PCR approaches for MRD in MM, based on the constancy and specificity of the CDR3 region.

Samples were obtained at diagnosis and progression (19 pairs) or at 2 different timepoints of progressive disease (7 pairs). Median time between sampling was 2 years. M-protein subtype remained stable in all pairs but 1, associated with a light-chain escape phenomenon. All samples proceeded from bone marrow (BM) except for 2 pairs, composed by BM and extramedullary disease (spleen and testes). Two major cytogenetic changes were identified: increased 13q14 deletion (from 7 to 54%) in 1 pair and increased 17p (p53) deletion (from 5 to 87%) in a further one. Treatments administered between sampling included most of the current approaches used in MM (data not shown).

Genomic DNA isolation, PCR amplification and sequencing were performed following conventional methods. Germline VH, DH and JH segments were identified by comparison with public databases. CDR3 region was first identified in all samples and then compared between the two samples in the 26 pairs: the sequence of nucleotides was constantly identical in each pair, including those associated with major cytogenetic changes, a light-chain escape, extramedullar vs. BM infiltration and relapsed (and therefore, treatment selected) vs. refractory disease. Therefore, we can first conclude that the main tumor clone in MM retains a specific signature across all stages of disease evolution that allows the identification of samples as evolutionary related. This major clone signature is not modified by clinical or biological changes in the disease nor under different treatment pressures and would thus identify disease relapse and progression.

Our results have also a clear impact on the validity of molecular MRD techniques. The high rate of complete responses (up to 50-60%) currently achieved in MM has prompted the use of new techniques for disease assessment. Today, ASO RQ-PCR, based on the use of specific primers and probes complementary of the VDJH rearrangement, continues to be the most sensitive approach. One pitfall of this technique would be the potential instability of PCR targets over time, which would induce false negative results. In B-cell precursor ALL, this is estimated to happen in 30-40% of cases but has not been deeply evaluated in MM yet. With the present study, we can also conclude that the junction region of the VDJH rearrangement in MM constantly identifies the myeloma cells responsible for relapse and therefore can be used as a reliable target for MRD assessment by ASO RQ-PCR and more recently, by NGS methods.

If the CDR3 region remains stable, the novel concept of clonal tiding in MM should not be interpreted as a poly- or oligoclonal but subclonal. In MM, tides can be subclonal, but the ocean remains monoclonal.