Kappa deleting element as an alternative molecular target for minimal residual disease assessment by real-time quantitative PCR in patients with multiple myeloma

Background and Objectives Minimal residual disease (MRD) assessment by PCR in multiple myeloma (MM) has several shortcomings, including the lack of a suitable target. Kappa deleting element (KDE) rearrangements occur in virtually all Ig‐lambda B‐cell malignancies and in 1/3 of Ig‐kappa are not affected by somatic hypermutation and, as in ALL, could be used as PCR targets. Methods We have first investigated the incidence, gene segment usage, and CDR3 composition of IGK‐KDE rearrangements in 96 untreated myeloma patients. Second, we tested 16 KDE gene rearrangements as molecular targets for MRD assessment by RQ‐PCR using a germline reverse primer and a germline Taqman probe in combination with allele‐specific oligonucleotides (ASO) as forward primers. Results Monoclonal KDE rearrangements were amplified in 45% (43/96) of cases, monoallelic in 2/3 of them (29 cases), and biallellic in the remaining 14 cases. Overall, 88% of cases were successfully sequenced, KDE being equally frequently rearranged with VK and with intron‐Recombination signal sequence (RSS). Median numbers of inserted and deleted nucleotides in the junctional region were one and five, respectively. Conclusions Using KDE rearrangements as additional PCR target for MRD assessment in MM improves the applicability of these studies in 9% of cases overall and in 20% of lambda cases. Its use in the latter subset could represent a significant advance.