Detection of N-Ras codon 61 mutations in subpopulations of tumor cells in multiple myeloma at presentation

Activating point mutations in codons 12, 13, or 61 of the K-ras and N-ras g enes have been reported to occur in up to 40% of patients with multiple mye loma at presentation. In a study of 34 presentation myeloma cases using a s ensitive polymerase chain reaction-restriction fragment length polymorphism strategy on enriched tumor cell populations, the present study detected N- ras codon 61 mutation-positive cells In all patients. Quantitative plaque h ybridization using allele-specific oligonucleotide probes showed that in th e majority of patients, ras mutation-positive cells comprise only a subpopu lation of the total malignant plasma cell compartment (range, 12%-100%). Us ing clonospecific point mutations in the 5 ' untranslated region of the BCL 6 gene to quantitate clonal B cells in FACS-sorted bone marrow populations from 2 patients, the representation of ras mutation-positive cells was inde pendent of immunophenotype. These observations imply that mutational activa tion of N-ras codon 61 is a mandatory event in the pathogenesis of multiple myeloma; such mutations provide a marker of intraclonal heterogeneity that may originate at an earlier ontologic stage than immunophenotypic diversif ication of the malignant B cell clone. (C) 2001 by The American Society of Hematology.