Analysis of multidrug-resistance (MDR-1) glycoprotein and CD56 expression to separate monoclonal gammopathy from multiple myeloma.

Monoclonal gammopathy of undetermined significance (MGUS) is different from multiple myeloma (MM) by a low proliferation and by its indolent clinical course. In this study, two biological parameters were investigated which mark the transition from MGUS to MM, i.e. expression of the P-170 glycoprotein associated with the multidrug resistance phenotype (MDR-1) and expression of the natural killer cell antigen. CD56. Strong MDR-1 expression was found in plasma cells of 32/38 untreated MGUS as compared with 33/105 untreated MM stage I-III (84% v 32%, P < 0.001) and in 0/10 normal plasma cell samples. CD56 expression in high density was present in 43/57 analysed untreated MM but in none of 23 MGUS (78% v 0%, P < 0.0001). Plasma cells did characteristically show a low Ki-67 proliferation index in 14/15 MGUS patients (mean 0.05%, range 0-0.2%) and a higher index in 25 analysed MM patients (mean 2.31%, range 1-7%, P < 0.03). These data indicate that MDR-1 expression together with absence of CD56 expression and a low proliferation index can be used to separate MGUS from MM.