EXPRESSION OF P53, P21, MDM2, RB, BAX AND KI67 PROTEINS IN LYMPHOMAS OF THE MUCOSA-ASSOCIATED LYMPHOID (MALT) TISSUE

We have investigated by immunohistochemistry 38 cases of B-cell MALT- NHL comprising 23 high grade (HG) and 15 low grade-(LG) tumours for th e expression of p53, mdm2, p21, Rb, Ki, 67, bcl2 and Bax proteins. P53 , mdm2 and p21 proteins were found in at least 5% of the tumour cells in 13/23 2/23 and 11/23 HG tumours, respectively. These proteins were detected in very rare tumour cells in LG tumours. The following patter ns were recorded in HG tumours: p53+/p21+/mdm2+ (2 cases), p53+/p21+/m dm2- (7 cases), P53+/p21-/mdm2- (4 cases), p53-/p21-/mdm2- (18 cases) and p53-/p21+/mdm2-(2 cases). Proliferative Ki67 index and Rb protein expression were higher in HG than in LG MALT-NHL. Bcl2 protein was exp ressed in all LG MALT-NHL, whereas only 2/23 HG MALT-NHL were bcl2 pos itive in most tumour cells. Bax protein was expressed in all MALT-NHL with HG tumours being positive in higher proportion of tumour cells th an LG rumours. These findings show that significant expression of p53, mdm2, p21,Ki67 and Rb proteins occurs more frequently in aggressive h istotypes of MALT-NHL. The parallel Rb/Ki67 expression suggests that R b protein expression in MALT-NHL is normally regulated in relation to the proliferative growth fraction of the tumours. The pattern p53+/p21 +/mdm2 +/- may represent MALT-NHL with wild type (wt) p53 gene since m dm2 and p21 proteins are inducible by wt p53 gene. The pattern p53+/md m2-/p21-may represent MALT-NHL with p53 gene mutations unable to activ ate expression of mdm2 and p21 proteins. MALT-NHL with the p53-/mdm2-/ p21 + pattern may be consistent with p53-independent p21 expression. B ar protein expression in all MALT-NHL suggests a role for this protein in the pathogenesis of these tumours.