Summary. Alteration of the tumour suppressor gene p53 is frequent in AIDS-related non-Hodgkin's lymphomas (AIDS-NHL), particularly in Burkitt's or Burkitt's-like lymphomas (BL/BLL). Since mechanisms of inactivation other than mutations have been advanced, the transcriptional activity of the p53 protein was studied in a functional assay in yeast in a series of AIDS-NHL lesions and compared with their morphology, immunohistochemistry (IHC) and single-strand conformation polymorphism (SSCP) analysis detection of other p53 abnormalities, Epstein-Barr virus (EBV) status, MDM-2 oncoprotein expression and c-MYC rearrangement. Polymorphic lymphoproliferations (PL), identified as precursors of NHL in HIV-patients, were also analysed in attempt to detect p53 modifications related to clonal progression.The functional assay detected p53 mutants in 40% (12/ 30) of the tumours: 50% (6/12) of BL/BLL, 40% (4/10) of diffuse large cell lymphomas (DLCL) and 25% (2/8) of PL. An oligoclonal or monoclonal population was identified in the two PL cases with mutant p53. An accumulation of the p53 protein was detected by IHC in 26% (8/30) of the tumours (five BL/BLL and three DLCL) and was associated with positive functional assay. In the 20 lesions tested by both of the screening methods for mutations, a p53 mutant pattern was detected in 55% of cases (11/20) and in 25% of cases (5/ 20) respectively with the functional assay and SSCP analysis of exons 5-8. There was no inverse correlation between the detection of EBV genome and the presence of p53 mutations and no overexpression of MDM-2 protein for the whole series. In conclusion, the functional assay was more sensitive than IHC and SSCP for the detection of p53 mutations in tumour samples. The mutations identified in AIDS-NHL lesions inactivate the p53 protein and in PL they could represent a selection of an aggressive clone.

It was recently demonstrated that classification of posttransplantation lymphoproliferative disorders (PT-LPDs) into morphological and molecular categories is clinically relevant. It was also reported that PT-LPD not associated with Epstein-Barr virus (EBV) had a more aggressive course than most lesions associated with EBV. Because the cyclin-dependent kinase inhibitor p16/INK4a has been reported to be frequently inactivated in high-grade lymphomas, we evaluated 17 PT-LPD to determine whether p16/ INK4a expression could be correlated to morphology, EBV detection, and a Ki-67 labeling index. We demonstrated that tumors with no p16/INK4a expression (n ‫؍‬ 8) had a predominantly monomorphic appearance, and most were EBV negative (respectively, 7/8 and 5/8), whereas lesions with p16/INK4a expression (n ‫؍‬ 9) were mostly polymorphic PT-LPD (6/9) (P ‫؍‬ 0.049) and associated with EBV (9/9) (P ‫؍‬ 0.015). In particular, strong p16/INK4a expression was observed in atypical immunoblasts and ReedSternberg-like cells. Furthermore, the proliferation index was significantly higher in tumors lacking p16/ INK4a expression than in other lesions (P ‫؍‬ 0.0008). Lymphoproliferative disorders (LPDs) are a major cause of morbidity in organ transplant recipients receiving immunosuppression, 1,2 varying between 1 and 12% of the graftees, for the most part according to the dose and duration of immunosuppressive therapy and the organ transplanted. 1,3-6 These tumors frequently involve extranodal sites and generally regress with reduction or withdrawal of immunosuppressive therapy, but some of them progress independently from the restoration of the immune system. It has been hypothesized that PT-LPDs arise from a polyclonal expansion of Epstein-Barr virus (EBV)-infected B cells and can evolve into a lymphoid tumor with the emergence of an oligoclonal or monoclonal dominant population. 7 PT-LPDs are usually classified into two subgroups, according to their appearance: polymorphic and monomorphic. 3 The latter closely resembles the diffuse large B-cell lymphoma (DLCL) of the Revised European American Lymphoma (REAL) classification. 8 Based on morphological and molecular analysis, Knowles et al 7 distinguished three main subgroups: 1) plasmacytic hyperplasia, 2) polymorphic B-cell hyperplasia and polymorphic B-cell lymphomas, and 3) malignant lymphoma or multiple myeloma, with the latter group being more frequently associated with genetic alterations, such as the inactivation of tumor suppressor gene p53 and the activation of c-myc and N-ras oncogenes. Moreover, Chadburn et al 9 recently showed the clinical relevance of this classification, as no response to aggressive clinical intervention was more frequently seen in patients with the lesions from the last group than those with the other tumors. Some patients with PT-LPDs that are not associated with EBV have a shorter survival time than those with EBV-positive lesions. 10 Expansion of B cells after organ transplantation should involve an excessive activation of the cell cycle, as in man...

To study the prevalence of p53 inactivation and MDM2/p21(WAFI/CIP1) expression in severe combined immunodeficient (SCID) mice Epstein-Barr virus (EBV)-induced lymphoproliferation, 19 samples obtained after ip injection of peripheral blood mononuclear cells (PBMCs) from EBV-seropositive donors or lymphoblastoid cell lines (LCL) were analyzed. In all samples tested, overexpression of Ki-67 antigen was shown by immunohistochemistry, indicating a high proliferative index of SCID mice EBV-induced lymphoproliferation. P53 mutations were screened by functional assay in yeast in 14 samples. With this test, a p53-inactivating mutation was found in only one case; the remaining cases exhibited a wild-type p53 pattern. However, an accumulation of p53 protein was detected by immunohistochemistry in six of 19 samples. P21 expression was found in seven of 19 samples but was not correlated with the rate of p53 protein in tumors. In contrast, high levels of nuclear accumulation of MDM2 were found in all samples by immunohistochemistry. These results suggest that a high Ki-67 proliferative index in SCID mice EBV-induced lymphoproliferation is not due to the inactivation of p53 by mutation, but could be associated with an overexpression of MDM2, which would act by a p53-independent mechanism.(J Histochem Cytochem 47:1315-1321, 1999)