Chronic leukemic B-cell disorders and trisomy 12 : a study of surface markers, protein expression and clinical course

Abstract

Chronic lymphocytic leukemia of B-cell type (CLL) is the most common disease entity of chronic leukemic B-cell disorders (CLBD). CLL is the most frequent leukemia in the Western world and the incidence is increasing. The clinical course is very variable and specific therapy is not always needed. Therefore, valid prognostic markers are of great importance managing the individual patient. Disease stage is determined in most cases and has a prognostic value. Chromosomal abnormalities have also been evaluated as prognostic markers in CLDB. The most common numerical chromosomal aberration is trisomy 12. The purpose of this thesis was to use fluorescence in situ hybridization (FISH) to detect trisomy 12 in CLBD and to characterize the patients with trisomy 12 from a biological, clinical, and prognostic perspective.

Totally 156 patients with CLBD were evaluated for the occurrence of trisomy 12. The findings were correlated with the morphology of the leukemic cells and the surface expression of CD25 (IL2- receptor), CD54 (ICAM-1), CD95 (Apo 1) and with the intracellular murine double minute (MDM2) protein. Repeat FISH analyses were performed to study the leukemic clone during the course of the disease.

Trisomy 12 was detected in 2-54% of the blood mononuclear cells (MNC) in 18/118 (15%) of the CLBD patients. The aberration was found in 24% of patients with tumor cells with atypical morphology including lymphoplasmacytoid features, while only 2% of the cases with a typical morphology had trisomy 12. The trisomic patients had higher S-LDH, expressed a monoclonal band more frequently, had shorter median time to treatment and reduced survival compared with the disomic cases.

Analyses of the occurrence of trisomy 12 in different morphologically defined lymphoid cells by combined conventional cell staining and FISH (MGG/FISH), showed that the aberration was as frequent in morphologically atypical as in typical cells.

By sequential FISH analyses, we found that acquisition of trisomy 12 during the course of the disease was a rare event (2/60). In all cases with trisomy 12 (n=17), the trisomic clone expanded as the disease progressed. Therapy with alkylators and combination regimens significantly reduced the trisomic clone. In contrast, after treatment with purine analogs a relative increase of the trisomic clone was observed.

Using flow cytometry, CLL patients had a higher frequency of B-cells expressing CD25 than controls. High expression of CD25 (>30% of the B-cells) was associated with a significantly shorter median time to treatment.

CLL patients had a significantly higher percentage of mononuclear cells (MNC) expressing the MDM2 protein than controls. Patients with trisomy 12 had significantly more MDM2 expressing cells than the disomic cases. This extra copy of chromosome 12 can not be the only explanation for the difference, since most cases had more MDM2 expressing cells than trisomic MNC.

Even though trisomy 12 was associated with several of the studied biological and clinical factors neither appeared specific for CLBD patients with trisomy 12.