CD2 EXPRESSION IN ACUTE PROMYELOCYTIC LEUKEMIA IS ASSOCIATED WITH MICROGRANULAR MORPHOLOGY (FAB M3V) BUT NOT WITH ANY PML GENE BREAKPOINT

In the t(15;17) translocation of acute promyelocytic leukemia (APL) at least three regions of the PML gene are involved in the reciprocal tr anslocation between the PML and the RAR-alpha loci. The chimeric PML/R AR-alpha fusion transcripts can be demonstrated in all cases of APL, b y a specific reverse-transcription PCR (PT-POP). Previous studies foun d a correlation between expression of CD2 and involvement of the PML b cr3. In this study, we assessed this association in 43 children and ad ults with APL. A blind morphologic review of all smears was performed by four experienced hemopathologists who agreed the diagnosis of M3 vs M3v APL. CD2 expression on APL was detected by using different monocl onal antibodies (MoAbs) directed against specific CD2 epitopes by flow cytometry and in selected cases by Northern blot by the use of a spec ific CD2 cDNA probe. Nineteen of 43 cases displayed the typical microg ranular features consistent with the diagnosis of M3v. Of these, 12 ha d the bcr3 breakpoint on chromosome 15, while seven had the bcr1 type. In 16 of the 19 patients, leukemic cells expressed both CD2 protein a nd the corresponding mRNA. Similarly, in the negative cases, Northern blot analysis failed to demonstrate the presence of specific mRNA. The remaining 24 patients, with the classic morphologic features of M3, w ere CD3 negative. These results point out that CD2 expression correlat es with the FAB M3v and not with the PML breakpoints. During the cours e of all-trans retinoic treatment a downmodulation of CD2 expression w as observed in three M3v cases. Overall, our findings might suggest a role of CD2 epitopes in the regulation of adhesion properties of APL b last cells.