Frequency and Dynamics of Bcr1/Bcr3 Isoforms of PML/RARA Fusion Gene - Results of Russian APL Multicenter Trial.

Detection and monitoring of minimal residual disease (MRD) in APL patients has become a routine practice and is essentially important for tailoring the therapeutical intervention. Two major isoforms of PML-RARa fusion gene (bcr1/2 and bcr3) are considered to determine diverse outcome with bcr3 to be less favorable. It was also shown by different study groups that among different ethnic populations the ratio of bcr1/2 and bcr3 varies. The prevalence of bcr1/2 isoform over bcr3 was reported in Latinos and Chinese population: 84% vs 16% and 73% vs 27%, respectively. Less evident but nevertheless convincing tendency was registered in USA (US Intergroup – 63% vs 37%; Memorial Sloan Kettering CRC – 66% vs 34%), Great Britain (MRC – 61% vs 39%), Italy (GIMEMA – 60% vs 40%), Spain (PETHEMA – 56% vs 44%) (Santillana S. et al Joint International Congress on APL, Rome, 2001, abstr P2.14; Dan Douer et al BJH, 2003, p.563–570). The analysis of PML-RARa transcript in 66 APL patients included in the Russian Multicenter trial revealed the inverse (statistically significant from mentioned above) ratio with 37 patients (56%) bearing bcr3 isoform and 29 (44%) – bcr1/2. There is no exact explanation for these findings but it may be due to low numbers of cases and perhaps to certain environmental and/or ethnic reasons. The treatment results in these 66 APL patients are very similar to usually reported with 7+3+ATRA protocols – 91% CR rate with 75% molecular CR after the first and 96% - after the second course. The OS and DFS do not differ according to the isoform and constitute 77% and 80% at 3 years, respectively. It is worth to stress that close monitoring of MRD (3–15 probes per patient) revealed different time adjusted dynamics in the molecular relapse incidence for bcr1/2 and bcr3 isoform. It was equal at each month during the whole period of follow-up (36 mo from treatment start) for bcr1/2 patients, and clustered in two periods (3–12 mo) and (20–36 mo) for bcr3 patients with 8 months of absence of positive signal. So we may suppose that each isoform of PML-RARa fusion gene determines its distinctive course of MRD while standard APL treatment. This data once again shows discrepancies in the biological features of APL with bcr1/2 and bcr3 isoforms.