Acute Promyelocytic Leukemia with T(15;17): Frequency of Additional Clonal Chromosome Abnormalities and FLT3

Acute promyelocytic leukemia (APL) is a distinct subtype of acute myeloid leukemia (AML) characterized by t(15;17)(q22;q21), a balanced reciprocal translocation involving PML and RARA genes. Studies conducted in animal models with PML/RARA suggest that this gene rearrangement is not sufficient for the development of APL, emphasizing the importance of additional mutations. Another gene that could be involved in APL leukemogenesis is FLT3, a Fms-like tyrosine kinase receptor which mutations have been observed in different AML subtypes as the internal tandem duplications (ITD), in around 18–37% of APL cases or less frequently, the D835Y mutation. The association of FLT3/ITD in APL gave rise to the proposition that it could provide a common class of cooperative mutation in the development of the disease. Indeed, APL represents 21% of all cases of AML with karyotype confirmation in our institution, located in São Paulo, Southeast region of Brazil. Bone marrow samples from 92 consecutive APL patients at diagnosis were enrolled in the study. Mean age was 29.5 years old, varying from 4 months to 67 years old, with the following age distribution: 35.7% of the patients were younger than 20 years old; 55.2% were between 20 and 49 years old and only 17.2% were older than 50 years. There were 50 males and 42 females. The WBC count demonstrated a wide variation, from severe leukopenia (<500 leukocytes/μL) until marked leukocytosis (174.600/μL), most of them presenting circulating blasts. Hemoglobin level varied from 3.7g/dL to 14g/dL and platelets counts were frequently low. Cases included five (5.4%) microgranular variant patients. We observed that 23 (25%) out of 92 cases presented chromosomal abnormalities in addition to t(15;17), among which trisomy 8 was the most frequent (21.7%, 5/23). Other abnormalities included tetraploidy, trisomy 6, trisomy 21 (non constitutional), nulissomy Y, ider(17q), trisomy 11, del(9)(q22q32), add(3)(p26), del(6)(p21), abnormalities of chromosome 7 and 1, t(1;3)(q21;q29); t(7;9)(q36;q31); monosomy of chromosomes 6, 7, 8 and 9; dup(1)(q23q42), del(10)(q24); add(7)(q36) and marker chromosomes. Seventy nine cases had enough DNA for complimentary molecular studies; in the remaining 13 cases we could not retrieve any DNA amount from fixed cells. Among those, FLT3/ITD was detected in 29 (36.7%), 5 of them showing additional karyotype abnormalities and D835Y mutation was present in only 2 (2.8%) of the cases. None of the two patients with D835Y showed additional chromosomal abnormality, but one of them presented ITD and D835Y mutation simultaneously at diagnosis. No correlation was found between age, sex, WBC count, hemoglobin level, platelet count or clinical symptoms and the presence or absence of additional karyotype abnormalities, FLT3-ITD and D835Y. The frequencies of additional karyotype abnormalities as well as FLT3/ITD in Brazilian APL patients were similar to those reported in several worldwide published papers. On the other hand, the incidence of D835Y was lower than that described elsewhere (2.8% versus 7% in AML or 5.7% in APL). So although Latin American population has an increased incidence of APL, it seems not to bear a higher frequency of clonal additional chromosome abnormalities or mutations in FLT3 gene. Moreover, FLT3/ITD was not over-represented in cases with additional chromosomal abnormalities (17.2% versus 36.7%, p=0,2), thus suggesting an independent role for each FLT3/ITD and extra clonal chromosomal abnormalities in t(15;17) APL. In the present study, a significant proportion of APL patients (56%) showed additional karyotype alterations and/or FLT3/ITD or D835Y mutation. In fact, one fourth of APL cases presented additional chromosomal abnormalities at diagnosis while 1/3 of the cases showed the FLT3/ITD and only two patients presented D835Y mutation. So, roughly more than one half of the cases presented at least one of the alterations investigated, raising the possibility that these abnormalities may be complimentary events to PML/RARA in the process of leukemogenesis. The influence of additional chromosomal abnormalities predisposing to genomic instability and cooperating with genetic mutations in acute leukemia still deserves investigation, since about 45% of APL cases have not been shown to bear any detectable molecular abnormality to date.