Abstract
Background: Cytogenetic data are essential not only for the diagnosis of acute myeloid leukemia but also for the evaluation of prognosis. Large systematic studies of cytogenetic aberrations in patients with acute myeloid leukaemia (AML) from Arab countries are not available.
Methods: We analysed 631 consecutive newly diagnosed AML patients by conventional cytogenetics and compared our results with reports from other regions of the world. There were 97 (15·4%) children and 534 (84·6%) adults.
Results: Abnormal karyotypes were found in 397 (62·9%) of all cases. T(15;17) and t(8;21) were the most frequent chromosomal abnormalities observed in 83 (13·2%) and in 78 (12·4%) patients, respectively. –5/del(5q) and −7/del(7q) were less frequent, seen in only 14 (2·2%) and 19 (3%) cases, respectively. Trisomy 8 was found in 44 (7%) of our patients followed by 11q23 rearrangements seen in 24 (3·8%) and then by inv(16) observed in only 22 (3·5%) of all cases. Unusual or novel cytogenetic abnormalities were found in 107 (17%) of our patients.
Discussion: Although we confirmed, as usually described, that some recurrent cytogenetic abnormalities are correlated with the FAB subtypes, we noted however that some of them vary in frequency among different geographical areas and ethnic groups. This finding suggests a geographic heterogeneity in the pathogenesis of AML but more extensive epidemiological studies are required to confirm this.
Introduction
Acute myeloid leukemia (AML) is a group of disorders with diverse morphological, immunophenotypic, cytogenetic and clinical features. Cytogenetic analysis is widely recognized as one of the most important prognostic determinants in AML.Citation1
Some chromosomal and molecular aberrations are found to be specifically associated with distinct morphological subtypes of the French-American-British (FAB) classification and thus, are important for revealing the underlying disease mechanisms.Citation2,Citation3 However, such a close relationship needs to be proven on a large sample series. Chromosome aberrations account for ∼52% of all de novo AML and a little higher in children.
Despite the substantial information on cytogenetic abnormalities in AML, the role of ethnic and geographic variability in cytogenetic abnormalities in patients with acute leukemia has been elucidated in only a few published studies.Citation4–Citation7 Such data will be crucial in determining the influence of ethnic and environmental factors in AML-associated genetic abnormalities. Our aim was to determine the frequencies and subtypes of chromosomal abnormalities among AML patients in the Tunisian population and to compare the cytogenetic profile with published data from other regions of the world.
This study is the largest consecutive series of AML cases reported in Tunisia.
Materials and methods
Patients
This study comprised 656 consecutive ethnic Tunisian patients with de novo AML during the 7-year period from January 2000 to December 2007. At the time of diagnosis, their bone marrow (BM) cells were sent to our cytogenetic laboratory at Farhat Hached University Hospital, Sousse (Tunisia) from the divisions of hematology of the university hospitals of Sousse, Tunis and Sfax (Tunisia).
Twenty five patients did not have adequate metaphase cells for analysis (4%). The remaining 631 patients were investigated in this study. Of these, 97 (15·4%) were children (⩽15 years) and 534 (84·6%) were adults. Morphological diagnosis and classification of AML were performed according to the French-American-British (FAB) criteria.Citation8
Cytogenetic investigation
Cytogenetic studies were performed using standard methods for preparations and R-Banding as previously described.Citation9 Chromosome identification and classification of chromosomal abnormalities were made according to the International System for Human Cytogenetic Nomenclature (ISCN).Citation10
At least 20 metaphases were analyzed after 24-hour and/or 48-hour bone marrow cell culture.
Clonal abnormalities were defined as two or more cells with the same whole chromosome gain or chromosome rearrangement, or three or more cells with the same chromosome loss.
Cases were grouped using the hierarchy: t(8;21)(q22;q22), t(15;17)(q22;q12), inv(16)(p13q22), or abnormal 16q22, –5/del(5q), −7/del(7q), trisomy 8, 11q23 rearrangements, other structural changes, and other numerical changes.
Chromosomal abnormalities were also studied according to ploidy (diploidy, pseudodiploidy, hypodiploidy, hyperdiploidy and tetraploidy); cases with a normal karyotype and with a complex aberrant karyotype (defined as three or more clonal abnormalities) represented their own categories.
Results
Patient data and morphology
Of the 631 patients, 353 were male (56%) and 278 were female (44%), with a median age of 37 years (range 8 days to 95 years). Ninety-seven (15·4%) of these patients were children (⩽15 years), and the remaining 534 (84·6%) were adults. Among the FAB types, M2 was the most frequent accounting for 31·2% and the M7 subtype was the least frequent with only 2·7%.
Cytogenetic analysis
Clonal abnormalities were observed in 397 of the 631 patients (62·9%): 51% of adults and 11·9% of children (). According to ploidy, 234 patients (37·1%) showed a diploid karyotype, 250 (39·6%) were considered as pseudodiploid, 55 (8·7%) as hypodiploid, 90 (14·3%) as hyperdiploid and 2 (0·3%) had a tetraploid karyotype (both were adults).
Table 1. Classification of AML patients within age groups by sex, ploidy, complexity and chromosome abnormalities
Complex karyotypes were detected in 68 (10·8%), 56 (8·8%) were adults and the 12 (2%) remaining were children.
The most frequent cytogenetic abnormality was t(15;17) (13·2%), followed by t(8;21) (12·2%), trisomy 8 (7%), 11q23 rearrangements (3·8%), inv(16) (3·5%), −7/del(7q) (3%), and −5/del(5q) in only 2·2% of all cases. These frequencies are distributed between children and adults and classified as sole or in combination with other anomalies as shown in and , respectively. Some rare abnormalities were also observed in less than 2% of all AML cases such as del(12)(p13), del(9)(q21q31), t(6;9)(p23;q34), inv(3)(q21;q26), i(17q), i(21q) and trisomy 21. Interestingly, several unusual or novel abnormalities were observed in 107 (17%) of our patients ().
Table 2. Distribution of sole and combination anomalies among specific karyotypes
Table 3. Rare and novel abnormalities observed in this study
Specific translocations were found associated with FAB type: t(8·21) with AML2 in 80·5% (62/77), t(15;17) with AML3 in 92·7% (77/83) and inv(16) with AML4 in 90·9% (20/22).
Discussion
To the best of our knowledge, this study presents the largest consecutive series of AML from an Arab country in which we report the cytogenetic results from 631 patients as an ethnic Tunisian population and compare our findings with previous reports. The results of this study compared with several population-based and regional studies are detailed in .
Table 4. Comparison of the karyotype patterns of the Tunisian AML patients with others from different geographic areas
The median age at presentation in our cohort was 37 years, which is significantly younger than for other populations which ranged from 58 to 63 years, especially from Europe and north America but it is older than the 25 years found in the Omani population.Citation3,Citation11
Among the FAB types, M2 was the most frequent subtype found among our cohort and accounted for 31·2%, comparable with that reported from China (29·9%),Citation3 Morocco (37%),Citation12 Germany (39·6%),Citation2 America (37%),Citation13 Singapore (32%),Citation7 and Oman (35%).Citation11
The incidence of M3 accounted for 17·1% in our cohort which is comparable to that reported by the Eastern countries such as SingaporeCitation7 and OmanCitation11 (14%) but lower than the 25·3% observed among the Chinese population.Citation3 However, this incidence was higher than that in Germany (5%)Citation2 and America (2%).Citation13
In contrast, the frequency of M4 was lower in our study with only 7·9% compared with 23%, 20·4%, and 17% reported from America,Citation13 GermanyCitation2 and Oman,Citation11 respectively.
Interestingly, the M5 subtype was higher at 18·5% of our patients compared with that previously reported from America (12%),Citation13 Singapore (7·5%),Citation7 Germany (7·3%),Citation2 and in Oman (6%),Citation11 but comparable to the 23·3% found among Chinese patients.Citation3
The reasons for the difference between age at presentation in subjects, and between the frequencies of different FAB subtypes from the various regions are not clear but may be explained the geographic/ethnic difference. In our cohort, 84·6% were adults and 15·4% of our patients were children compared to 29% among Omani patients reported recently by Udayakumar et al.Citation11
Although the disease is evenly distributed between males and females among children (47·4% vs 52·5%), we note a predominance of the disease in males among adults (56·7% vs 43·2%) and this was in agreement with previous reports.Citation6,Citation7,Citation14
Cytogenetic abnormalities were detected in 62·9% of the cases, compared with that previously reported in the literature ranging from 52% to 80%Citation7,Citation11,Citation12,Citation15 and closer to the 61% and 62% reported from Singapore and Oman,Citation11 respectively. However, this incidence is slightly higher than the 55% previously reported by Sendi et al.Citation16 among Tunisian patients and this can be explained by the smaller cohort analyzed with only 139 patients.
In our study, M3 with t(15;17) is the most prevalent aberration found in 13·2% of patients. This was in accordance with what was found in Singapore,Citation7 China,Citation3 Spain,Citation14 and Austria as shown in .Citation6 However, reports from AmericaCitation13 and UKCitation17 found this translocation in only 7% and 8%, respectively although it also represents the most frequent anomaly. Thus, the overall frequency of t(15;17) appears to be higher among Tunisian and Asian subjects and this may be explained by the inclusion of children in these studies.
Geographical differences have also been reported for t(8;21), which is reported as the most common aberration in the Japanese AML population with 13·2%. In our cohort, this translocation was found in 12·2% of patients. This is closer to that observed in Oman (11%),Citation11 but higher than that reported from other populations as shown in , for example, from China (8·3%),Citation3 America (6%)Citation13 and Spain (2·7%).Citation14
In our study, t(8;21) was found as the sole abnormality in 42·8% (33/77) of cases and in combination with secondary abnormalities in 57·2% (44/77) of cases. As usually described, this translocation is found associated with deletion of sexual chromosomes in 35% (27/77) of cases, or with del(9p) in 13% (10/77), or to the trisomy 8 in only one case (1·2%; 1/77). This was in accordance with what is reported in the literatureCitation19 with 50%, 11% and 7·5%, respectively.
We found inv(16) in 3·8% of patients, consistent with the results from previous studies (ranging from 2% to 8%).Citation4,Citation5,Citation7,Citation19,Citation20
The most frequent numerical abnormality was the gain of chromosome 8 occurring in 7% of our patients. This was comparable with earlier reports from other geographic regionsCitation2,Citation7,Citation11,Citation18 but lower than that found among the Spanish population with 11·4%.Citation14 Reports from AmericaCitation13 and GermanyCitation2 showed that the trisomy 8 was the most prevalent aberration observed in 9% and 6·7%, respectively.
The abnormalities −5/del(5q) and −7/del(7q) were seen in 2·2% and 3% of our cases comparable to that in GermanyCitation2 and in China,Citation3 whereas most studies report a frequency of 4–10% of elderly de novo AMLCitation7,Citation13,Citation14,Citation18,Citation19 and 11% was reported from Morocco.Citation12
11q23 occurred in 3·8% of our subjects and is similar to earlier studies where it occurred in less than 5% of adult AML patients,Citation3,Citation4,Citation21,Citation22 but lower than the 6% found in Morocco.Citation12
Several rare or novel abnormalities were seen in 107 (17%) of our patients as shown in . Further investigations may lead to the discovery of novel putative genes which can be implicated in the oncogenic transformation.
Finally, we confirmed the relationship between some specific abnormalities and the FAB classification usually reported in the literature. Indeed, we found t(8;21) with AML2 in 80·5%, t(15;17) with AML3 in 92·7% and inv(16) with AML4 in 90·9%.
In conclusion, this study describes in detail the pattern of cytogenetics in a large series of AML patients in Tunisia. This is the largest cohort reported in the Tunisian population in which we found some differences in prevalence of chromosomal aberration when compared with other reports from different regions. However, we noted that our findings are closer to those found in Eastern countries than to those reported in Western populations. The differences mentioned above may be because of environmental and ethnic factors.
Thus, more epidemiological studies are needed involving different ethnic populations and geographic regions of the world and these should help unfold the true nature of the environmental and genetic interplay in the development of AML.
We are grateful to the physicians of the three institutions who collaborated with us in this work. The authors thank Mouna Zarrad, Zeineb Mrad, Sondes Meksi, and Rym Khattat for excellent technical assistance in the cytogenetic experiments.
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