INTRODUCTION
The t(8;21)(q22;q22) is a frequently occurring aberration in acute myeloid leukemia (AML) (18-20%), and usually correlates with the French-America-British (FAB) M2 subtype (Shinagawa et al. 1999). The translocation involves the RUNX1 gene on chromosome 21q22 and the RUNX1T1 gene on chromosome 8q22. The translocation resulted in fusion of transcripts RUNX1/RUNX1T1 on the derivative of chromosome 8. It is reported that patients with AML carrying the t(8;21)(q22;q22) exhibit good response to chemotherapy with a high remission rate and long-term disease-free survival. However, it may be associated with a high incidence of early relapse in some cases (Shinagawa et al. 1999). Trisomy 4 is a recurrent but rare chromosome abnormality which appears to occur in a broad range of hematologic malignancies, mainly cases of AML with FAB M2 and M4 subtypes, and some cases with M1 and M5 subtypes (Kwong et al. 1993; Shinagawa et al. 1999; Beghini et al. 2000). Trisomy 4 could also be secondary to an environmental mutagen or drug exposure (Shinagawa et al. 1999; Nishii et al. 2003). It may also present with a sole cytogenetic abnormality or concomitantly with other aberrations. The significance of its occurrence is still undefined.
CASE REPORT
We report a case of a 33-year-old female presenting with fever and anemia. Her full blood picture (FBP) showed pancytopenia. The morphology of the circulating blasts showed presence of Auer rods with hypergranulation. Full blood count showed a white blood cell (WBC) count of 2.3 × 109 /L, with a hemoglobin (Hb) concentration of 6.4 g/L and a platelet count of 37× 109 /L. There was no history of long term medication and toxic chemical exposure. She was suspected as having AML, FAB M3 (APL). However, the bone marrow aspirate (BMA) was reported as only AML. BMA was sent for cytogenetic analysis to confirm the diagnosis. A trans-retinoic acid (ATRA) regimen was given to her as the initial treatment. She died soon after the diagnosis.
CYTOGENETICS ANALYSIS
Chromosome analysis of bone marrow aspirate was performed using the standard cytogenetic protocol. The karyotype was described according to the International System for Human Cytogenetic Nomenclature (Shaffer & Tommerup 2005). Thirteen metaphases were analysed. Cytogenetic analysis demonstrated 46,XX,+4, t(8;21)(q22;q22) (Figure 1). This finding excluded the diagnosis of AML M3, but indicated that the patient is having AML with trisomy 4 in addition to t(8;21).
DISCUSSION
The t(8;21)(q22;q22) AML present at diagnosis is associated with a better prognosis and generally demonstrated prolonged disease-free survival (Nishii et al. 2003). AML with the t(8;21) is frequently associated with a loss of sex chromosome Y in males and an inactive X in females, and 3.4% of the cases are variant translocations (Shinagawa et al. 1999; Nishii et al. 2003). A case of FAB M2 subtype with anomalies, complex translocation of t(6;21;8) and trisomy 4 have been reported. However, the presence of trisomy 4 has not demonstrated any influence in clinical prognosis (Shinagawa et al. 1999).
Aberrations in chromosome 7 and chromosome 8 are reported to be associated with secondary AML or during clonal progression of AML (Beghini et al. 2000; Trivedi et al. 2008). Trisomy 4 is rarely reported as the abnormality in AML (<1%) (Trivedi et al. 2008). Nishii et al. (2003) showed that t(8;21) AML with trisomy 4 is very rare. Out of 94 cases, only three were found to have concomitant aberrations of t(8;21) with trisomy 4 (3.2%) (Nishii et al. 2003). t(8;21) AML with trisomy 4 has a poor prognosis and was speculated that the presence of KIT mutation may be responsible for the adverse effect of the disease (Langabeer et al. 2003; Trivedi et al. 2008). Few studies have demonstrated that patients with the t(8;21) and a trisomy 4 showed a significantly high frequency ofKIT mutation (10%) than patients with normal and complex aberrant karyotype (Shimada et al. 2006). Patients with activation of KIT gene have been shown to confer drug resistance, which could explain the relatively poor prognosis (Beghini et al. 2000; Langabeer et al. 2003; Shimada et al. 2006). KIT is localized at chromosome 4q12, thus trisomy 4 leads to an increased gene dosage of KIT. It is believed that the occurrence of t(8;21) represents the initial step to initiate the disease, followed by activation of KIT pathway that promotes disease progression (Beghini et al. 2000; Shimada et al. 2006).
In our case, the patient apparently had clinical and hematological features suggestive of acute promyelocytic leukemia (APML) in whom the cytogenetic analysis revealed the presence of trisomy 4 with an additional t(8;21) hence, excluding the diagnosis of APML. Inclusion of cytogenetic analysis in addition to clinical and morphological criteria will produce a reliable clinical diagnosis of the disease. This is because of variable morphology due to the presence of complex or additional chromosomal alteration, also the multiple genetic events which may affect the various maturation stages of cells. Thus, cytogenetic analysis is one of the useful tools to confirm the clinical diagnosis. Detection of the t(8;21) can be further investigated with florescence in situ hybridization (FISH) analysis to determine the AML1 gene rearrangement. Other diagnostic tests include immunophenotyping which may help to identify the subtype of t(8;21) AML.
A study by Nishii et al. (2003) reported that most of the cells carrying trisomy 4 showed low expressions of CD19 and IL-17 receptor, and high expressions of CD33, CD18, and CD 56 when compared to t(8;21) AML without other additional chromosomal abnormality (Nishii et al. 2003). CD56 expression in AML is reported in multidrug resistance (Trivedi et al. 2008). There was no flow cytometric analysis done in our case. Therefore, we cannot explain the relation between CD56 expression and trisomy 4 in t(8;21) AML.
In conclusion, the present case of AML FAB M2 is a rare case in terms of cytogenetic findings. Concomitant t(8;21) with trisomy 4 may constitute a distinctive subtype of t(8;21) AML (Nishii et al. 2003). Hence the acquisition of trisomy 4 in AML with AML1 rearrangement might promote disease progression and an unfavourable prognosis. This case illustrated the essential roles of cytogenetics studies in confirming the diagnosis. Cytogenetics analysis provides simultaneous analysis of genetic rearrangements within leukemic cell. Meanwhile FISH analysis approached is a complementary test for conventional cytogenetics that helps to determine the chromosomal rearrangements such as t (8;21), t(15;17) and inv(16) in AML cases.
ACKNOWLEDGMENTS
We would like to thank the Director General of Health Malaysia for permission to publish this paper. We would like to acknowledge the Director of Ampang Hospital, Datin Dr Aishah bt Ahmad Mukinuddin, members of IMR’s genetic laboratory for their technical assistance and staff of the Haematology Department, Hospital Ampang for their support.