Acute myeloid leukemia with t(7;21)(p22;q22) and 5q deletion: a case report and literature review

The gene RUNX1 at chromosome 21q22 encodes the alpha subunit of Core binding factor (CBF), a heterodimeric transcription factor involved in the development of normal hematopoiesis. Translocations of RUNX1 are seen in several types of leukemia with at least 21 identified partner genes. The cryptic t(7;21)(p22;q22) rearrangement involving the USP42 gene appears to be a specific and recurrent cytogenetic abnormality. Eight of the 9 cases identified in the literature with this translocation were associated with acute myeloid leukemia (AML), with the remaining case showing refractory anemia with excess blasts, type 2. Herein, we present a patient with two preceding years of leukopenia and one year of anemia prior to the diagnosis of AML, NOS with monocytic differentiation (myelomonocytic leukemia) whose conventional cytogenetics showed an abnormal clone with 5q deletion. Interphase FISH using LSI RUNX1/RUNXT1 showed three signals for RUNX1. FISH studies on previously G-banded metaphases showed the extra RUNX1 signal on the short arm of chromosome 7. Further characterization using the subtelomeric 7p probe showed a cryptic 7;21 translocation. Our case and eight previously reported leukemic cases with the t(7;21)(p22;q22) appear to share similar features including monocytic differentiation, immunophenotypic aberrancies (often with CD56 and/or CD7), and a generally poor response to standard induction chemotherapy. About 80% of these cases had loss of 5q material as an additional abnormality at initial diagnosis or relapse. These findings suggest that t(7;21) may represent a distinct recurrent cytogenetic abnormality associated with AML. The association between the t(7;21) and 5q aberrancies appears to be non-random, however the pathogenetic connection remains unclear. Additional studies to evaluate for RUNX1 partner genes may be considered for AML patients with RUNX1 rearrangement and 5q abnormalities; however knowledge of the prognostic implications of this rearrangement is still limited.


Introduction
Gene fusion and chromosomal abnormalities play vital roles in tumorigenesis, including hematologic malignancies [1]. More than 100 fusion genes have been reported in acute myeloid leukemia (AML) [2]. Many of these fusion genes involve the translocation of Runt-related transcription factor 1 (RUNX1), also known as acute myeloid leukemia 1 protein (AML1) [3]. The gene RUNX1 at chromosome 21q22 encodes the alpha subunit of core binding factor (CBF), a heterodimeric transcription factor involved in the development of normal hematopoiesis. At least 21 partner genes of RUNX1 have been identi-fied. Translocations of RUNX1 have been reported in acute lymphoblastic leukemia (ALL) [4,5], acute myeloid leukemia (AML), and myelodysplastic syndromes (MDS). In AML, the t(8;21)(q22;q22) is the most frequent translocation involving RUNX1. In contrast, the much rare t(7;21)(p22;q22) involving a rearrangement with the USP42 gene, is cryptic, but appears to be a specific and recurrent cytogenetic abnormality.
The deletion of all or part of long arm of chromosome 5 (del(5q)) is a recurrent abnormality in AML and MDS patients. While the association between del(5q) and AML are still unclear, a deletion of 1.5 Mb region at 5q31.1 flanked by genes IL-9 and EGR-1 has been reported to be related with AML [6]. In addition, del(5q) is mostly part of the complex karyotype in many AML cases, which makes it more difficult to reveal its significance.
In this study, we describe a new AML case with a cryptic t(7;21)(p22;q22) and chromosome 5q abnormality. The case reported here shares the common features with the few other cases reported in the literature, suggesting that the t(7;21)(p22;q22) may represent a distinct entity in AML.

Clinical presentation
The patient is a 57-year-old female who was referred for hematology consult after presenting with a two-year history of chronic leukopenia with progression to include anemia of several months duration. A bone marrow biopsy was performed, revealing approximately 24% myeloblasts by immunohistochemistry, with the blasts expressing monocytic differentiation with aberrant CD56 expression by flow cytometry. She was diagnosed with AML, and was referred to an academic center for inpatient chemotherapy and possible transplant. Following evaluation, she was placed on Revlimid therapy as opposed to induction chemotherapy for about 5 months without improvement or significant deterioration of her blood counts. She was referred to our institution to be evaluated for allogenic stem-cell transplantation. A repeat bone marrow biopsy (about 6-7 months post diagnosis) confirmed persistent AML, and the patient was started on 7 + 3 AML induction (Cytarabine 320 mg IV continuous days 1-7, Idarubicin 19 mg IV on days 3-6). A day 16 repeat bone marrow biopsy showed persistent presence of abnormal myeloblasts. Biopsy about 6 weeks following induction therapy showed remission with no excess or abnormal myeloblasts. She completed consolidation chemotherapy with high-dose cytarabine (3 gm/m2 IV q 12 hours on days 1, 3, 5 for 6 doses), and recently had a successful allogenic stem-cell transplant.

Histologic methods
Particle smears and touch preparations were prepared upon receipt of specimen, air dried, and stained with Wright-Giemsa. Trephine biopsy and aspirate clot sections were fixed in B5 and buffered formalin, respectively. Following fixation, tissues were paraffin-embedded, and 4-μm thick sections were cut and stained with hematoxylineosin.

Flow cytometry
A WBC suspension is created from the bone marrow aspirate sample using bulk lysing with BD Pharmlyse (ammonium chloride base) and PBS wash. 6-Color custom cocktails (FITC/PE/PerCPCy5.5/APC/APCH7/V450 fluoro chrome format) for immunophenotyping were manufactured by the CTT group of Becton Dickinson. The antibody staining and fixation include a 15 minute incubation in the dark, PBS washing, and resuspension & fixation with 1% paraformaldehyde. The Becton Dickinson FACSCanto II (3-laser) flow cytometer with BD FACSDiva Software were used for event acquisition, and FCS Express Version 3.00.0821 (DeNovo Software, Los Angeles, CA) was used for analysis.

Cytogenetic studies
Conventional chromosome analysis was performed on bone marrow cells using standard cytogenetic techniques and G-banding. A total of 20 metaphase cells were analyzed. Karyotypes were prepared using Applied Imaging CytoVision software and described according to the guidelines of International System for Human Cytogenetic Nomenclature (ISCN, 2013) [7].

Cytogenetic analysis
Chromosome analysis of the bone marrow revealed additional material of unknown origin at 5q13. This unbalanced rearrangement characterized as "add" (ISCN, 2013) led to partial deletion of 5q in 5 of the 20 metaphase cells examined. The initial karyotype was described as: 46,XX,add(5) (q13) [5]/46,XX [15] (Figure 4a). This finding was confirmed by the concurrent interphase FISH study (Figure 4b).
One of the genes located on chromosome 5q is ribosomal protein S14 (RPS14), which is essential for the maintenance of normal erythroid progenitor cells [10]. Loss of chromosome 5q causes haploinsufficiency of RPS14 [18]. Boultwood et al. [19] hypothesized that this haploinsufficiency may be related with failure of erythropoiesis. In our case report, the patient had a history of anemia for one year, and the karyotype showed loss of chromosome 5q. We speculate that the anemia in this patient is related with haploinsufficiency of RPS14, however, this speculation would need to be validated by further molecular testing.
As previously noted, deletion of 5q is a common cytogenetic abnormalities in 8 of the 9 patients with t(7;21) (p22;q22). The high incidence of t(7;21)(p22;q22) found together with del(5q) suggest that this combination is a non-random abnormality. Many questions need to be answered, including: Why does this combination happen with such a high frequency? What is the etiology? What is the significance of these aberrancies in relation to AML? Although a recent large scale cooperative study of 514 MDS patients showed that 5q deletion with additional cytogenetic abnormalities is a marker of poor prognosis [20], additional study is needed for this combination in AML. While we cannot provide further experimental evidence, we think one possibility is that two (or more) functionally redundant genes were disrupted by the t(7;21)(p22;q22) and del(5q) respectively. Subsequently, the affected cells would have lost important    (7) is missing a green signal. Two normal chromosomes 14 are seen as indicated by the signal pattern (one yellow and one aqua signals on two normal chromosomes 14 respectively). functions associated with drug resistance, apoptosis, and/ or DNA repair. All previous reports about the t(7;21)(p22;q22) rearrangement have detected a RUNX1-USP42 fusion protein. It is reasonable to assume that our case also has the associated RUNX1-USP42 fusion protein, although we did not pursue the exact partner of RUNX1 in this report due to the limited specimen. RUNX1 is a vital regulator of hemopoiesis [21]. Various hemopoietic signaling molecules, growth factors, and transcription factors are regulated by RUNX1 [22]. RUNX1 translocation is a recurrent feature in many AML and MDS patients, and numerous translocation partners have been described. RUNX1 rearrangment's leukemogenicity lies in the retention of RUNX1's DNA binding "Runt homology domain" (RHD) domain, and associated loss of its transactivation domain. The resultant fusion protein acts as a dominant negative inhibitor of wild-type RUNX1 in transcription activation [11,22].
While most studies focus on RUNX1, ubiquitin specific peptidase 42 (USP42) may also play an important leukemogenic role in t(7;21)(p22;q22) cases. USP42, belonging to the ubiquitin-proteasome system (UPS), is a deubiquitinating enzyme (DUB). The process of ubiquitination and deubiquitination decides the fate of many proteins, many of which are related with cell cycle regulation, DNA repair, and differentiation. USP and the DUBs contain many potential drug targets for the treatment. USP42 itself was reported to increase the stability of p53 [23]. We propose that RUNX1-USP42 fusion protein loses wild type USP42 function and decreases the stability of p53. Further studies to characterize the involvement of USP42 in our case were not possible due to insufficient sample.
In conclusion, we report a new case of AML with a cryptic t(7;21)(p22;q22) and chromosome 5q abnormality. To the best of our knowledge, this represents one of the few AML patients with this genetic aberrancy. Review of the literature showed that our case shares many common features with other reported cases, which suggests that t(7;21)(p22;q22) may represent a new distinct entity in AML.

Consent
Consent for publication of this case report and any accompanying images was obtained from the patient.