Cancer Genetics and Cytogenetics 162 (2005) 122–126

3# CBFb deletion associated with inv(16) in acute myeloid leukemia

Johanna Kellya,1, Nicola J. Footb,1, Eibhlin Conneallyc, Helen Enrightd,Mervyn Humphreyse, Karen Saundersf, Michael J. Neata,*

aNational Centre for Medical Genetics, Our Lady’s Hospital for Sick Children, Crumlin, Dublin 12, Republic of IrelandbMedical Oncology Unit, John Vane Science Building, Queen Mary and Westfield College, Charterhouse Square, London, England

cDepartment of Haematology, St James’s Hospital, Dublin, Republic of IrelanddDepartment of Haematology, The Adelaide and Meath Hospital, Tallaght, Dublin 24, Republic of Ireland

eRegional Genetics Centre, Floor A, Belfast City Hospital, Lisburn Road, Belfast, Northern IrelandfLeukaemia Science Unit, The Rayne Institute, 123 Coldharbour Lane, London, England

Received 18 October 2004; received in revised form 21 February 2005; accepted 3 March 2005

Abstract Recent reports have shown that concomitant submicroscopic deletions can occur in association withchromosomal translocations/inversions in several leukemia subtypes. Detectable by fluorescence insitu hybridization (FISH), these losses of sequence include deletion of the 5# region of the ABL geneand the 3# region of BCR in chronic myeloid leukemia (CML) and acute lymphoblastic leukemia(ALL), as well as the 5# region of ETO in acute myeloid leukemia (AML) French–American–British type M2 associated with t(8;21), 3# MLL in AML and ALL, and 3# core-binding factorb (CBFb) in AML associated with inv(16). While it has been widely reported that submicroscopicdeletions of the derivative 9 in CML have an adverse prognostic impact, the clinical significance, ifany, of deletions associated with t(8;21), inv(16)/t(16;16), or MLL rearrangement is yet to bedetermined. We analyzed a series of 39 patients diagnosed with AML who had cytogeneticallydetectable inv(16)/t(16;16) by using a FISH probe for the CBFb region to determine the incidenceof the 3# CBFb deletion. Deletions were detected in three patients (8%), all associated with inv(16),bringing the number of cases reported so far to seven. The prognostic significance of this findingremains unclear. � 2005 Elsevier Inc. All rights reserved.

1. Introduction

The cytogenetic rearrangements inv(16) and t(16;16)have a well-established association with the acute myeloidleukemia (AML) subtype M4EO [1] although they havebeen detected in a number of other French–American–British (FAB) types [2]. These rearrangements have anoverall incidence of approximately 4% in patients with denovo AML [3,4] and 11% in secondary AML [4]. The useof polymerase chain reaction (PCR) has reported a higheroverall incidence of approximately 10% [2], while otherstudies report an incidence much closer to that seen byconventional cytogenetics [5,6]. The result of theseabnormalities of chromosome 16 is the fusion of themyosin heavy chain gene (MYH11) at 16p13 and core-binding factor b (CBFb) at 16q22 [7,8], with the

1 Both authors contributed equally to this work.

* Corresponding author. Tel.: 1353-1-409-6970; fax: 1353-1-456-

0953.

E-mail address: mneat@olhsc.ie (M.J. Neat).

0165-4608/05/$ – see front matter � 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.cancergencyto.2005.03.001

transcriptionally active fusion gene comprising 5#-CBFb/MYH11-3# [9,10].

AML with the CBFb/MYH11 rearrangement has beenassociated with a high rate of complete remission (CR) anda favorable prognosis when compared to other AMLsubtypes [3,11–13]. These patients have also been shownto benefit from post-remission intensification with high-dose cytarabine [13–15], highlighting the importance ofcytogenetic and molecular detection of such abnormalitiesupon initial diagnosis.

It has been reported that increasing numbers ofapparently balanced recurrent disease-specific cytogeneticabnormalities are in some cases associated with concom-itant submicroscopic deletions [9,10,16–30]. These dele-tions may have an adverse prognostic impact, as has beensuggested in cases of Philadelphia-positive (CML) carryingdeletions of the derivative 9 [17–21,25,32], although thisprognostic association has been disputed recently [31]. Theimpact, if any, of other submicroscopic deletions remains tobe determined [25,26]. In AML with inv(16)/t(16;16), it iswell established that deletions occur proximally to MYH11

123J. Kelly et al. / Cancer Genetics and Cytogenetics 162 (2005) 122–126

[9,10] which involve deletion of the multidrug resistance–associated protein gene (MRP1), but it would seem thatthey do not carry prognostic significance [22,23]. Threerecent reports have described four cases of deletion of 3#CBFb associated with inv(16) in patients with AML M4(one case) [24] or M4EO (three cases) [25,26] detected byfluorescence in situ hybridization (FISH). In this study,FISH data were collected from 39 patients with inv(16)/t(16;16) to assess the incidence of the 3# CBFb deletion.

2. Materials and methods

2.1. Patients

3# CBFb FISH analysis data were collected from 39patients diagnosed with AML M4EO (18 cases), AML M4(16 cases), secondary AML (3 cases), AML M2EO (1 case),or CML in myeloid blast crisis (1 case), and havingcytogenetically detectable inv(16)(p13q22) (35 cases) ort(16;16)(p13;q22) (4 cases), as analyzed in four cytogeneticlaboratories from the Republic of Ireland and the UnitedKingdom. Of the 35 cases with inv(16), this was the soleabnormality in 26, trisomy 22 was seen in 3 cases, andtrisomy 8 in 3 cases. The remaining four cases had t(16;16),which was the sole abnormality in three cases and wasassociated with both trisomies 8 and 22 in one case. Theincidence of inv(16)/t(16;16) in this series was 3.5%.

2.2. Cytogenetic analysis

Standard cytogenetic analysis was carried out on allsamples at diagnosis using bone marrow or unstimulated

peripheral blood. Short-term cultures were established incomplete medium (RPMI 1640 with glutamax, 20% fetalcalf serum, and 1% penicillin and streptomycin), meta-phases were harvested, and GTG banded analysis per-formed [33]. Whenever possible, at least 10 metaphaseswere fully analyzed for each patient, and karyotypes weredescribed according to the International System for HumanCytogenetic Nomenclature [34].

2.3. FISH analysis

FISH was carried out according to manufacturer’sinstructions, using the Vysis LSI CBFb dual-color break-apart probe (Vysis, Downers Grove, IL). This probecontains a mixture of 3# CBFb labeled with SpectrumGreenand 5# CBFb labeled with SpectrumRed, mappingtelomeric and centromeric to the breakpoint region,respectively. At least 100 interphase cells were analyzedfor each sample, and metaphases were also analyzedwhenever possible.

3. Results

3.1. FISH analysis

Analysis of normal interphase cells using the dual-colorbreak-apart probe results in two fusion signals, while cellscarrying inv(16) or t(16;16) show one fusion and separatered and green signals (Fig. 1b). Analysis of metaphaseswith inv(16) shows one fusion on 16q22 and separate redand green signals on opposite arms of the inverted 16(Fig. 1a). Deletion of 3# CBFb results in the loss of one

Fig. 1. FISH analysis of a metaphase cell (a) and interphase cell (b) of a patient with inv(16) without deletion of 3# CBFb hybridized with the CBFb dual-

color break-apart probe. The fusion signal can be seen on the q arm of the normal 16, with red 5# and green 3# signals on the p arm and q arm, respectively of

the inv(16) and separated signals in interphase.

124 J. Kelly et al. / Cancer Genetics and Cytogenetics 162 (2005) 122–126

Fig. 2. FISH analysis of a metaphase cell (a) and interphase cell (b) of a patient with inv(16) and deletion of 3# CBFb hybridized with CBFb dual-color

break-apart probe. The fusion signal can be seen on the q arm of the normal 16, and the red 5# CBFb signal can be seen localized to the p arm of the inv(16),

with loss of the green 3# CBFb signal in both metaphase and interphase.

green signal, giving a one fusion/one red signal pattern(Fig. 2, a and b).

The standard one fusion/one red/one green pattern wasdetected in 36/39 samples; 3/39 samples showed a onefusion/one red pattern, with loss of the green signalindicative of the 3# CBFb deletion.

3.2. Data for patients with 3#CBFb deletion

Patient 1: A 76-year-old woman was referred forhematologic assessment with a 3- to 4-week history ofmalaise. Full blood count (FBC) revealed a white cell count(WCC) of 34.6 � 109/L, hemoglobin 9.7g/dL, and platelets35 � 109/L. Blood film showed circulating blasts. The bonemarrow aspirate showed a mixture of myeloblasts andmonoblasts. There was also dysplasia of eosinophilicprecursors. Immunophenotyping was consistent withAML, and a diagnosis of AML M4Eo was made (FABclassification). Cytogenetic analysis showed the karyotype46,XX,inv(16)(p13q22)[10], and FISH analysis confirmedCBFb rearrangement with concomitant 3# CBFb deletion.The patient underwent induction chemotherapy on theMedical Research Council AML 12 protocol and achievedCR. Although the planned fourth course of consolidationchemotherapy was not given due to severe toxicity, thepatient has remained in CR.

Patient 2: A 20-year-old woman presented with a 2-month history of anergia and a 1-week history of mucosalbleeding. Upon clinical examination, she had multipleecchymoses. Her FBC showed a WCC of 187�109/L,hemoglobin of 8.8g/dL, and a platelet count of 31�109/L.Peripheral blood film revealed that the majority of the whitecells were circulating blast cells. Flow cytometric analysis

confirmed these cells to be myelomonocytic in origin.Cytogenetic analysis of an unstimulated peripheral bloodsample showed the karyotype 46,XX,inv(16)(p13q22)[8]/46,XX[2], and FISH analysis detected deletion of 3# CBFb.

The patient underwent induction chemotherapy withstandard-dose cytosine arabinoside and daunorubicin. Abone marrow aspirate was performed 4 weeks after the startof chemotherapy and showed morphologic evidence ofpersistent disease with 20% blasts. At this point, however,FISH analysis detected no evidence of CBFb rearrange-ment in 100 metaphase and interphase cells analyzed,a finding that cannot be explained but may have resultedfrom a hemodilute sample.

Patient 3: A 68-year-old man who had been diagnosed 3months previously with terminal cancer of the larynx wasreferred to the hematology service for investigation ofpancytopenia, and a bone marrow aspirate was performed.Morphology and immunophenotyping of his bone marrowindicated a diagnosis of AML M4 (FAB classification).This was confirmed by the karyotype 47,XY,18,inv(16)(p13q22)[12]/46,XY[3]. FISH analysis showedCBFb rearrangement with deletion of 3# CBFb. Due tothe patient’s antecedent diagnosis of carcinoma of larynx,chemotherapy for AML was not pursued and he diedshortly afterward.

4. Discussion

A number of recent reports have identified submicro-scopic deletions in association with a number of recurrentcytogenetic rearrangements [9,10,16–30]. While it has beenlong established that 16p deletions occur in association

125J. Kelly et al. / Cancer Genetics and Cytogenetics 162 (2005) 122–126

with inv(16)/t(16;16) [9,10], the suggestion being that theseevents are not of prognostic significance [22,23], only fourcases of concomitant deletion of 3# CBFb with inv(16)have been reported previously [24–26]. In this study, wehave analyzed 39 patients with AML and inv(16)/t(16;16),and found deletions of 3# CBFb in 3/39 (8%), all associatedwith inv(16).

In all three cases, all abnormal cells analyzed uponpresentation of disease contained the deletion, suggestingthat it occurred at the same time as the inversion, and whatappears to be the same disease in a number of patients can,in fact, be molecularly heterogeneous from early in itsevolution. This is certainly borne out by the CMLparadigm; t(9;22) can produce various BCR/ABL fusiongenes according to the BCR breakpoint. In turn, these resultin the transcription of p190, p210, or p230 fusion proteins,which can lead to differences in disease phenotype.Furthermore, alternative splicing can lead to the simulta-neous expression of p190 and p210 mRNA in the samepatient [35], and the more recent identification of t(9;22)patients with concomitant deletions of the der(9) suggestthat disease-specific cytogenetic abnormalities may bemore complex than had been thought previously. Themechanism by which these deletions arise has yet to bedefined. One suggestion implicates the presence of Alusequences; the occurrence of these repetitive sequencesflanking chromosomal breakpoints may increase genomicinstability and facilitate further rearrangement [25]

It has not been possible to draw any conclusions fromthis group of patients regarding the prognostic implicationsof the 3# CBFb deletion associated with inv(16) because ofthe short period of follow-up (two patients) or lack oftreatment (one patient). While submicroscopic deletionshave been identified in association with a number ofcytogenetic abnormalities, prognostic significance has beenimplicated only in the case of 5# ABL and/or 3# BCRdeletions in CML, where a less favorable response totherapy has been reported when compared to the non-deleted group of patients [17–21]. This issue remainscontroversial, however, and there exists contradictory data[31]. The mechanisms by which poor prognosis may beconferred have yet to be elucidated but could result fromhaploinsufficiency of one or more genes in the region [21].Where other deletions are concerned, the number ofreported cases is currently too small for statistical analysis,and it will require large clinical trials that incorporate FISHanalysis as routine follow-up to the detection of certaincytogenetic abnormalities to determine the clinical signif-icance of these submicroscopic deletions.

Acknowledgments

We would like to thank Owen Smith, Derville O’Shea,and Raymond Stallings for critical reading of themanuscript.

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