Leukemia (1999) 13, 671–678  1999 Stockton Press All rights reserved 0887-6924/99 $12.00 http://www.stockton-press.co.uk/leu MINI-REVIEW

Centric and pericentric rearrangements in hematopoietic malignancies R Berger and M Busson-Le Coniat

INSERM U434 and CNRS SD 401 No. 434, Institut de Ge´ne´tique Mole´culaire, Paris, France

Cytogenetic and fluorescence in situ hybridization (FISH) lymphoblastic leukemia, have been described as nonrandom analysis of 10 patients with various hematopoietic malig- abnormalities in hematopoietic malignancies.6–10 Other nancies revealed the presence of dicentric or pericentric chromosome rearrangements. Dicentrics were only examples of dicentric chromosomes implying various chro- ascertained by FISH studies in six patients. Two types of peri- mosomes occurring as clonal abnormalities, have been centric chromosome rearrangements have been observed: reported to be present in hematopoietic disorders (Table 1). ‘classical’ dicentrics with two clearly separated centromeric Dicentrics may be difficult to detect with banding techniques regions, and more unusual rearrangements with a breakpoint if the two are very closely located on rearranged within the centromeric or heterochromatic area, but outside the chromosomes. This difficulty can be overcome by use of FISH, alphoid domain. FISH analysis of partial q dupli- cations present in three lines confirmed and it has been shown that a large number of the partial involvement of the non-alphoid centromeric domain identified by chromosome banding analysis actually are iso- in the duplicated chromosome segment. The incidence of cen- dicentric chromosomes (Table 1). The overall frequency of tromeric and pericentromeric rearrangements in hematopoietic isochromosomes in malignancies was 9.9% in 18 160 malignancies may be higher than hitherto admitted. The neoplasms11 and unevenly distributed according to the type chromosomal localization of these rearrangements suggests of tumors. several mechanisms possibly involved in the malignant process and deserves more systematic study. The present report underlines the importance of FISH tech- Keywords: hematopoietic malignancies; chromosomes; dicentric; niques to detect dicentric chromosomes and identify pericen- pericentric rearrangements tric rearrangements occurring as clonal abnormalities in malignant blood disorders. This study was initially based upon

Introduction Table 1 Dicentric chromosomes in hematopoietic malignancies

Dicentric chromosomes classically are instable structures Disease Ref.a prone to be broken at anaphase. While dicentric chromo- somes are rare in normal cells, their frequency is increased Dicentric chromosomes when the cells are exposed to mutagenic agents and ionizing dic(1;15)(p11;p11) MDS, PV 56 radiations. Dicentrics resulting from irradiation from various dic(5;7)(p13;p11) AML 57 sources (X-rays, gamma rays) were also shown to be instable dic(5;17) MDS, CML-BC 58–60 in somatic cells since they disappear throughout successive dic(7;9)(p11–p13;q11) ALL 61,62 mitoses. Dicentric chromosomes, however, have been dic(7;12)(p11;p12) ALL 62 dic(9;12)(p11–13;p11–12) ALL, sAML, 6,7, 63–65 observed in malignant cells as clonal abnormalities with a CML-BC, variable incidence according to the type of tumor examined. ATL, NHL Since such dicentrics persist during cell proliferation, it has dic(9;20)(p11–13;q11) ALL 8–10 been hypothesized that one was functionally inac- dic(12;13)(p11;p13) AML, MDS 66 tive. The abnormal chromosome could consequently escape dic(12;17)(p11;p12) ALL 62 breakage at anaphase. The hypothesis of two kinds of cen- dic(16;22)(q11;p11) MDS, sAML 67 dic(17;18)(p11;p11–12) AML, CML-BC, 68,69 tromeres, active and inactive, was supported by the fact that APL the CENP-C and CENP-E centromere constitutive binding pro- dic(17;22) CML 70 teins are necessary components of functional centromeres but whole arm chromosome various hemato- 18 not of inactive ones.1,2 The centromere is, indeed, a complex translocations poietic disorders structure, associating various DNA subtypes and , dif- ferently associated within the different chromosomes. Alpha Isodicentric chromosomes idic(8)(p11) T-PLL 71 satellite (alphoid) DNA, which is believed to play an idic(12)(q12) MDS, ALL 64, 72 3,4 important role in the function of the centromere, is a family idic(14)(q11) T-CLL, sMDS 73, 74 of satellite including several subtypes.5 Some idic(17)(p11) AML, CML 75–81 sequences are chromosome-specific, allowing their use as idic(21)(p11) AML 82 markers in techniques of fluorescence in situ hybridization idic(Ph) CML, AML 70, 83 (FISH) to human cells. idic(X)(q13) AML, MDS, MPD 84 Some recurrent dicentric chromosomes, such as dic(9;12)(p11–13;p11–12) and dic(9;20)(p11;q11) in acute ALL, acute lymphoblastic leukemia; AML, acute myeloblastic leuke- mia; sAML, secondary AML; APL, acute promyelocytic leukemia; CML, chronic myeloid leukemia; CML-BC, blastic crisis of CML; MDS, myelodysplastic syndrome; MPD, myeloproliferative disorder; Correspondence: R Berger, U434, 27 rue Juliette Dodu, 75010, Paris, NHL, non-Hodgkin lymphoma; PV, polycythemia vera; T-PLL, T cell France; Fax: 33 1 5372 51 92 prolymphocytic leukemia. Received 7 December 1998; accepted 21 January 1999 aSee also Ref. 17. Mini-review R Berger and M Busson-Le Coniat 672 the finding of aberrant segments in D9Z1) and beta satellite probes (Oncor, D9Z5) specific to rearranged chromosomes, either in possible or obvious dicen- , YACs (from the CEPH library, Paris, France) tric chromosomes, or in translocations mainly involving the 882b3 covering the BCL9 ,15 978e4 covering the ARNT- long arm of chromosome 1. Ten patients and three cell lines AF1q loci, 742f9 covering the MLL locus, and 936e2 covering with these criteria were chosen for the study. While the inci- the TEL/ETV6 locus, cosmid 19q covering the telomeric part dence of these abnormalities in hematopoietic malignancies of (L Kearney, MRC Molecular Haematology, cannot be ascertained at the present time, the aim of this John Radcliffe Hospital, Oxford, UK), BAC 16091 and PAC report is to focus on these rearrangements which suggest 16093 corresponding to chromosome bands 7q11–7q21 (G several working hypotheses. Gilliland, Brigham and Women’s Hospital, and Howard Hughes Institute, Philadelphia, USA).

Materials and methods Results Patients Analysis of FISH studies of patients (and cell lines) roughly The clinical and hematological data of 10 patients examined confirmed the results of conventional banded for malignant blood disorders in the Department of Hematol- analysis in most of the cases (Figure 1). However, some abnor- ogy of the Saint-Louis Hospital (Paris) are summarized in malities were found to be better or differently defined with Table 2. In addition three Burkitt lymphoma (BL) cell lines, FISH techniques (Table 3). Dicentric chromosomes or pericen- BL2, BL3, and LY66,12,13 were re-examined with FISH tric rearrangements were present in all patients examined. techniques.

Patient 1, therapy-induced AML, M4 after multiple mye- Chromosome studies loma: The karyotype was complex. The painting probe showed various rearrangements of chromo- Chromosome studies were performed on bone marrow and/or some 22, including add(22)(q13), and del(22)(q11). FISH peripheral blood cells after short-term culture, and on cultures analysis with several probes (, 12, and 22 of BL cell lines. GTG and/or RHG banding techniques were whole chromosome painting, YAC 936e2) showed that the applied and the chromosomes were classified according to the add(22)(q13) chromosome was dicentric, including cen(22) international nomenclature. The results of banded karyotype and cen(8) with insertion of a segment of 12p including the analyses are summarized in Table 3. ETV6/TEL locus (YAC 936e2) between the 22 and 8 fragments. A second marker, add(12)(p12), was also dic, dic(5;12)(q12:p12). Fluorescence in situ hybridization (FISH)

FISH techniques14 were applied on metaphase chromosomes Patient 2, ALL: Chromosomes 1 were analyzed with DAPI using various molecular probes depending on the abnormali- staining and a chromosome 1-specific alpha satellite DNA ties studied: whole chromosome painting probes for chromo- probe. A faint heterochromatin-like band was present on the somes 5, 9, 12, 17, 18, 22 (U301 INSERM) and 1 (STAR*FISH long arm of the rearranged chromosome 1, at the limit of its 1066-1B, Cambio, Byosis, Compie`, France), 8 (p5201 partial duplication suggesting that the breakpoint was located Coatsome, Oncor, Illkirch, France), alpha satellite probes spe- within band 1q12. Moreover, this band was not labeled with cific to chromosomes 1, 12, 17, and 18 (Oncor, D1Z5, the ‘alphoid’ probe as the normal centromeric regions were. D12Z3, D17Z1, and D18Z1), ‘classical’ satellite (Oncor, This pattern of labeling indicates that the DNA breakpoint

Table 2 Clinical and hematological data of 10 patients with hematopoietic malignancies

No. CG Sex/Age Diagnosis Bone marrow Peripheral blood Follow-up % blasts Leukocytes Hemoglobin Platelets ×109/l (blasts g/dl ×109/l

1 3462 M/42 y tAML-M4 (MM) 82 35 (36) 8.9 110 — 2 98033 M/3 y ALL-L2 (B) 93 45.6 (84) 9.5 33 CR, 1 m+ 3 2559 F/18 m AML-M4 50 60 (31) 12.9 27 CR, R:7 m, D:8 m 4 97290 M/18 m ALL-L1 (B) 93 47 (83) 8.3 96 CR 5 95276 F/25 y AML-M1 98 159 (94) 9.3 50 CR 6 97220 M/2.5 y AML-M5 79 65 (66) 10.2 280 CR 7 8513 F/2 y ALL (B) 32 4.3 (40) 7.9 50 CR, 6y+ 8 8136 F/8 y ALL-L2 (B) 68 1.3 (0) 7.5 43 CR F/18 y ALL-L2 (R) 89 8.9 (49) 11 6 CR, R:19 m F/19 y ALL-L2 (R2) D: 5 y 4 m 9 4082 F/11 y ALL-L2 (B) 100 7.6 (68) 9.6 46 CR 10 97192 F/62 y AML-M6 21 4.3 (1%) 7.7 89 CR, 6 m+

AML, acute myeloblastic leukemia; tAML, therapy-related, AML; MM, ; ALL, acute lymphoblastic leukemia; M1, M4, M5, M6, L1, L2, FAB subclass; B, B lineage; R, relapse; R2, second relapse; CR, complete remission; D, dead. Mini-review R Berger and M Busson-Le Coniat 673 Table 3 Cytogenetic and FISH data

Case Status Materiala Karyotypeb MD, 24, 48–B24,48

1 D M48,B24,48 44,XY,−5,der(8)t(8;?)(p11;?),−9,add(9)(q11),add(12)(p12), add(16)(q24),add(22)(q13)[cp23] 44,XY. ish dic(5;12)(q12;p12)(wcp5+, wcp12+,D12Z3+),−9,dic(22;12;8) (p13q12;p12;p12q24) (wcp8+,wcp12+,YAC936e2+,wcp22+)[cp 31͖ 2 D S17,24 62,XXYY,dup(1)(q21q44),+3,+4,+5,+6,+7,+8,+10, +11,+14,+17,+18,+21,+21, +22[20]/46,XY[2] 62,XXYY; ish dup(1;1)(31;q12)(YAC882b3+, YAC978e4+) [11] 3 D M24,48 45,XX,t(7;12)(p11;q11),−12[47]/46,XX[19] 45,XX,ish.dic(7;12)(p11;p12)(D7Z1+,D12Z3+),−12,t(12;19)(p13;p13)(wcp12+,cos12q+)[6] 4 D M17,24 46,XY,−9,t(9;9)(p11;q11),+min [32]/46,XY[5] 46,XY. ish dic(9;9)(p11;cen)(wcp9+,D9Z1+,D9Z5+),−9,+cen(9)(D9Z1+D9Z5+)[28] 5 D M24,S24 46,XX,−17,−18,+t(17;18)(cen;cen)[6]/46,XX[11] 46,XX. ish −17,−18,+dic(17;18)(p11;p11)(wcp17+,wcp18+,D17Z1+,D18Z1+) [6] 6 D M48 45,XY,t(10;11)(p12;q23),t(12;18)(p13;q11)[6]/46,XY,[9] 45,XY. ish t(10;11)(wcp10+,wcp11+, YAC742f9s),dic(12;18)(p13;p11)(D12Z3,D18Z1)[12] 7 D M17 56,XXX,+4,+6,+8,+10,+14,+17,+18,+der(19)t(1;19)(q23;p13),+21,+21 [9]/46,XX[5] 56,XXX. ish dic(1;19)(q12;p13)(YAC882b3+, YAC978c4+,cos19q+) [17] 8 D Md,17,24 46,XX [49] R M24 45,XX,−9,dic(9;12)(q33;p11)[15] 45,XX. ish −9,dic(9;12)(q33;p11) [18] R2 S17,24 45,XX,−9,dic(9;12)(q33;p11)(wcp9+,wcp21+,YAC936e2+)[9]/46,idem,+mar[4]/46,XX[4] 9 D Md,24 45,XX,−8,der(12)t(8;12)(p12;p11) [30] 45,XX. ish −8,der(12)t(8;12)(p12;p11)(wcp8+,wcp12+ YAC936e2+), dic(9;12)(q12;p11) (wcp9+, YAC936e2+) [26] 10 D M24 46,XX,del(5)(q14),dic(5;7)(q15;p22),+8,−16,−17,+22 [cp20]/idem,add(3)(q26)(3]/ id,der(3)t(3;?)(p12;?)[cp3]/46,XX [8] 46,XX. ish del(5)(wcp5+),dic(7;16)(p22;p11)(BAC16091,PAC16093,wcp16+),+8,− del(16)(p11)(wcp16+),−17 [21] CR M24,48 46,XX[19]

D, at diagnosis; R, relapse; R2, second relapse. aMd, 17, 24, 48: Bone marrow direct examination and after 17, 24, 48 h in vitro culture. S17, 24: 17 and 24 h in vitro blood cell culture. bFor each case: upper lines(s): banded karyotype, lower line(s): FISH analysis. within the heterochromatin was located outside the alpha a signal, centromeric to that given by the D9Z1 probe on the stellite sequence site. Two hybridization signals were nonrearranged centromere of dic(9;9) and another hybridiz- observed with the YAC 882b3 and 978c4 probes on the der(1) ation signal on the minute chromosome. chromosome, one at the normal localization and the other distal to the duplicated heterochromatin part. This labeling pattern confirmed that the breakpoint of the duplication was Patient 5, AML, M1: Dicentric (17;18)(p11;p11) was con- proximal to the BCL9 locus on the rearranged chromosome 1. firmed by FISH with alpha satellite probes D17Z1 and D18Z1 showing a signal of each probe on the derivative (17;18) chromosome. Patient 3, AML, M4: A translocation t(12;19)(p13;p13) was evidenced with whole chromosome painting probes. The involvement of 19p was ascertained with a 19q telomeric cosmid probe which allowed the unambiguous recognition of Patient 6, AML, M5: Tranlocation t(10;11) was confirmed short and long arm of chromosome 19. Hybridization with by FISH analysis showing the splitting of the hybridization sig- YAC 936e2 probe showed only one signal on normal chromo- nal of the YAC 742f9 probe, covering the MLL locus, on some 12 short arm, indicating the loss of the TEL/ETV6 locus derivatives chromosomes 10 and 11. Translocation t(12;18) and the location of the breakpoint centromeric to this locus was confirmed by whole and 18 painting on the translocated 12. With painting and alpha satellite probes. Using alpha satellite probes, t(12;18) was shown to probes, it was shown that the t(7;12) recognized by banding be dic(12;18)(p11;p11). The ETV6/TEL locus was lost on analysis actually was dic(7;12)(p11;p11). dic(12;18) as shown by hybridization with YAC 936e2 which only hybridized to the normal chromosome 12.

Patient 4, ALL: Dic(9;9)(p11;p11) was confirmed by FISH using probe D9Z1 corresponding to satellite DNA specific to Patient 7, ALL: The initial karyotype with t(1;19)(q23;p13) chromosome 9. Two spots, one larger than the other, were was re-examined because the heterochromatin pattern of the distinct on the , and another hybridiz- der(19) appeared to be unusual. The chromosome 1-specific ation signal was present on the minute chromosome. The alpha satellite probe did not label the whole heterochromatin breakpoint was thus within the satellite sequence site in one area on one chromosome 1, and the small band of heteroch- chromosome 9 centromere and outside the centromeric area romatin on the rearranged 19 chromosome was not labeled. in the other. FISH with the D9Z5 probe (beta satellite) showed This finding shows that the breakpoint on chromosome 1 was Mini-review R Berger and M Busson-Le Coniat 674

Figure 1 Partial (R-bands) of patients 1 to 10 and Burkitt lymphoma cell lines BL2, BL3, and LY66. Black triangles show abnormal chromosomes. Patient 1: dic(5;12) and dic(8;12;22); patient 2: dup(1)(q21); patient 3: dic(7;12); patient 4: dic(9;9) and minute; patient 5: dic(17;18); patient 6: t(10;11) and dic (12;18); patient 7: t(1;19); patient 8: dic(9;12)(q33;p11); patient 9: der(8;12) and dic(9;12)(q12;p11); patient 10: add(3)(p11), del(5)(q14),dic(7;16); BL2: der(6)t(1;6); BL3: t(1;19); LY66: der(1).

outside the DNA alpha satellite site, as in patient 3. The area of chromosomes 9 and 12, as expected from the rearrangement was thus t(1;19)(qh;p13). banded chromosome analysis. In a second relapse, the same dicentric chromosome was present, as well as new marker chromosomes. Patient 8, ALL in relapse: dic(9;12)(p13;q34) has been pre- viously reported.16 DNA satellite probes D9Z1 and D12Z3 showed two distinct spots corresponding to the centromeric Patient 9, ALL: Use of an alpha satellite probe specific to Mini-review R Berger and M Busson-Le Coniat 675 showed the centromeric area of 8 in the The BL3 cell line was initially described as having 72 chro- t(8;12)(p12;p12) chromosome. With chromosomes 9 and 12 mosomes with t(1;19)(q23;q13),t(8;14)(q24;q32).12 This cell painting, chromosome 12 alpha satellite, and YAC 936e2 line was re-examined with FISH technique because of the probes, it was shown that t(9;12) was dic(9;12)(p11–12;q12) presence of t(1;19) associated with t(8;14)(q24;q32). As in (Figure 2), and that the ETV6/TEL locus was translocated on case 7, the der(19) exhibited a part of 1 qh without labelling to the derivative t(8;12). FISH with D9Z5 (beta satellite) of the heterochromatin from chromosome 1 in FISH experi- showed a weak signal on the normal chromosome 9. The spot ments with specific DNA alpha satellite probe. The der(19) observed in FISH experiments with D9Z5 was centromeric was actually der(19)t(1;19)(q12;p13), with a breakpoint within to the signal obtained with D9Z1 on the dicentric t(9;12) 1 qh, as in patient 7. chromosome. A derivative chromosome 1 was present in LY66 with 46,XY, t(2;8)(p12;q24).13 The der(1) chromosome had a nor- mal heterochromatin band, and another smaller band marking Patient 10, AML, M6: Whole chromosome painting the limit of the duplicated chromosome 1 region. This band probes showed that the der(7) was t(7;16)(p15–22;p11), and was not labeled with the alpha satellite probe (Figure 3). The that one chromosome 22-like was actually derived from chro- breakpoint was thus outside the alpha satellite domain of the mosome 16. It was thus del(16)(p11). With alpha satellite 1 qh area. probes, the t(7;16) could be redefined as dic(7;16)(p15;p11), and the der(16) chromosome was also labeled. This result indicates that the breakpoint on der(16) was outside the DNA Discussion alpha satellite site, and that the rearrangement resulted from a non-homologous homologous breakage on Recurrent dicentric, dic(9;12)(p12–13;p11–12) and with duplication of the alpha satellite domain since it is dic(9;20)(p11;p11) have been described in patients with acute present on the two rearranged 16 s. lymphoblastic leukemia, dic(17;18)(p11;p11) in myeloid pro- liferations, and other more uncommon rearrangements in other patients (Table 1, and Ref. 17). Dicentric chromosomes Burkitt lymphoma cell lines have also been observed in solid tumors. Centromeric break- age is particularly frequent in squamous cell carcinoma and Three Burkitt lymphoma established cell lines were chosen it has been claimed to be the major cause of cytogenetic for FISH analysis: In BL2, the rearranged chromosome initially abnormality in oral squamous cell carcinoma.18 Whole chro- described was 46,XY,trp(1)(q23q25),t(1;6)(q23;q26),t(8;22).12 mosome translocations have also been observed in various By FISH, the der(6)t(1;6) was shown to be t(1;6)(q12;q26) types of tumours.19 An unknown percentage of acquired without labeling of the heterochromatin part from der(1) with isochromosomes observed in malignant cells are indeed the alphoid probe. This finding indicates that the breakpoint dicentric chromosomes. The recurrent isodicentric X present is within the non-alpha satellite area of chromosome 1 qh. in myeloid proliferations is the more common in hematopo- Moreover, it is worth noting that the two chromosome 1 non- ietic malignancies (Table 1 and Ref. 11). rearranged centromeres were asymmetrically labeled with the In the present study, dicentric chromosomes or pericentric centromeric probe: the heterochromatin of one was not entirely labeled with the probe while the other was, confirming the existence of heterochromatin variant.

Figure 3 Burkitt lymphoma cell line LY66. FISH sudies with probe DIZ5. The hybridization signals (arrows) on the centromeric area of chromosome 1 as grey on the white background which is the DAPI- counterstained heterochromatin. The arrowhead shows the heteroch- Figure 2 Patient 9: FISH analysis with probes D9Z1 and D12Z3. romatin of the rearranged chromosome 1. Mini-review R Berger and M Busson-Le Coniat 676 rearrangements were observed in 10 patients with hematopo- than decondensation of chromosome 1.36 Moreover, human ietic malignancies examined prior to treatment and in three ␣-satellite DNAs have been claimed not to be susceptible to Burkitt lymphoma cell lines. Eight patients had de novo malig- undercondensation after azacytidine exposure.45 The role of nancies, one exhibited clonal chromosome abnormalities only changes in methylation patterns is well established in malig- in relapse (case 8) and only one presented with therapy- nancies, and such changes could favor the rearrangements of related acute myeloblastic leukemia (case 1). Two different the second type as defined above. In these dicentrics, the par- subtypes of chromosomal rearrangements of centromeric and tially duplicated part of chromosome 1 conserved non-␣-het- pericentromeric areas could be distinguished according to the erochromatin, as shown by FISH analysis, suggesting that the results of FISH experiments using DNA satellite probes: in the chromosome breakpoint occurred outside the ␣-satellite first subtype (cases 1, 3, 5, 6, 8, 9, and 10), the hybridization domain of heterochromatin. signals on metaphase chromosomes were distinct, and each The DNA sequences surrounding the centromeric areas are centromeric region was labeled by its chromosome specific less well characterized at the present time, and it is difficult probe. Using bicolor FISH, the two probes, one colored in to guess if homologies of the DNA sequences located close green and the other in red, did not overlap on the rearranged to the centromere of the rearranged chromosomes could chromosome. In two cases, however, a very faint yellow sig- explain the mechanisms of translocations resulting in dicentric nal due to the apparent superimposition of the two colors was structures. The nonrandom involvement of the short arm of seen between the green and red signals. This pattern of labe- chromosomes 12, 9, 17, and 18 in recurrent translocations ling results more likely from the close proximity of the labeled can only be noticed, and instability of some pericentromeric centromeric regions rather than from partial centromeric regions like 22q11, 15q11, 16p11, and 17p11 has been noted fusion because the yellow signal was occasionally seen only in constitutional abnormalities.46 It has been reported that after image acquisition by the image analyzer, while it was beta-satellite DNA47 contains retrotransposons in mam- not visible at direct microscopic observation. In the second mals.48,49 Beta-satellite DNA is present in the centromere of subtype of pericentric rearrangement all or almost all the human chromosome 9 but not of No. 1.20,47,50,51 However, alpha satellite component of the centromere were not the presence of retrotransposons, which could favor DNA and involved, as shown by the absence of hybridization signal chromosome rearrangements, is not proven in human chro- with the alphoid probe (cases 2, 7, and BL cell lines). In the mosome 9 centromeric DNA, at the present time. The pro- rearranged chromosomes 1 belonging to this subtype, the het- duction of isodicentric chromosomes can obviously been fav- erochromatin of the non-rearranged centromeres of the two ored by DNA homologies. Whatever the mechanisms could chromosomes 1 was asymmetric (1qh variants), and the be, the presence of abnormal somatic pairing of some cen- alphoid probe did not hybridize to the whole heterochromatin tromeres (chromosomes 1, 7, 17, 7, and 10) observed in on one chromosome. This labeling pattern is probably due to interphase nuclei could favor chromosomal rearrangements a larger amount of non-alpha satellite DNA in the heterochro- within homologs.52–54 The consequences of the presence of matin area. It is worth noting that the localization of break- dicentrics depend on the whole karyotype of the cells: they points to satellite III has already be reported to occur in can contribute to genic dosage abnormality due to loss or gain inversions of chromosome 920 and in Robertsonian translo- of one or two chromosome arms, and they can also result in cations.21 The incidence and distribution of C-band variants rearrangements of located close to the centromere. It in patients with malignancy has been studied since the late has also been shown that a , brown, positioned adjacent seventies.22–26 At that time, differences in the distribution of to heterochromatin was silenced by contact with centromeric size variants, asymmetry between homologs, and inversions heterochromatin in Drosophila.55 As far as we know, how- or partial inversions between patients with malignancy and ever, there is no available proof that the same mechanism can controls were reported in several studies.27–29 Whether these occur in . Whatever they are, so-called primary or sec- variants could indicate malignancies could not be firmly ondary chromosome abnormalities in malignancies, dicentric established at that time.30 Such studies only provided statisti- chromosomes and pericentric rearrangements appear to occur cal data on a series of patients. FISH analysis gives a better more frequently than expected in malignant cells. Finally, the opportunity to search for heterochromatin variants which question now arises whether the more systematic use of FISH could be prone to rearrangements in these particular cases. analysis to study chromosome rearrangements involving the The present study cannot answer this question, although the centromeric area can contribute to detecting dicentric chro- presence of chromosome 1 heterochromatin length variants mosomes. Similarly, the possibility of the preferential involve- was noted in a few cases and that partial inversion of chromo- ment of some centromeric areas, such as those of chromo- some 9 could be detected in patient 9. somes 1, 9, 12, in rearrangements of hematopoietic Clonality of dicentric chromosomes of malignant cells malignancies has to be considered. implies that these chromosomes are stable through successive cell divisions. The stability of dicentrics can be explained by the functional inactivation of one of the two centromeres. The References structure of human centromeres is complex and includes vari- ous components such as chromosome-specific and non-spe- 1 Sullivan BA, Schwartz S. Identification of centromeric antigens in 31,32 dicentric Robertsonian translocations: CENP-C and CENP-E are cific repeated DNA sequences, and constitutive proteins. necessary components of functional centromeres. Hum Mol Genet Even the so-called DNA alpha satellite sequences are hetero- 1995; 4: 2189–2197. geneous and each centromere differs in DNA subtypes.33–39 2 Faulkner NE, Vig B, Echeverri J, Wodeman L, Vallee RB. Localiz- Moreover functional centromeres can lack alphoid DNA.40–43 ation of motor-related proteins and associated complexes to It has been shown that the heterochromatin of some chromo- active, but not inactive, centromeres. Hum Mol Genet 1998; 7: somes such as chromosome 1 could be uncoiled by the hypo- 671–677. 44 3 Masumoto H, Sugimoto K, Okazaki T. Alphoid satellite DNA is methylating agent azacytidine. 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