Disruption and aberrant expression of HMGA2 as a consequence of diverse chromosomal translocations in myeloid malignancies MD Odero 1 , FH Grand 2 , S Iqbal 2 , F Ross 2 , JP Roman 1 , JL Vizmanos 1 , J Andrieux 3 , JL Laı ¨ 3 , MJ Calasanz 1 and NCP Cross 2 1 Department of Genetics, School of Science, University of Navarra, Pamplona, Spain; 2 Wessex Regional Genetics Laboratory, Salisbury and Human Genetics Division, University of Southampton, Southampton, UK; and 3 INSERM Unite ´ 524, Institut de Recherche sur le Cancer de Lille and Laboratoire de Ge ´ne ´tique Me ´ dicale, Ho ˆ pital Jeanne de Flandre, CHRU de Lille, Lille, France Chromosomal translocations that target HMGA2 at chromo- some band 12q14 are seen in a variety of malignancies, notably lipoma, pleomorphic salivary adenoma and uterine leiomyoma. Although some HMGA2 fusion genes have been reported, several lines of evidence suggest that the critical pathogenic event is the expression of truncated HMGA2 isoforms. We report here the involvement of HMGA2 in six patients with myeloid neoplasia, dysplastic features and translocations or an inversion involving chromosome bands 12q13–15 and either 7p12, 8q22, 11q23, 12p11, 14q31 or 20q11. Breaks within or very close to HMGA2 were found in all six cases by molecular cytogenetic analysis, leading to overexpression of this gene as assessed by RT-PCR. Truncated transcripts consisting of HMGA2 exons 1–2 or exons 1–3 spliced to intron-derived sequences were identified in two patients, but were not seen in controls. These findings suggest that abnormalities of HMGA2 play an important and previously unsuspected role in myelo- dysplasia. Leukemia (2005) 19, 245–252. doi:10.1038/sj.leu.2403605 Published online 16 December 2004 Keywords: HMGA2; MDS; MDS/MPD Introduction The high-mobility group (HMG) proteins HMGA1a, HMGA1b and HMGA2 comprise a subgroup of nonhistone chromatin accessory proteins, often referred to as architectural transcrip- tion factors. 1–3 They are low molecular mass nuclear proteins of about 100 amino acids, which each possess three copies of a nine amino-acid motif (AT-hook) that interacts with the minor groove of many promoter and enhancer DNA regulatory elements. 4 HMG proteins possess no intrinsic transcriptional activity, but instead function to orchestrate the assembly of nucleoprotein structures involved in gene replication, transcrip- tion and overall chromatin structure through a complex network of protein–DNA and protein–protein interactions. 1–7 The HMGA2 gene (formerly known as HMGI-C) spans a genomic region of 160 kb at chromosome band 12q13 and consists of five exons. The translated protein contains the three AT hooks, encoded by exons 1–3, and an acidic C-terminal domain, encoded by exon 5. 8 HMGA2 is expressed predomi- nantly during embryonic development, 8–14 but is overexpressed in many malignant cell lines, and is thought to contribute to the transformation process. 9–11 The role of HMGA2 in mouse development is underscored by the finding that inactivation of this gene results in the pygmy mouse, the phenotype of which exhibits growth retardation and a significant reduction of overall body adipose tissue. 12 Rearrangements of HMGA2 have been detected frequently in human mesenchymal tumors, resulting in fusion to diverse partner genes. In lipomas, HMGA2 has been shown to fuse to LPP at 3q27–q28, a gene encoding an LIM domain-containing protein; 8,15 to LHFP at 13q12, the function of which is still unknown; 16 to the G-protein-coupled receptor RDC1 at 2q35– 37 17 and to a putative gene at 15q24 predicted to encode a protein with a serine/threonine-rich domain. 18 In pleomorphic adenomas of the parotid gland, HMGA2 is fused to FHIT at 3p14, the gene that is frequently disrupted in gastrointestinal tumors 19,20 and to NFIB at 9p24.1, a member of the human nuclear factor I gene family. 21 In osteosarcoma, HMGA2 is fused to the proteoglycan LUM gene at 12q22–23, 22 whereas in uterine leiomyomas fusion partners include the mitochondrial aldehyde dehydrogenase gene ALDH2 at 12q24.1, 23 the recombinational repair gene RAD51L1 at 14q23–24 24 and the cytochrome c oxidase subunit COX6C at 8q22–23. 25 Of these fusions, only HMGA2-RAD51L1 and HMGA2-LPP yield in frame chimeric transcripts. All other fusions are out of frame and predicted to be translated into truncated variants of HMGA2. The breakpoints in the gene are typically located in introns 3 or 4, with the translated truncated products predicted to retain the AT hooks but to have lost the C-terminal acidic domain. In addition to direct truncation by gene fusion, several chromosomal translocations involving HMGA2 have been shown to result in the expression of aberrantly spliced transcripts that may also encode truncated forms of HMGA2. These mRNAs are structurally similar to those formed by gene fusion, typically consisting of HMGA2 exons 1–3 fused to intronic sequences from the same gene. 8,26 These mRNA variants have been found in all the tumor types described above. 8,26–29 These findings, and the fact that there is no obvious functional relationship between the various partner genes, suggest that overexpression of the N-terminal part of HMGA2 may be the critical transforming event in tumors with 12q13 rearrange- ments, rather than the formation of particular fusion genes. This idea is supported by functional analysis using both cell lines and animal models. 30–32 However, overexpression of wild-type HMGA2 also predisposes to malignancy 33 and it has been suggested that truncated HMGA2 proteins are transforming by virtue of the fact that they activate expression of the wild-type HMGA2 allele. 34 Chromosomal translocations are common in hematological malignancies and typically generate transforming oncogenes by gene fusion or overexpression of one or more genes near the breakpoints. Despite the diversity of chromosomal transloca- tions in patients with leukemia or lymphoma, direct involve- ment of HMGA2 has only been reported in four cases, one with Richter transformation of chronic lymphocytic leukemia (CLL), 35 one with acute lymphoblastic leukemia (ALL) 27 and two with myelofibrosis with myeloid metaplasia (MM). 28 Here, we report six cases of myeloid malignancy with reciprocal translocations involving 12q13–15 and involvement of HMGA2. Received 2 August 2004; accepted 31 August 2004; Published online 16 December 2004 Correspondence: Professor NCP Cross, Wessex Regional Genetics Laboratory, Salisbury District Hospital, Salisbury SP2 8BJ, UK; Fax: þ 44 1722 338095; E-mail: ncpc@soton.ac.uk Leukemia (2005) 19, 245–252 & 2005 Nature Publishing Group All rights reserved 0887-6924/05 $30.00 www.nature.com/leu