HMGA1 Protein Research Articles

NMR resonance assignments of the human high mobility group protein HMGA1

Human HMGA1 is a 107-residue, non-histone chromatin nuclear factor with a wide sphere of influence including embryogenesis, apoptosis, differentiation, cell proliferation, and cancer development (Reeves, 2001). Because of the repetitive nature of the three DNA-binding domains, strings of glutamic acid residues at the C-terminus, and its unstructured nature in the absence of A-T rich regions of DNA and/or other proteins, backbone assignment for HMGA1 was challenging. Especially useful was the HNN experiment (Planchal et al., 2001), a set of truncated HMGA1 constructs, and some high resolution data collected at a ¹H resonance frequency of 900 MHz. Except for absolute assignment of R60 and R86, all 82 amide were assigned to cross peaks in the ¹H-¹⁵N HSQC spectrum and many of the side chain ¹³C and ¹H resonances were assigned (BMRB code xxxx). The intensity of the amide cross peaks for residues E3 – S9 and S64 – K67 were much weaker than the other amide cross peaks in the ¹H-¹⁵N HSQC spectrum suggesting that even in this unstructured protein there are regions experiencing motion different from the molecule as a whole.

HMGA2 Rearrangement in a Case of Vulvar Aggressive Angiomyxoma

The histogenesis of aggressive angiomyxoma, an uncommon mesenchymal neoplasm of the vulvar, pelvic, and perineal soft tissues, is poorly understood, although the neoplastic cells exhibit a myofibroblastic phenotype. Cytogenetic studies of aggressive angiomyxoma are scarce; however, a nonrandom involvement of the 12q15 region where the high mobility group (HMG) protein HMGA2, an architectural transcription factor expressed primarily during embryogenesis, is located has been suggested. HMGA2 involvement has also been described in a variety of benign gynecologic mesenchymal neoplasms. We report an additional case of HMGA2 rearrangement in vulvar aggressive angiomyxoma, using fluorescence in situ hybridization.

Nuclear HMGA1 nonhistone chromatin proteins directly influence mitochondrial transcription, maintenance, and function

We have previously demonstrated that HMGA1 proteins translocate from the nucleus to mitochondria and bind to mitochondrial DNA (mtDNA) at the D-loop control region [G.A. Dement, N.R. Treff, N.S. Magnuson, V. Franceschi, R. Reeves, Dynamic mitochondrial localization of nuclear transcription factor HMGA1, Exp. Cell Res. 307 (2005) 388-401.] [11]. To elucidate possible physiological roles for such binding, we employed methods to analyze mtDNA transcription, mitochondrial maintenance, and other organelle functions in transgenic human MCF-7 cells (HA7C) induced to over-express an HA-tagged HMGA1 protein and control (parental) MCF-7 cells. Quantitative real-time (RT) PCR analyses demonstrated that mtDNA levels were reduced approximately 2-fold in HMGA1 over-expressing HA7C cells and flow cytometric analyses further revealed that mitochondrial mass was significantly reduced in these cells. Cellular ATP levels were also reduced in HA7C cells and survival studies showed an increased sensitivity to killing by 2-deoxy- d-glucose, a glycolysis-specific inhibitor. Flow cytometric analyses revealed additional mitochondrial abnormalities in HA7C cells that are consistent with a cancerous phenotype: namely, increased reactive oxygen species (ROS) and increased mitochondrial membrane potential (ΔΨ m). Additional RT-PCR analyses demonstrated that gene transcripts from both the heavy (ND2, COXI, ATP6) and light (ND6) strands of mtDNA were up-regulated approximately 3-fold in HA7C cells. Together, these mitochondrial changes are consistent with many previous reports and reveal several possible mechanisms by which HMGA1 over-expression, a common feature of naturally occurring cancers, may affect tumor progression.

E2F1 activation is responsible for pituitary adenomas induced by HMGA2 gene overexpression

The High Mobility Group protein HMGA2 is a nuclear architectural factor that plays a critical role in a wide range of biological processes including regulation of gene expression, embryogenesis and neoplastic transformation. Several studies are trying to identify the mechanisms by which HMGA2 protein is involved in each of these activities, and only recently some new significant insights are emerging from the study of transgenic and knock-out mice. Overexpression of HMGA2 gene leads to the onset of prolactin and GH-hormone induced pituitary adenomas in mice, suggesting a critical role of this protein in pituitary tumorigenesis. This was also confirmed in the human pathology by the finding that HMGA2 amplification and/or overexpression is present in human prolactinomas. This review focuses on recent data that explain the mechanism by which HMGA2 induces the development of pituitary adenomas in mice. This mechanism entails the activation of the E2F1 protein by the HMGA2-mediated displacement of HDAC1 from pRB protein.

Haploinsufficiency of the Hmga1 Gene Causes Cardiac Hypertrophy and Myelo-Lymphoproliferative Disorders in Mice

The HMGA1 protein is a major factor in chromatin architecture and gene control. It plays a critical role in neoplastic transformation. In fact, blockage of HMGA1 synthesis prevents rat thyroid cell transformation by murine transforming retroviruses, and an adenovirus carrying the HMGA1 gene in the antisense orientation induces apoptotic cell death in anaplastic human thyroid carcinoma cell lines, but not in normal thyroid cells. Moreover, both in vitro and in vivo studies have established the oncogenic role of the HMGA1 gene. In this study, to define HMGA1 function in vivo, we examined the consequences of disrupting the Hmga1 gene in mice. Both heterozygous and homozygous mice for the Hmga1-null allele show cardiac hypertrophy due to the direct role of HMGA1 on cardiomyocytic cell growth regulation. These mice also developed hematologic malignancies, including B cell lymphoma and myeloid granuloerythroblastic leukemia. The B cell expansion and the increased expression of the RAG1/2 endonuclease, observed in HMGA1-knockout spleen tissues, might be responsible for the high rate of abnormal IgH rearrangements observed in these neoplasias. Therefore, the data reported here indicate the critical role of HMGA1 in heart development and growth, and reveal an unsuspected antioncogenic potential for this gene in hematologic malignancies.

HMGA proteins in malignant peripheral nerve sheath tumor and synovial sarcoma: preferential expression of HMGA2 in malignant peripheral nerve sheath tumor

Histological separation of synovial sarcomas from malignant peripheral nerve sheath tumors can be difficult and available immunohistochemical markers sometimes give rise to overlapping staining patterns. Additional markers are needed to better define the two entities in the routine surgical pathology practice. To this end, we explored diagnostic applications of HMGA (HMGA1 and HMGA2) protein immunohistochemistry in comparable groups of synovial sarcoma and malignant peripheral nerve sheath tumors. The histological diagnosis of these cases was confirmed by the presence or absence of synovial sarcoma specific SYT-SSX fusion transcript analyzed by real-time reverse transcription polymerase chain reaction. In all, 13 malignant peripheral nerve sheath tumors and 15 synovial sarcomas were included in this study. Immunohistochemically, most malignant peripheral nerve sheath tumors expressed both HMGA1 and HMGA2 protein (12/13 and 12/13 cases, respectively) with moderate to strong nuclear staining patterns. Most cases of synovial sarcomas demonstrated variable expression of HMGA1. However, significant immunoreactivity for HMGA2 was present in the glandular component of a biphasic tumor (1/1) and rarely detected in monophasic synovial sarcomas (1/14). In summary, expression of HMGA2 is a feature of MPNST but not of synovial sarcoma and immunohistochemical staining of HMGA2 may be a useful marker to separate malignant peripheral nerve sheath tumor from synovial sarcoma.

Determination of High Mobility Group A1 (HMGA1) Expression in Hepatocellular Carcinoma: A Potential Prognostic Marker

Our objective was to investigate the expression of HMGA1 mRNA and protein in hepatocellular carcinoma (HCC) and the correlation between its expression and clinical pathological characteristics and prognosis. HMGA1 expression was determined at both the mRNA level and the protein level in 30 HCC tissues and their corresponding paracancer liver tissues (PCLTs) and 2 normal liver tissues by RT-PCR and IHC. Follow-up study was done on the 30 patients involved in this research. HMGA1 mRNA was detected in nine cases of HCC tissues and two PCLTs, for a positivity rate of 30% and 6.7%, respectively (P < 0.05), whereas no HMGA1 mRNA expression was found in normal liver tissues. Clinicopathological analysis revealed that HMGA1 mRNA expression was significantly correlated with Edmondson's grade (P < 0.05). HMGA1 protein was detected in four HCC tissues by IHC and located mainly in the nuclei; no positive staining was found in PCLTs. Follow-up study showed that HMGA1 mRNA-positive patients had a higher risk of recurrence/metastasis and a shorter survival than negative cases (P < 0.05). Our findings indicate that HMGA1 may be involved in the carcinogenesis and invasiveness of HCC and the determination of HMGA1 can be of great value in predicting the prognosis of patients with HCC.

Inhibition of Nucleotide Excision Repair by High Mobility Group Protein HMGA1

The mammalian non-histone "high mobility group" A (HMGA) proteins are the primary nuclear proteins that bind to the minor groove of AT-rich DNA. They may, therefore, influence the formation and/or repair of DNA lesions that occur in AT-rich DNA, such as cyclobutane pyrimidine dimers (CPDs) induced by UV radiation. Employing both stably transfected lines of human MCF7 cells containing tetracycline-regulated HMGA1 transgenes and primary Hs578T tumor cells, which naturally overexpress HMGA1 proteins, we have shown that cells overexpressing HMGA1a protein exhibit increased UV sensitivity. Moreover, we demonstrated that knockdown of intracellular HMGA1 concentrations via two independent methods abrogated this sensitivity. Most significantly, we observed that HMGA1a overexpression inhibited global genomic nucleotide excision repair of UV-induced CPD lesions in MCF-7 cells. Consistent with these findings in intact cells, DNA repair experiments employing Xenopus oocyte nuclear extracts and lesion-containing DNA substrates demonstrated that binding of HMGA1a markedly inhibits removal of CPDs in vitro. Furthermore, UV "photo-foot-printing" demonstrated that CPD formation within a long run of Ts (T(18)-tract) in a DNA substrate changes significantly when HMGA1 is bound prior to UV irradiation. Together, these results suggest that HMGA1 directly influences both the formation and repair of UV-induced DNA lesions in intact cells. These findings have important implications for the role that HMGA protein overexpression might play in the accumulation of mutations and genomic instabilities associated with many types of human cancers.

HMGA1 protein expression sensitizes cells to cisplatin-induced cell death

HMGA1 proteins belong to a family of nonhistone chromatin proteins able to bind DNA in AT-rich regions and to interact with various transcription factors thus enhancing or inhibiting gene transcription by acting as architectural proteins. Although their expression is very low or absent in many adult tissues, HMGA1 proteins have been frequently found to be upregulated in human cancers and are expressed at high levels during embryogenesis, suggesting they could have a role in highly proliferating cells. We have previously demonstrated that HMGA1 expression in primary breast cancer and mammary carcinoma derived cell lines inversely correlated with BRCA1 expression and that HMGA1 is able to downregulate the expression of BRCA1 gene by binding directly to its promoter region. Being BRCA1 protein expression strictly linked to the DNA repair activity of the cell, we investigated whether HMGA1 expression was able to influence cellular responses to DNA damage. Here, we report that high expression levels of HMGA1 proteins in MCF-7 or mouse embryonic stem cells results in diminished BRCA1 expression and enhanced sensitivity to Cisplatin and Bleomycin. The increased DNA damage-induced cell death in HMGA1-expressing cells is likely due to a diminished cellular DNA repair activity. Therefore, we propose that high expression of HMGA1 protein in human malignant neoplasias, acting on BRCA1 expression, could contribute to the progression of malignant transformation influencing the response of the cells to the damaged DNA.

Molecular mechanism of HMGA1 deregulation in human neuroblastoma

Very soon after their original identification in HeLa cells in 1983, HMGA proteins appeared as interesting cancer-related molecules. Indeed, they were immediately noted as a sub-class of High Mobility Group proteins induced in fibroblast or epithelial cells transformed with sarcoma viruses. After more than 20 years, the association between HMGA protein expressions and cellular transformation has been largely confirmed and HMGA are among the most widely expressed cancer-associated proteins. Nevertheless, their functional contribution to tumour development and progression is far from being completely understood. Furthermore, although HMGA1 expression has been reported to be inducible by a number of factors and circumstances, the question of how their expression is deregulated in cancer is even less clear and somehow has been ignored from most researchers. An active AP1 site is the only characterized element of the HMGA1 human promoter, that remains a rather complicated and unexplored source of information to answer this question. Following the indication that c-Myc might bind and activate the mouse HMGA1 gene promoter, we have demonstrated that HMGA1 is a new target for MYCN in human neuroblastomas. In this report, we overview part of the current information on HMGA1 and focus our attention on the analysis of its human promoter.

HMGA1 proteins in human atherosclerotic plaques

One of the major characteristics of atherosclerosis is the migration of smooth muscle cells (SMC) from the tunica media to the intima, caused by alterations in the environment, e.g. mechanical, chemical, or immunologic injuries of the arterial walls. A group of molecules that may act as a main regulator of SMC phenotype switching is formed by the so-called HMGA1 high-mobility group proteins. One target gene of the HMGA1 protein, playing a major role in the development of atherosclerotic lesions, is CD44. The expression of CD44 is regulated by IL-1 β , but binding of HMGA1 potentiates the transactivation of the CD44 promoter. In this study, the HMGA1 expression of human atherosclerotic plaque samples was examined. Compared to the non-active components, all major components of the well-developed atherosclerotic plaques showed strong positivity of the high-mobility group protein HMGA1 in their activated areas, e.g. neointimal SMCs, macrophages, newly built blood vessels. This report is the first to describe HMGA1 as one of the first mediators in the development of human atherosclerotic plaques.

HMGA1 Protein Overexpression in Human Breast Carcinomas

We measured, by immunohistochemistry, HMGA1 protein expression in 212 breast tissue specimens: 6 normal samples, 28 hyperplastic lesions (13 with cellular atypia), 11 fibroadenomas, 10 in situ ductal carcinomas, 144 ductal carcinomas, and 13 lobular carcinomas. HMGA1 was not expressed in normal breast tissue; HMGA1 staining was intense in 40% of hyperplastic lesions with cellular atypia and in 60% of ductal carcinomas and weak in fibroadenomas and in hyperplastic lesions without cellular atypia. Because HMGA1 expression was similar among ductal breast carcinomas with different histologic grading, we evaluated the association between HMGA1 expression and that of other markers of breast carcinoma invasion (estrogen and progesterone receptors, Ki-67 antigen, and ErbB2) in 21 cases of grade 3 breast ductal carcinomas and 7 cases of breast lobular carcinomas. We found that HMGA1 expression tended to be associated only with c-erbB-2 expression (Spearman rho: 0.36; P=0.065). Taken together, these results suggest that HMGA1 expression might be a novel indicator for the diagnosis and prognosis of human breast cancer.

Therapy of human pancreatic carcinoma based on suppression of HMGA1 protein synthesis in preclinical models

Pancreatic carcinoma is one of the most aggressive tumors, and, being refractory to conventional therapies, is an excellent target for new therapeutic approaches. Based on our previous finding of high HMGA1 expression in pancreatic cancer cells compared to normal pancreatic tissue, we evaluated whether suppression of HMGA1 protein expression could be a treatment option for patients affected by pancreatic cancer. Here we report that HMGA1 proteins are overexpressed in pancreatic carcinoma cell lines, and their downregulation through an adenovirus carrying the HMGA1 gene in an antisense orientation (Ad Yas-GFP) results in the death of three human pancreatic carcinoma cell lines (PANC1, Hs766T and PSN1). Pretreatment of PANC1 and PSN1 cells with Ad Yas-GFP suppressed and reduced, respectively, their ability to form xenograft tumors in nude mice. To further verify the role of HMGA1 in pancreatic tumorigenesis, we used a HMGA1 antisense phosphorothioate oligodeoxynucleotide (ODN); its addition induced a decrease in HMGA1 protein levels and a significant reduction of the proliferation rate of PANC1-, Hs766T- and PSN1-treated cells. Therefore, suppression of HMGA1 protein synthesis by an HMGA1 antisense approach seems to be a feasible treatment strategy in pancreatic carcinomas.

Meiotic progression of isolated mouse spermatocytes under simulated microgravity.

Progression through the prophase of the first meiotic division can be obtained in culture by treatment of mouse spermatocytes with the serine/threonine phosphatase inhibitor okadaic acid. Chromosome condensation during this G2/M transition involves the activation of the MAPK pathway, which causes the activation of Nek2 and the phosphorylation of the chromatin architectural protein Hmga2. In an effort to set up conditions to allow a spontaneous progression of mouse spermatocytes through meiosis, we have investigated the cell-cycle features of these cells cultured for 24 h with a rotary cell culture system in a humidified atmosphere in a thermostatic incubator to simulate a microgravity environment. Morphological analysis of nuclear squashes indicated a 2-fold increase in late-pachytene spermatocytes with highly condensed chromosomes, and a contemporaneous decrease of mid-pachytene cells with less condensed chromatin. Microgravity induced a 2-fold activation of the cyclinB-cdc2 complex, confirming at the molecular level that cell-cycle progression had occurred. Moreover, using immuno-kinase assays with specific substrates we have demonstrated that the meiotic progression obtained under microgravity conditions is accompanied by activation of the Erk1/p90Rsk2 pathway. These data indicated that activation of the MAPK pathway correlates with chromatin condensation even under conditions in which meiotic progression occurs spontaneously and is not induced by a drug. We suggest that culture under microgravity conditions might help to release the block that inhibits isolated spermatocytes from progressing through prophase at unit gravity, and to study the physiological events of germ cell differentiation in vitro.

The functional interaction between HMGA1 and the estrogen receptor requires either the N- or the C-terminal domain of the receptor

We have previously shown that HMGA1 enhances the transcriptional activity of promoters containing the estrogen response element (ERE) and increases binding of the estrogen receptor (ER) to ERE. Herein, we have assessed the transcriptional activity and ERE-binding ability of deleted ER fragments in absence or in presence of HMGA1. The HMGA1 protein stimulated binding and transcriptional activity by a factor of about 2-fold compared to the wild-type ER and both the N- and C-terminal ER deleted domains, but had no effect when both domains were deleted. These data show that HMGA1 cooperates with either the N- or the C-terminal transcriptional activation domain of the ER.

Phosphorylation of high-mobility group protein A2 by Nek2 kinase during the first meiotic division in mouse spermatocytes.

The mitogen-activated protein kinase (MAPK) pathway is required for maintaining the chromatin condensed during the two meiotic divisions and to avoid a second round of DNA duplication. However, molecular targets of the MAPK pathway on chromatin have not yet been identified. Here, we show that the architectural chromatin protein HMGA2 is highly expressed in male meiotic cells. Furthermore, Nek2, a serine-threonine kinase activated by the MAPK pathway in mouse pachytene spermatocytes, directly interacts with HMGA2 in vitro and in mouse spermatocytes. The interaction does not depend on the activity of Nek2 and seems constitutive. On progression from pachytene to metaphase, Nek2 is activated and HMGA2 is phosphorylated in an MAPK-dependent manner. We also show that Nek2 phosphorylates in vitro HMGA2 and that this phosphorylation decreases the affinity of HMGA2 for DNA and might favor its release from the chromatin. Indeed, we find that most HMGA2 associates with chromatin in mouse pachytene spermatocytes, whereas it is excluded from the chromatin upon the G2/M progression. Because hmga2-/- mice are sterile and show a dramatic impairment of spermatogenesis, it is possible that the functional interaction between HMGA2 and Nek2 plays a crucial role in the correct process of chromatin condensation in meiosis.

High mobility group A1 is expressed in metastatic adenocarcinoma to the liver and intrahepatic cholangiocarcinoma, but not in hepatocellular carcinoma: its potential use in the diagnosis of liver neoplasms.

An increased level of high mobility group A (HMGA) gene/protein expression has been demonstrated to be associated with many human neoplasms originating from a variety of tissues. However, HMGA1 expression has not yet been studied in hepatic tumors. In this study, we analyzed HMGA1 expression in hepatic primary and metastatic tumors in order to verify whether determination of the HMGA1 expression level could provide any diagnostic advantages in the pathological diagnosis of hepatic tumors. Twenty samples of hepatocellular carcinoma, 5 samples of intrahepatic cholangiocarcinoma, and 21 samples of metastatic adenocarcinoma to the liver (15 metastatic tumors from colorectal carcinoma and 6 metastatic tumors from pancreatic carcinoma) were analyzed immunohistochemically using an HMGA1-specific antibody. While no significant nuclear immunoreactivity was found in hepatocytes of non-neoplastic liver tissue, 40% (2/5) of intrahepatic cholangiocarcinomas, 53.3% (8/15) of metastatic lesions from colorectal carcinoma, and 100% (6/6) of metastatic lesions from pancreatic carcinoma showed positive immunoreactivity. In contrast, all 20 samples of hepatocellular carcinoma were negative for HMGA1 nuclear immunoreactivity. Thus, hepatocellular carcinoma represents the first case of malignant neoplasia in which HMGA1 expression is not induced, which presents a striking contrast to several previous studies demonstrating the significance of increased HMGA gene/protein levels in carcinogenesis and/or tumor progression. Based on these findings, we conclude that the HMGA1 protein level could serve as a potential diagnostic marker that may enable the differential diagnosis between hepatocellular carcinoma and intrahepatic cholangiocarcinoma or metastatic adenocarcinoma to the liver.

High mobility group protein HMGA1 expression in breast cancer reveals a positive correlation with tumour grade.

Members of the HMGA protein (high mobility group protein A) family act as master switches of the chromatin structure by bending DNA and thus modulating the formation of transcription factor complexes of a number of target genes. Accordingly, HMGA proteins have been shown to be associated with the development and/or progression of a variety of benign and malignant tumours. Nevertheless, the HMGA1 expression studies published so far have not included primary breast cancer samples. In this study we have investigated the HMGA1 expression patterns in a series of 170 breast cancer samples by immunohistochemistry. We have found a strong variation in HMGA1 expression between the tumours. Based on an immunoreactive score (IRS) 14.1% of the tumour samples were scored to IRS 8-12 (strong positivity for HMGA1), 24.7% were scored to IRS 4-6 (moderate positivity), 25.3% were scored to IRS 1-3 (weak positivity), and 35.9% showed no positivity at all. Immunoreaction could be detected in all histological types of breast cancers analysed with the exception of invasive papillary and cribriform carcinoma. Statistical analysis revealed a strong correlation between tumour grade and HMGA1 expression (rs=0.3516, p<0.0001). Thus, the HMGA1 expression level can be considered a potential prognostic marker for breast cancer.

A truncated HMGA1 gene induces proliferation of the 3T3-L1 pre-adipocytic cells: a model of human lipomas.

The high mobility group A (HMGA) proteins are non-histone chromosomal proteins implicated in the organization of chromatin structure and in the assembly of protein complexes on the promoters of several inducible genes. Rearrangements of HMGA1 and HMGA2 genes, consequent to chromosomal translocation, have been frequently detected in human benign tumours of mesenchymal origin including lipomas. We have demonstrated previously that 3T3-L1 adipocytic differentiation is associated with increased HMGA1 protein levels, and that the block of HMGA1 synthesis dramatically increases the growth rate of 3T3-L1 cells and suppresses adipocytic differentiation. Here we have examined the role of a truncated HMGA1 gene in adipocytic cell growth. We have found that expression of the truncated Hmga1 gene (Hmga1/T) dramatically increases 3T3-L1 cell growth without blocking adipocytic differentiation. The Hmga1/T 3T3-L1 cells had higher E2F activity than the wild-type cells, and a deregulated cell cycle. In fact, the Hmga1/T cells had a reduced G0/G1 fraction, and a greater number of cells in S-phase. However, consistent with the benign nature of tumours associated with HMGA1 rearrangements, the Hmga1/T 3T3-L1 cells did not acquire the malignant phenotype. These results suggest a critical role played by HMGA1 rearrangements in the generation of human lipomas.

HMGA2 is expressed in an allele-specific manner in human lipomas

The architectural transcription factor HMGA2 is almost exclusively expressed in undifferentiated mesenchymal cells. Interestingly, it has been mapped to the translocation site in a variety of human mesenchymal tumors that reveal a terminally differentiated phenotype. The expression of chimeric HMGA2 transcripts encoding three DNA-binding domains fused to novel transcriptional regulatory domains was previously described in lipomas. In this study with lipoma ST91-198, we report the expression of truncated HMGA2 transcripts that gained no functional domains. The highly polymorphic region in the 5′ untranslated region (UTR) of HMGA2 was used to determine the allele-specific expression of HMGA2 in lipomas. Microsatellite PCR revealed a monoallelic expression pattern, and only the translocated allele was expressed when the DNA-binding domains of the rearranged allele were fused with transcription activation domains. Surprisingly, a diallelic expression pattern of HMGA2 was observed in lipoma ST91-198, and the wild-type allele was also expressed. In conjunction with studies involving rearrangements of HMGA genes in other benign mesenchymal tumors, our results support a model in which the expression of the wild-type HMGA allele is critical for the pathogenesis of mesenchymal tumors and in which rearrangements of HMGA do not lead to a gain of function in the chimeric HMGA protein.