Large deletion of part of the HMGIC locus accompanying a t(3;12)(q27 approximately q28;q14 approximately q15) in a lipoma

Cell Adhesion and Transcriptional Activity - Defining the Role of the Novel Protooncogene LPP

Integrating signals from the extracellular matrix through the cell surface into the nucleus is an essential feature of metazoan life. To date, many signal transducers known as shuttle proteins have been identified to act as both a cytoskeletal and a signaling protein. Among them, the most prominent representatives are zyxin and lipoma preferred (translocation) partner (LPP). These proteins belong to the LIM domain protein family and are associated with cell migration, proliferation, and transcription. LPP was first identified in benign human lipomas and was subsequently found to be overexpressed in human malignancies such as lung carcinoma, soft tissue sarcoma, and leukemia. This review portrays LPP in the context of human neoplasia based on a study of the literature to define its important role as a novel protooncogene in carcinogenesis.

Loss of let-7 binding sites resulting from truncations of the 3' untranslated region of HMGA2 mRNA in uterine leiomyomas

A subset of uterine leiomyomas (UL) shows chromosomal rearrangements of the region 12q14 approximately q15, leading to an overexpression of the high-mobility group protein A2 gene (HMGA2). Recent studies identified microRNAs of the let-7 family as post-transcriptional regulators of HMGA2. Intragenic chromosomal breakpoints might cause truncated HMGA2 transcripts lacking part of the 3' UTR. The corresponding loss of let-7 complementary sites (LCS) located in the 3' UTR would therefore stabilize HMGA2 mRNA. The aim of this study was to check UL with rearrangements of the chromosomal region 12q14 approximately 15 for truncated HMGA2 transcripts by real-time reverse-transcription polymerase chain reaction. In 8/13 leiomyomas with aberrations of chromosomal region 12q15, the results showed the presence of the complete 3' UTR with all LCS. A differential expression with highly reduced 3' untranslated region levels was found in 5/13 myomas. In two of these, full-length transcripts were almost undetectable. Truncated transcripts were apparently predominant in roughly one-third of UL with chromosomal rearrangements affecting the HMGA2 locus, where they lead to a higher stability of its transcripts and subsequently contribute to the overexpression of the protein. The assay used is also generally suited to detect submicroscopic alterations leading to truncated transcripts of HMGA2.

Truncation and fusion of HMGA2 in lipomas with rearrangements of 5q32-->q33 and 12q14-->q15

Chromosome segment 12q13-->q15 recombines with many different chromosome bands in lipomas and at least ten recurrent translocations have been identified. The HMGA2 gene is often rearranged, but little is known about the molecular consequences at other breakpoints. Fusion genes between HMGA2 (12q14-->q15) and LPP (3q27-->q28), LHFP (13q12) and CMKOR1 (2q37) have been reported. In the present study, eight lipomas with rearrangements involving chromosome bands 12q14-->q15 and 5q32-->q33 were analyzed. In chromosome 5, five of the cases had a breakpoint in the 5' part of EBF in 5q33, while three cases had breakpoints located about 200 kb 3' of EBF. In chromosome 12, the breakpoints clustered to the region of HMGA2. Four cases had breaks within the gene and four had breaks 5' to HMGA2 where the gene BC058822 is located. Two versions of an HMGA2/EBF fusion transcript were detected in one case; one transcript was in frame and the other out of frame. Identical EBF/BC058822 fusion transcripts, seen in two cases, one of which also had the HMGA2/EBF transcript, were out of frame and resulted in truncation of EBF. Since EBF and HMGA2 have different orientations, the findings must be explained by complex aberrations including multiple breaks. The combined data indicate that the pathogenetically significant event is fusion, truncation or transcriptional activation of HMGA2, but it can not be excluded that EBF, which has been implicated in adipogenesis, contributes to the tumor development.

Expression patterns of the LPP-HMGA2 fusion transcript in pulmonary chondroid hamartomas with t(3;12)(q27 approximately 28;q14 approximately 15)

The high frequency of the t(3;12)(q27 approximately 28; q14 approximately 15) in lipomas and pulmonary chondroid hamartomas (PCHs) makes the HMGA2-LPP fusion gene the most frequent fusion gene in human tumors. We analyzed 11 PCHs with a t(3;12)(q27 approximately 28;q14 approximately 15) for the expression of the LPP-HMGA2 fusion transcript. In a previous study, all of these tumors were shown to express the HMGA2-LPP fusion transcript, composed of exons 1-3 of HMGA2 and exons 9-11 of LPP. In the present study, reverse transcriptase-polymerase chain reaction revealed the expression of the reciprocal fusion transcript in 8 of 11 cases. In all positive tumors, the reciprocal fusion transcripts had the same structure, namely, exons 1-8 of LPP and exons 4-5 of HMGA2 encoding a protein composed of the proline-rich region and the first LIM-domain of LPP and the acidic tail of HMGA2. To our knowledge, this is the first report of the expression of the LPP-HMGA2 fusion transcript in a series of PCHs. Its frequent occurrence in PCHs indicates the absence of a larger deletion of the LPP locus accompanying the translocation, such as has been described in a lipoma. Thus, based on this one finding, a role of LPP-HMGA2 in PCH should be considered.

Extensive expression studies revealed a complex alternative splicing pattern of the HMGA2 gene

Chromosomal rearrangements of the HMGA2 locus belong to the most common aberrations in human benign tumors. HMGA2 rearrangements often result in chimeric genes expressing transcripts consisting of the first three exons of HMGA2 followed by ectopic sequences derived from intron 3 of that gene. RT-PCR-based expression studies of 4 of these HMGA2 transcripts revealed a co-expression with the "wild-type" HMGA2a in tumor samples as well as in normal tissues. Northern blot hybridizations of the lipoma cell line Li-14 revealed the expression of five additional HMGA2 transcripts consisting of exons 1 to 3 but not exons 4 to 5 besides the full-length HMGA2a transcript. In silico analyses have been performed showing a high homology to well-established consensus sequences for the 3' splice acceptor site, the branch site, and poly(A) signal. Thus, it is quite obvious that the HMGA2 transcripts described herein are alternative, not aberrant, splice-products of the HMGA2 gene. It is hypothesized that HMGA2-dependent tumorigenesis is caused by a disturbed equilibrium in the co-expression of the HMGA2 splice variants leading to aberrant cell proliferation and/or malignant transformation of cells.

Expression of the HMGA2-LPP fusion transcript in only 1 of 61 karyotypically normal pulmonary chondroid hamartomas

The HMGA2 gene has been found to be rearranged in a variety of benign solid tumors. However, in all tumor entities where aberrations of the corresponding chromosomal region have been found, a large percentage of tumors do not show any detectable cytogenetic deviation. Thus, the question arises whether or not in some of these cases, small subpopulations of tumor cells characterized by HMGA2 rearrangements exist. The existence of these populations would strongly suggest a secondary nature of the chromosomal aberrations. Herein, we have addressed this question by RT-PCR analyses of the HMGA2-LPP fusion resulting from t(3;12)(q27 approximately q28;q14 approximately q15) in a series of 61 pulmonary chondroid hamartomas (PCH) with an apparently normal karyotype. As a result, the HMGA2-LPP fusion transcript was amplified in only one of 61 PCH with a normal karyotype. In this case, fluorescence in situ hybridization analysis revealed a hidden chromosomal aberration. The absence of the HMGA2-LPP fusion in small populations of tumors with a normal karyotype suggests the primary nature of chromosomal rearrangements in the development of PCH affected by those aberrations.