Chromosomal translocations of HMGA2 are common in mesenchymal tumors, whereas the regulatory mechanisms of HMGA2 in malignant epithelial tumors are much more complex.
Fluorescence in situ hybridization with a BAC clone covering the 5' region produced three signals versus two signals with a BAC clone covering the 3' region including intron 3 of HMGA2, which harbors most of the breakpoints described in benign mesenchymal tumors.
The observed pattern is similar to rearrangements of HMGA2 found in several other benign mesenchymal tumors, i.e., disruption of the HMGA2 locus leaves intact exons 1-3 which encode the AT-hook domains and separates them from the 3'-terminal part of the gene.
Recent data suggest that mutations of the mediator subcomplex 12 gene (MED12) and rearrangements of the gene-encoding high-mobility group protein AT-hook 2 (HMGA2) characterize major genetic subtypes of these tumors, which, for example, differ by their average size.
To shed more light on the pathobiology of aggressive angiomyxoma and to investigate the molecular mechanisms behind the involvement of the HMGA2 gene in this tumor type (fusion transcript vs deregulated expression), we investigated, cytogenetically and with molecular techniques, one such tumor which presented a t(11;12)(q23;q15) as the sole karyotypic aberration.
The authors hypothesized that histogenetic classification of salivary duct carcinoma (SDC) could account for de novo tumors and those with morphologic or molecular evidence (pleomorphic adenoma gene 1 [PLAG1], high-mobility group AT hook 2 [HMGA2] rearrangement, amplification) of pleomorphic adenoma (PA).
Akin to the HMGI-C rearrangements observed in benign solid tumors with 12q14-15 abnormalities, the HMGI(Y) gene has been assumed to play a crucial role in tumors with 6p21 abnormalities.
Two high-mobility group (HMG) protein genes, HMGIC and HMGIY, located at 12q15 and 6p21.3, respectively, are involved in rearrangements in various mesenchymal tumors including UL.
Identification of these novel variants suggested that aberrant splicing can join chromosomal translocation and inversion as a mechanism for producing abnormal HMGIC transcripts, and that separation of the DNA binding domains of HMGIC from its acidic carboxyl-terminal regulatory domain can lead to development of benign mesenchymal tumors.
These two and several other previously reported tumors containing HMGA2 3' UTR rearrangements show breakpoints after let-7 miRNA CBS 1, which suggests that the elimination of this miRNA binding site is not critical for driving HMGA2 transcriptional upregulation.
The tumor with the 12q14-15 aberration as the sole alteration and the leiomyoma with 12q14-15 rearrangement plus deletion of the long arm of chromosome 7 were shown to express HMGIC.
In a totally benign endometrial polyp, double minute chromosomes were shown to contain an amplified and apparently nonrearranged HMGIC gene, expressed in the tumor cells, suggesting amplification of HMGIC through double minute chromosome formation as another hitherto unreported mechanism associated with the development of some mesenchymal tumors.
The presence of activated pCHK1(Ser296) coincided with prolonged G2/M block and increased tumor cell survival, which was enhanced further in the presence of HMGA2.
Previously, HMGA2 was shown to be misexpressed in a number of benign, differentiated mesenchymal tumors including lipomas, uterine leiomyomas, and pulmonary chondroid hamartomas.
Thus, intragenic rearrangements within the LHFP gene leading to its fusion to HMGIC are not a consistent finding in mesenchymal tumors with clonal aberrations of both chromosomal regions 12q13 through q15 and 13q12 through q14.
By FISH with cosmids spanning the gene encoding the high-mobility-group protein HMGIC, we were able to show a rearrangement within or close to HMGIC in all tumors with 12q14-15 abnormalities tested, in 11 tumors with an apparently normal karyotype, and in 4 tumors with complex abnormalities without cytogenetically visible alterations of chromosomes 12.