An intragenic rearrangement of HMGA2 is not necessary for lipoma formation

Translocation (Y;12) in lipoma

Lipomas are the most common benign mesenchymal neoplasm in adults, and have been extensively characterized at the cytogenetic level. Chromosomal aberrations have been observed in the majority of lipomas, two-thirds of which involve chromosomal region 12q14.3. To date, structural rearrangements have been reported affecting every chromosome except chromosome Y. Here we report a case of a lipoma that shows a novel apparently balanced translocation involving chromosomes Y and 12. Fluorescence in situ hybridization using a break-apart HMGA2 in-house probe set detected a single signal on the normal chromosome 12 but not on either the derivative chromosome Y or 12, indicating a cryptic loss of 12q14.3, where HMGA2 is mapped. Immunohistochemical studies, however, revealed overexpression of HMGA2 with nuclear expression in the majority of tumor cells, whereas MDM2 and CDK4 were negative. The overexpression of HMGA2 may be caused by a cryptic chromosomal aberration affecting either the cytogenetically unaltered HMGA2 allele or HMGA2 regulators elsewhere. The current case broadens our knowledge about the translocation partners of HMGA2 in lipomas and highlights the biological complexity in regulating HMGA2 expression.

HMGA2 is the partner of MDM2 in well-differentiated and dedifferentiated liposarcomas whereas CDK4 belongs to a distinct inconsistent amplicon

Data concerning the fine structure of the 12q13-15 amplicon which contains MDM2 and CDK4 in well-differentiated and dedifferentiated liposarcomas (WDLPS/DDLPS) are scarce. We investigated a series of 38 WDLPS/DDLPS using fluorescence in situ hybridization analysis with 17 probes encompassing the 12q13-15 region. In addition, using quantitative RT-PCR we studied the expression of MDM2, CDK4, DDIT3 (CHOP/GADD153), DYRK2, HMGA2, TSPAN31 and YEATS4 (GAS41) in 11 cases. We showed that CDK4 (12q14.1) belonged to a distinct amplicon than MDM2 (12q15). There was no continuity in the amplified sequences between MDM2 and CDK4. Moreover, while MDM2 was amplified and overexpressed in all cases, CDK4 was not amplified or overexpressed in 13% of cases. The centromeric border of the CDK4 amplicon was located immediately downstream the 5' end of DDIT3, a gene known for being involved in myxoid liposarcoma translocations. DDIT3 was amplified in 3 cases and overexpressed in 9 cases. The overexpression of DDIT3 was correlated to the CDK4 amplification and not to its own amplification status. This suggested that the CDK4 amplicon, as well as the overexpression of DDIT3, might be generated by the disruption of a fragile region in 5' DDIT3. HMGA2 was always amplified and rearranged indicating that it plays a central role in WDLPS/DDLPS. HMGA2 rearrangement frequently resulted in a loss of the 3' end region that is a binding site for let-7. We also found a frequent amplification and overexpression of YEATS4, an oncogene that inactivates P53, suggesting that YEATS4 might play an important role together with MDM2 in WDLPS/DDLPS oncogenesis.

Gains and complex rearrangements of the 12q13-15 chromosomal region in ordinary lipomas: the "missing link" between lipomas and liposarcomas?

Well-differentiated liposarcomas (WDLPS) classically contain high-level amplification of 12q14-15 sequences, including the MDM2 and CDK4 genes, while lipomas are characterized by simple structural chromosome aberrations often involving HMGA2 at 12q15. Previous studies have shown that low-level gain of the 12q14-15 region, such as trisomy 12 and 12q15-24 duplication, might be sufficient for the development of minimal atypia and formation of WDLPS. Moreover, because some features, such as overexpression of HMGA2, are shared by both lipomas and WDLPS, it has been hypothesized that lipomas and WDLPS may form a genetic and morphological continuum. We report here the results of molecular cytogenetic analysis of 8 lipomas that had unusual chromosomal features resulting in gains of 12q14-15. While 3 cases had simple numerical rearrangements (trisomy 12) or structural rearrangements (unbalanced translocations with 12q gains), 5 cases were particularly intriguing because of peculiar features such as giant chromosomes, supernumerary chromosomes or neocentromeres that usually are the hallmark of WDLPS. Gain of 12q14-15 sequences including extra copies of MDM2 and CDK4 were detected by fluorescence in situ hybridization analysis in all analyzed cases but no expression of MDM2 and CDK4 was observed suggesting that these genomic imbalances had no functional consequence. We observed rearrangements of HMGA2 in 5 out 8 cases. Altogether, our results indicate that moderate gains of 12q are not always associated with a malignant phenotype, and that some intermediary forms exist between classical lipomas and classical WDLPS. Some of these intermediary forms may correspond to a genomic premalignant condition while some may have no malignant potential.