Evaluation of MDM2 and CDK4 gene amplification and expression as potential tools for the diagnosis of atypical lipomatous tumors/well-differentiated liposarcomas in paraffin-embedded material

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Evaluation of MDM2 and CDK4 gene amplification and expression as potential tools for the diagnosis of atypical lipomatous tumors/well-differentiated liposarcomas in paraffin-embedded materialMeulemans EV¹, Speel EJM^1,2, Rüland AM¹, Debiec-Rychter M³, Sciot R⁴, Pauwels P¹Department of Pathology¹, University Hospital Maastricht, and ²Department of Molecular Cell Biology, Research Institute Growth and Development (GROW), University of Maastricht, the Netherlands, and Departments of ³Cytogenetics and ⁴Pathology, Leuven University Hospital, BelgiumThe distinction between lipomas and atypical lipomatous tumours/well differentiated lipo-sarcomas (WDLs) is difficult to make based on histology. Lipomas are cytogenetically characterized by a variety of balanced rearrangements often including the chromosome 12q13-15 region, whereas in WDLs supernumerary ring and giant marker chromosomes have been reported containing amplified 12q sequences. The MDM2 and CDK4 genes, located within the 12q13-15 region, are important candidates for amplification, which may result in protein over expression.The goal of this study was to determine if over expression of MDM2 and/or CDK4 as well as gene amplification can be used as (a) diagnostic marker(s) to solve differential diagnostic problems between a lipoma and WDL.We analyzed paraffin-embedded material of a series of 31 lipomas, 19 WDLs, and 3 unclassified lesions for MDM2 and CDK4 protein accumulation by routine immunohistochemistry and for MDM2 and CDK4 gene amplification by multicolor FISH using gene-specific and centromere 12-specific probes.Six of the 31 lipomas showed MDM2 protein accumulation, particularly in areas with fat necrosis, whereas CDK4 immunostaining was negative. All lipomas harbored two copies of the chromosome 12 loci per cell nucleus. The 19 WDLs were also disomic for centromere 12, but in 11 cases both MDM2 and CDK4 amplification was detected. Of these tumors, 10 exhibited MDM2 and 9 CDK4 over expression. Accumulation of these proteins was also observed in 5 and 2 of the remaining 8 WDLs, respectively, and again fat necrosis correlated with MDM2 immunostaining. In the 3 unclassified lesions, 2 harbored amplification of both genes associated with protein over expression, implicating that these cases most probably are WDLs. The third case only showed MDM2 protein accumulation and no amplified loci.In conclusion, strong immunostaining of both MDM2 and CDK4 proteins is compatible with amplification of both gene loci in WDLs. In contrast, lipomas show no gene amplification and predominantly no protein expression. Because MDM2 protein expression, however, is often up regulated in case of inflammatory infiltrates present in the tumor, gene amplification detection by FISH or CDK4 immunostaining is indicated in the differential diagnosis of lipoma and WDL. In addition, a subset of WDLs shows no MDM2 and CDK4 amplification, suggesting other chromosome loci to be involved in tumor genesis.