Expression of HMGI-C and HMGI(Y) in ordinary lipoma and atypical lipomatous tumors: immunohistochemical reactivity correlates with karyotypic alterations.

The high mobility group proteins (HMGs) are a class of low molecular weight, nonhistone, nuclear proteins that bind DNA and function as transcription cofactors. This class includes the HMGI family members HMGI-C and HMGI(Y). Both are not significantly expressed in differentiated adult tissues, including fat, but their expression is induced in proliferating and transformed cells. Their involvement in the development of lipomatous tumors has been recently demonstrated for HMGI-C, which is encoded by a gene located at 12q15, the chromosomal segment often rearranged in ordinary lipomas. The same chromosomal segment is consistently amplified in the ring and giant marker chromosomes of atypical lipomatous tumors (ALTs), a term used to designate tumors previously labeled as well differentiated liposarcomas or atypical lipomas. The involvement of HMGI(Y) is strongly suspected as the gene coding for HMGI(Y) is located at 6p21, a chromosomal segment rearranged in a subset of ordinary lipomas. HMGI-C or HMGI(Y) protein expression was analyzed immunohistochemically in a group of 39 well differentiated adipose neoplasms (19 lipomas and 20 ALTs) of known karyotype using polyclonal antibodies raised against a recombinant protein (HMGI-C) and against a synthetic peptide (HMGI(Y)). The results of this study demonstrate that HMGI proteins are commonly expressed in well differentiated adipose neoplasms. Seventeen of twenty ALTS (85.0%), all of which had ring or giant marker chromosomes with amplification of 12q13-15, strongly expressed HMGI-C. HMGI-C expression was detected in 7 of 11 ordinary lipomas (63.6%) with alterations at 12q14-15 and in one case with an abnormal karyotype that included double minute chromosomes. HMGI-C immunoreactivity correlates with 12q13-15 chromosomal alterations (P = 0.001). HMGI(Y) reactivity was demonstrated in only two ordinary lipomas: one with 6p21 rearrangement and one with normal karyotype. No significant HMGI(Y) expression was found in the ALT group. The finding of aberrant expression of HMGI proteins in well differentiated adipose neoplasms in association with 12q13-15 and 6p21 chromosomal changes supports the proposed pathogenetic role of this group of proteins in the development of adipose tissue tumors.