The concept of tumor suppressor gene dysregulation is a central theme in cancer biology. A powerful method for identifying potential tumor suppressor genes is loss of heterozygosity (LOH) analysis. Several recent studies have demonstrated an area of LOH on chromosome 6 band q22 in T-cell leukemias, Non-Hodgkin's lymphomas refractory to chemotherapy, breast cancer and melanoma suggesting the presence of a tumor suppressor gene at this locus. Using a c-myb specific probe, we have previously cloned a rearrangement from this region, designated myb recombination region (MRR). In recent work, we have demonstrated that MRR1 is the human homolog of the recently described Ahi-1 (Abelson helper integration) gene altered by viral insertion in a variety of murine T and B cell neoplasms. Based on a prototype human cDNA clone, MRR is a novel nuclear WD-protein with an associated SH3 domain. Genomic analysis reveals MRR to be a 250 kB gene containing 28 primary exons in addition to multiple alternative exons and at least two polyadenylation sites. Analysis of MRR expression by RT-PCR reveals evidence of multiple alternatively spliced transcript forms in normal human tissues and cell lines. In normal human tissues, maximum expression is seen in brain, testis and ovary with variable expression in a variety of other tissues. Expression in human tumor cell lines reveals variable exprssion across given tumor types. Southern blotting experiments using a panel of exon-specific probes demonstrates a minimal frequency of gross structural alterations of MRR in a variety of human leukemia and lymphoma cell lines. However, additional alterations have been identified including a frame shift mutation in a cDNA from a uterine sarcoma and an aberrant mRNA species in a non-small cell lung cancer harboring a t1:6 translocation. Further analysis of ests from a variety of CGAP tumors reveals truncations and other message alterations in prostate and other tumor types. In our work in human T-cell leukemia cell lines, consistent alterations via aberrant splicing have been identified in a number of cell lines. These results together indicate that an important and novel tumor suppressor gene has been isolated that resides in a region of chromosome 6q that is the site of frequent alteration in a wide range of tumor types. In addition to analysis of expression and alterations in tumor cell types, work has begun on the expression of the MRR1 proteins using a panel of three peptide specific anti sera. Work to date demonstrates both nuclear and cytoplasic anti-peptide antibody specific forms of MRR1 protein with a spectrum of molecular weights. This is consistent with the finding of multiple mRNA forms produced based on RT-PCR data. Future work will focus on the function of this gene product and its' alternatively spliced forms in control of normal and tumor cell proliferation/ differentiation.
