Microsatellite deletion mapping on chromosome 10q and mutation analysis of MMAC1, FAS, and MXI1 in human glioblastoma multiforme.

Glioblastoma multiforme (GBM) is an end-stage brain tumor of glial origin. Allelic deletions encompassing all or part of chromosome 10q occur frequently in GBMs, indicating that loss of one or more tumor suppressor genes on 10q plays a role in GBM formation. One of these genes is MMAC1 (PTEN), a gene on 10q23 which encodes a dual-specificity protein phosphatase. We carried out a loss of heterozygosity (LOH) analysis of 66 GBM patients using microsatellite markers for 27 loci on 10q. Overall, LOH was detected in 70% of cases, most showing LOH with every informative marker. Eleven patients showed partial 10q deletions, the smallest spanning a 35 cM region distal to D10S187. Sequence analysis of the MMAC1 gene in 45 of these tumors revealed mutations in eleven cases (24%), all with LOH on 10q. None of these mutations was present in normal DNA from the same patients. In addition, we utilized SSCP analysis to test two other candidate genes on 10q: FAS, a cell surface receptor which transduces an apoptotic, cell death signal and MXI1, a transcriptional repressor. The absence of mutations in these genes suggested that FAS and MXI1 are not likely to be tumor suppressor genes physiologically relevant to GBM. These data do support a significant role for MMAC1 in GBM.

MMAC1/PTEN mutations in primary tumor specimens and tumor cell lines.

A candidate tumor suppressor gene, MMAC1/PTEN, located in human chromosome band 10q23, was recently identified based on sequence alterations observed in several glioma, breast, prostate, and kidney tumor specimens or cell lines. To further investigate the mutational profile of this gene in human cancers, we examined a large set of human tumor specimens and cancer cell lines of many types for 10q23 allelic losses and MMAC1 sequence alterations. Loss of heterozygosity (LOH) at the MMAC1 locus was observed in approximately one-half of the samples examined, consistent with the high frequency of 10q allelic loss reported for many cancers. Of 124 tumor specimens exhibiting LOH that have been screened for MMAC1 alterations to date, we have detected variants in 13 (approximately 10%) of these primary tumors; the highest frequency of variants was found in glioblastoma specimens (approximately 23%). Novel alterations identified in this gene include a missense variant in a melanoma sample and a splicing variant and a nonsense mutation in pediatric glioblastomas. Of 76 tumor cell lines prescreened for probable LOH, microsequence alterations of MMAC1 were detected in 12 (approximately 16%) of the lines, including those derived from astrocytoma, leukemia, and melanoma tumors, as well as bladder, breast, lung, prostate, submaxillary gland, and testis carcinomas. In addition, in this set of tumor cell lines, we detected 11 (approximately 14%) homozygous deletions that eliminated coding portions of MMAC1, a class of abnormality not detected by our methods in primary tumors. These data support the occurrence of inactivating MMAC1 alterations in multiple human cancer types. In addition, we report the discovery of a putative pseudogene of MMAC1 localized on chromosome 9.