The 10q25-26 region between the dinucleotide markers D10S587 and D10S216 is deleted in glioblastomas and, as we have recently shown, in low-grade oligodendrogliomas. We further re®ned somatic mapping on 10q23-tel and simultaneously assessed the role of the candidate tumor suppressor gene PTEN/MMAC1 in glial neoplasms by sequence analysis of eight low-grade and 24 high-grade gliomas. These tumors were selected for partial or complete loss of chromosome 10 based on deletion mapping with increased microsatellite marker density at 10q23-tel. Three out of eight (38%) low-grade and 3/24 (13%) high-grade gliomas exclusively target 10q25-26. We did not ®nd a tumor only targeting 10q23.3, and most tumors (23/32, 72%) showed large deletions on 10q including both regions. The sequence analysis of PTEN/MMAC1 revealed nucleotide alterations in 1/8 (12.5%) low-grade gliomas in a tumor with LOH at 10q21-qtel and in 5/21 (24%) high-grade gliomas displaying LOH that always included 10q23-26. Our re®ned mapping data point to the 10q25-26 region as the primary target on 10q, an area that also harbors the DMBT1 candidate tumor suppressor gene. The fact that we ®nd hemizygous deletions at 10q25-qtel in low-grade astrocytomas and oligodendrogliomas ± two histologically distinct entities of gliomas ± suggests the existence of a putative suppressor gene involved early in glial tumorigenesis.

It is important to understand how low grade tumors recur and progress to malignant lesions since this dramatically shortens patient survival. Here, we evaluated the concept that malignant progression and poor prognosis of low grade astrocytic tumors are TP53 dependent through clonal expansion of mutated cells. TP53 status was established in primary and recurrent tumors from 36 patients with WHO grade II astrocytic tumors and two tumor types were found. Tumors from 14 patients (39%; type 1) had TP53 mutated cells, and 92% of these recurred with 57% progressing to malignancy. The evolution of TP53 mutated cells before and after progression was examined using a clonal analysis procedure in yeast. Malignant progression was accompanied by an increased percentage of mutant TP53 (red) yeast colonies resulting from monoclonal expansion of cells with mutated TP53. The presence of TP53 mutations in WHO grade II astrocytic tumors was associated with malignant progression (P=0.034, w 2 test) and shorter progression-free survival (PFS; 47.6+9.6 months for TP53-mutated tumors vs 67.8+8.2 months for TP53-wild type tumors, P50.05, log-rank test). Tumors from 22 patients (61%; type 2) were without TP53 mutations, and 64% of these recurred without a change in TP53 status, although 41% progressed to malignancy. This suggests that TP53 mutation is not an initiating or progression event in the majority of low grade astrocytic tumors. Our study also indicates that irradiation for WHO grade II astrocytic tumors might be associated with poor outcome (P50.0001) and this was independent of TP53 status. These ®ndings have important implications in the clinical management of patients with low grade astocytoma and provide new support to the clonal evolution model for tumor progression.