High frequency of mitochondrial DNA mutations in glioblastoma multiforme identified by direct sequence comparison to blood samples

In an earlier study, we showed that heteroplasmy in the mitochondrial genom e of gliomas sometimes occurs in a D-loop polycytosine tract. We extended t his study by pairwise comparisons between glioma samples and adjacent brain tissue of 55 patients (50 glioblastomas, I astrocytoma WHO grade III, 4 as trocytomas WHO grade II). We used a combination of laser microdissection an d PCR to detect and quantify variations in the polycytosine tract, New leng th variants undetectable in the adjacent brain tissue were observed in 5 gl ioblastomas (9%), In 2 of these cases, samples from a lower tumor stage (WH O grade II) could be analyzed and revealed the early occurrence of these mu tations in both cares. Since the mitochondrial D-loop contains additional r epeats and highly polymorphic non-coding sequences, we compared 17 glioblas tomas with the corresponding blood samples of the same patients by direct s equencing of the complete D-loop, In 6 of these tumors (35%), instability w as detected in I or 2 of 3 repeat regions; in I of these repeats, the insta bility was linked to a germline T-to-C transition. Furthermore, of 2 tumors (12%) 1 carried I and the other 9 additional transitions, in the latter pa tient, 6.7 kb of the protein coding mtDNA sequence were analyzed, Six silen t transitions and 2 missense mutations (transitions) were found. All base s ubstitutions appeared to be homoplasmic upon sequencing, and 89% occurred a t known polymorphic sites in humans, Our data suggest that the same mechani sms that generate inherited mtDNA polymorphisms are strongly enhanced in gl iomas and produce somatic mutations. (C) 2001 Wiley-Liss, Inc.