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translocation (Chapter V). These hybrid cell lines were subsequently used to map the
breakpoint with probes from the relevant region of chromosome 22. A combination of pulsed
field analysis and chromosome walking resulted in both the recognition and crossing of this
breakpoint on chromosome 22 respectively. From this region a gene, designated MNl, was
isolated that was disrupted by the translocation (Chapter VI). Northern blot analysis of this
gene in meningiomas showed a very irregular expression pattern. Some, including
meningioma 32, showed no expression and others a relatively high expression level. This
might suggest that the MNl gene acts as a tumor suppressor gene at least in the meningiomas
without expression. However, on the other copy of chromosome 22 in meningioma 32 the
MNl gene was intact as judged from Southern blotting and because the breakpoint of the
der(22)t(I;22)(pll;q II) was mapped just distal to this gene. Southern blot analysis of DNA
isolated from 71 meningiomas with probes from the region spanning the MNl gene revealed
alternative fragments in one case that will be discussed below. The MNl gene is highly
conserved in evolution, though sequence analysis and data base searches have not yet
provided clues to its function. The nucleotide sequence of the 8 kb MNI mRNA would
suggest 2 distinct and physically separated open reading frames (ORF). Computer analysis
of these sequences (GRAIL program) suggests that they are excellent exon candidates.
Preliminary in vitro transcriptionltransiation experiments suggest that only one large protein
is formed. We speculate that both identified ORFs are part of this large protein and that the
sequence in between still contains some sequence artifacts. The 5' leader sequence of the
MNI transcript is very GC-rich and contains 4 AUG codons upstream of the putative
translational start site, both features suggest that this gene is poorly translated. These
characteristics are also found in mRNAs for proto-oncogenes, transcription factors, receptors
and signal transduction components. This suggests that such leader sequences are important
in the regulation of expression at the level of translation.
In another patient (patient 55) who suffered from multiple meningiomas, which suggests a
genetic predisposition, we observed a germ-line deletion and point mutation close to the MNl
gene on chromosome 22. The other copy of chromosome 22 was lost in all four investigated
tumors (Chapter VII). This might suggest that the constitutional mutations and the loss of
chromosome 22 inactivate the postulated predisposing gene, which could be the MNI gene.
Although the position of the germ-line alterations are very suggestive we found a high
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