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probe A is conserved in hamster DNA and detects a fragment of about 7 kb (Fig. 3A, lane
A3). Therefore, DNA from 14010 shows hybridization with two fragments of about 7 kb,
the 4p+ breakpoint fragment and the hamster homologue (Fig. 3A, lane 14010). The 7 kb
fragments observed in lanes 6A and 9B and the 20 kb fragment in PgMe25NU are also the
result of hybridization with conserved hamster and mouse sequences, respectively (Fig. 3A).
Oenomic probe B, located at the centromeric site of the 8 kb fragment, detects in
meningioma 32 DNA the normal 8 kb band and a 2.5 kb band (Fig. 3B, lane MN32). The
2.5 kb band is also found in DNA from hybrid 6A indicating that this band is produced by
the translocation breakpoint (Fig. 3B, lane 6A). These results prove that the translocation
breakpoint is located between probe A and B (Fig. 3, physical map). The localization of the
MNl gene relative to the t(4;22) in meningioma 32 was investigated with cDNA probe C,
which is part of the 5' exon and is located between the genomic probes A and B. Figure 3C
shows the EcoRI restriction fragments which hybridize with cDNA probe C. This probe
recognizes both products of the reciprocal t(4;22), as the 2.5 kb fragment is derived from
the 22q- marker (Fig. 3C, compare lanes MN32 and 6A) and the 7.0 kb fragment is the
result of the hybridization with the 4p+ marker (Fig. 3C, compare lanes MN32 and 140-
10). These results show that the 5' exon of the MNI gene is disrupted by the translocation
in meningioma 32. Figure 3C also shows weakly hybridizing fragments of approximately 6
and 7.5 kb (lanes human, MN32). This is the result of hybridization with homologous human
sequences that are not located on chromosome 22. In PgMe25NU, a hybrid ceHline with
only human chromosome 22 in a mouse background, these fragments were not observed (Fig
3C, lane PgMe25NU). cDNA probe C detects in both hamster and mouse DNA an EcoRI
fragment of 8.5 kb, suggesting that this DNA fragment is highly conserved (Fig 3C, lanes
A3, mouse). In the hybridizations shown in figure 3 it is obvious that the normal 8 kb EcoRI
band is underrepresented in tumor DNA (lane MN32). The normal copy of the MNI gene
in tumor 32 is located on a der(22)t( I ;22)(p II ;q II) with the breakpoint on chromosome 22
between cos 76A4 and the NF2 gene. The underrepresentation of this chromosome in MN32
DNA preparations was also observed when we used RFLP probes for the more proximal
region of chromosome 22. However, in in situ hybridizations with cosmids from the MNI
gene region on MN32 metaphase spreads, no deviation from the contribution of both marker
chromosomes 22 was observed (results not shown). Therefore, we must conclude that the
apparent underrepresentation of the der(22)t( I ;22) in DNA preparations is an artefact and is
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