Tight association of loss of merlin expression with loss of heterozygosity at chromosome 22q in sporadic meningiomas

Mutations of NF2, the gene for neurofibromatosis 2, are detected in 20-30% of sporadic meningiomas, and almost all mutations Lead to loss of merlin expression. However, loss of heterozygosity (LOH) at chromosome 22q Is found at a much higher frequency, up to 50-70%, and the possibility of another tumor suppressor gene in this region has not been excluded. Furthermore, a recent report proposed that abnormal activation of a protease mu-calpain can be an alternative pathway for merlin loss in meningiomas and schwannomas. To determine the correlation of merlin loss with NF2 genetic alteration or mu-calpain activation, we performed a molecular genetic analysis of 50 sporadic meningiomas and also examined the expression status of merlin and active form mu-calpain. LOH assay of five microsatellite markers franking NF2 revealed LOH in 22 cases, and single-strand conformation polymorphism assay detected six frameshift mutations, two splicing mutations, one nonsense mutation, and one missense mutation, all accompanied by 22q LOH. In addition, a multiplex PCR assay indicated homozygous deletion of NF2 in two cases. Interestingly, a marked decrease of merlin expression was seen exclusively in the 22 cases with 22q LOH. Activated mu-calpain expression was observed in 28 cases at various levels but showed no correlation with merlin status. These data strongly support the notion that NF2 is the sole target of 22q LOH in meningiomas and that loss of merlin expression is always caused by genetic alteration of NF2, following the classic "two hit" theory.