Previously, we have mimicked human neurofibromatosis type 2 (NF2) in conditional Nf2 mutant (P0Cre;Nf2 flox2/flox2 ) mice. Schwannomas, characteristic for NF2, were found at low frequency in older mice. Here, we report that these mice, upon additional hemizygosity for p53, rapidly develop multiple tumours showing features consistent with malignant peripheral nerve sheath tumours. Thus, p53 hemizygosity promotes tumorigenesis of mutant Nf2 peripheral nerve cells. In contrast, young P0Cre;Nf2 flox2/ þ ;p53 þ /À cis mice mainly succumb to Nf2/p53-related osteogenic tumours. Therefore, Cre-mediated early biallelic loss of Nf2 function in neural crest-derived cells hemizygous for p53 results in resistance to osteogenic tumours and increased susceptibility to peripheral nerve sheath tumours. Keywords: MPNST; neural crest; P0 promoter; conditional knockout mice; compensatory signalling Introduction Vestibular schwannomas are benign neural sheath tumours of the vestibular branch of the eighth cranial nerve. They occur either as bilateral tumours, the hallmark of the human hereditary cancer syndrome neurofibromatosis type 2 (NF2), or as sporadic unilateral tumours. Loss or mutational inactivation of both alleles of the NF2 gene in the Schwann cell is the ratelimiting step in neoplastic transformation. NF2 patients carrying one mutant NF2 allele are predisposed not only to bilateral vestibular schwannomas but also to schwannomas of other cranial, spinal and cutaneous nerves, to cranial and spinal meningiomas, as well as to presenile lens opacities and cerebral calcifications (McKusick, 1994). Loss of NF2 function is also frequently found in mesotheliomas (Bianchi et al., 1995). The product of the NF2 gene, called schwannomin or merlin (Rouleau et al., 1993;Trofatter et al., 1993), has structural similarity with members of the ezrin-radixin-moesin (ERM) family of proteins that link the cytoskeleton and the cell membrane (for a review, see, Bretscher et al., 2000). Together with ezrin and moesin, schwannomin and the transmembrane receptor CD44 form a molecular switch that specifies cell growth arrest or proliferation (Morrison et al., 2001).The role of loss of Nf2 function in development and tumorigenesis has been studied in various knockout mouse models. Embryos homozygous for Nf2 mutations causing either indirect skipping of exon 3 or 2-3 or direct deletion of exon 2 are lethal (McClatchey et al., 1998;Giovannini et al., 2000). They fail to initiate gastrulation between embryonic days 6.5 and 7.0 (McClatchey et al., 1997). Heterozygous Nf2 mutant mice do not mimic human NF2 but develop later in life (10-30 months) mainly osteomas and osteosarcomas showing loss of heterozygosity (LOH) for Nf2 (McClatchey et al., 1998;Giovannini et al., 2000). In addition, indications for a role of Nf2 loss in metastatic potential have been found (McClatchey et al., 1998). Previously, we have mimicked NF2 in conditonal Nf2 mutant mice with P0 promoter-directed Cre recombinase activity in Schwann cells (Giovannini et al., 2000). P0Cre;Nf2 fl...