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Editor—Neurofibromatosis type 2 (NF2) is an autosomal dominant disorder that predisposes to bilateral vestibular schwannomas and other nervous system tumours. Two clinical subtypes have been proposed. The severe type (Wishart) has an onset before 25 years of age, a rapid course, and multiple nervous system tumours. The mild type (Gardner) has a later onset with a more benign course, often restricted to bilateral vestibular schwannomas.1
The NF2 tumour suppressor gene is localised on chromosome 22q12 and encodes a protein called merlin or schwannomin, which is related to a family of cytoskeleton associated proteins.2 3 Since the identification ofNF2, various germline mutations have been identified,4-6 as well as somatic mutations.7 In general, germline mutations associated with a mild phenotype include missense mutations and small in frame deletions or insertions.5
In our study, the proband (III.2) is an 18 year old boy with a mild facial palsy. Slight enlargement of both vestibular branches of the eighth cranial nerve was observed after MRI scanning, but no other clinical features have been observed. His mother (II.4) was operated on at the age of 28 for bilateral vestibular schwannomas; she died at the age of 45 from breast cancer but with stable vestibular schwannomas. The grandmother (I.2) of the proband had progressive deafness followed by surgery for a right sided vestibular schwannoma at the age of 48 after which she died owing to postoperative complications. At necropsy a right and left sided vestibular schwannoma were found.
DNA from the proband and the available family members was prepared from peripheral blood according to standard procedures. Microsatellites were amplified from genomic DNA by the polymerase chain reaction (PCR). Eight polymorphic markers in the NF2 gene region were studied in the family: nf2GAI (intron 1), D22S929 (intron 1), nf2CT3I (intron 1), nf2CAII (intron 3), nf2CAIV (intron 8), nf2CAV (intron 10), nf2CAVI (intron 13), and nf2GAII.8
Cytogenetic studies with G banding were normal. PCR fragments from the NF2 gene were used as FISH probes, including a pool of exon 1 and intron 1 fragments (2 and 0.7 kb ), a pool of intron 1 fragments (2-2.5 and 3.5 kb), and a pool of intron 15 fragments (2-2.8 and 3.6 kb)9 (for primer sequences seehttp://www.cephb.fr/nf2deletion/). The different PCR fragment pools were used separately and combined as FISH probes. Normal controls were studied in parallel.
In the family we studied, five polymorphic markers were informative. Using these markers we identified the proband (III.2) as a carrier of a deletion in the NF2 region. He had not inherited any allele from his mother (fig 1). The deleted region extends at least from intron 1 to intron 10 of theNF2 gene, as measured by the microsatellites.
The FISH experiment on peripheral lymphocytes from II.4 confirmed these results (fig 2). The mother of the proband has only one copy of theNF2 gene, suggesting that the deletion extends at least from exon 1 to intron 15. TheNF2 gene contains only 17 exons.2 This method has been shown to be a very efficient way to detect the large deletion in the NF2gene.
Affected subjects in this family are heterozygous for a deletion in theNF2 region, which cosegregates with a relatively mild phenotype. Mild phenotypes have recently been reported in NF2 families10 11 and in five isolated cases with a complete gene deletion.9
The NF2 genotype-phenotype correlation is not completely clear yet; however, frameshift and stop mutations which produce a truncated protein are usually associated with severe phenotypes.5 On the other hand, large deletions with complete inactivation of theNF2 gene have been observed in patients with a milder phenotype.9
In contrast, large NF1 gene deletions are usually associated with a more severe phenotype than point mutations or small deletions or insertions in the NF1gene. In NF1, however, the deletions are substantially larger than theNF1 gene, indicating the presence of a possible contiguous gene deletion syndrome and thus explaining the more severe phenotype in these patients.12 13 In the case ofNF2 gene deletions, the nature and the exact size of the deletions remains to be determined. Hemizygosity for theNF2 gene seems to have less severe consequences for the phenotype than the presence of a truncated protein. It has been shown that merlin function depends on the formation of an intra- and intermolecular complex (homo- and heterodimers).14 Truncated proteins may block these molecular interactions and might have an effect on the growth of schwann cells even before the normal NF2 allele has been inactivated.
We would like to thank Renilde Thoelen for expert technical assistance with FISH. CLC is funded by COLCIENCIAS (Colombia). EL is a part time Clinical Researcher of the Fonds voor Wetenschappelijk Onderzoek Vlaanderen (FWO-Vlaanderen).
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