Mutation of the NF2 gene is the only known cause of this disorder. Linkage analysis is clinically available for at-risk individuals and fetuses with multiple family members of unambiguous clinical status regarding NF2 disease who are willing to participate in the testing process. The availability of highly informative intragenic NF2 polymorphisms increases the specificity of this method. For certain families, linkage analysis will be the most cost- and time-effective test that gives a definitive diagnosis. It can sometimes be an option when mutation-scanning or sequencing test results are negative. See "Interpretation of Test Results," below, for cautions regarding linkage test interpretation.
Identifying an NF2 mutation typically requires a multi-pronged testing protocol due to the high frequency of private constitutional mutations, the high frequency of postzygotic mutations, the different types of NF2 mutations, and the distribution of mutations throughout the gene. Wallace et al.50 describe a comprehensive testing service that includes four PCR reactions using a meta-PCR technique to link the amplicons into chimeric concatemers for direct sequencing, gene dosage PCR for deletions, loss of heterozygosity (LOH) studies, and subsequent sequencing of the gene in tumor tissue. In prospective studies, this approach yielded an 88% detection rate in familial NF2 cases and a 59% detection rate in sporadic NF2 cases. Direct sequencing or exon-scanning techniques (e.g., SSCP, TGGE, and HA; see chapter 2) of DNA from peripheral leukocytes, followed by direct sequencing to identify the underlying NF2 mutation, generally have a lower detection rate.46,47,51-53 The detection rate of either sequencing or exon scanning methods is significantly lower (34-51%) in sporadic cases in part due to the high frequency of postzygotic NF2 mutations, which can be masked by the presence of normal alleles.47,48,51,53 The mutation detection rate of mosaic cases can be increased significantly by analysis of tumor tissue.
Because schwannomas are clonal tumors with minimal cellular admixture, NF2 mutations can be detected at high frequency in tumor tissue. Testing of tumor tissue is available clinically (see GeneTests, http://www.geneclinics.org/) and is most useful in cases where a mutation is not detected in primary lymphoblasts, where clinical manifestations are suggestive of somatic mosaicism, or where constitutional tissue is not available.43,50,53 Moyhuddin et al.53 nearly doubled the mutation detection rate among mosaic cases using vestibular schwannoma tissue rather than peripheral leukocytes. Mutations are likely to be germline (rather than somatic) if the identical mutation is detected in two or more pathologically or anatomically distinct tumors or if a tumor shows LOH for NF2 intragenic or flanking loci, while constitutional tissue is heterozygous at these loci. Mutational analysis of tumor tissues is expected to have the greatest sensitivity for NF2 somatic mosaic mutations43 and sporadic cases with negative results from mutation scanning or sequencing tests.54
Efficient detection of the 11% to 30% of constitutional NF2 deletions (typically multiexonic in nature) has been accomplished using numerous techniques, including FISH, various gene dosage PCR assays, multiplex ligation probe amplification (MLPA), and high-resolution genomic arrays.53,55-57
Note that for mutation scanning tests and deletion-detection assays, detection rates will be laboratory specific due to the varying degrees of optimization of the technique; therefore, a survey of testing laboratories is recommended prior to sample submission.
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