Classification of NF1 microdeletions and its importance for establishing genotype/phenotype correlations in patients with NF1 microdeletions

An estimated 5–11% of patients with neurofibromatosis type-1 (NF1) harbour large deletions encompassing the NF1 gene and flanking regions. These NF1 microdeletions are subclassified into type 1, 2, 3 and atypical deletions which are distinguishable from each other by their extent and by the number of genes included within the deletion regions as well as the frequency of mosaicism with normal cells. Most common are type-1 NF1 deletions which encompass 1.4-Mb and 14 protein-coding genes. Type-1 deletions are frequently associated with overgrowth, global developmental delay, cognitive disability and dysmorphic facial features which are uncommon in patients with intragenic pathogenic NF1 gene variants. Further, patients with type-1 NF1 deletions frequently exhibit high numbers of neurofibromas and have an increased risk of malignant peripheral nerve sheath tumours. Genes located within the type-1 NF1 microdeletion interval and co-deleted with NF1 are likely to act as modifiers responsible for the severe disease phenotype in patients with NF1 microdeletions, thereby causing the NF1 microdeletion syndrome. Genotype/phenotype correlations in patients with NF1 microdeletions of different lengths are important to identify such modifier genes. However, these correlations are critically dependent upon the accurate characterization of the deletions in terms of their extent. In this review, we outline the utility as well as the shortcomings of multiplex ligation-dependent probe amplification (MLPA) to classify the different types of NF1 microdeletion and indicate the importance of high-resolution microarray analysis for correct classification, a necessary precondition to identify those genes responsible for the NF1 microdeletion syndrome.