Extensively high load of internal tumors determined by whole body MRI scanning in a patient with neurofibromatosis type 1 and a non-LCR-mediated 2-Mb deletion in 17q11.2

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Abstract
Deletions in 17q11.2 affecting the NF1 gene and surrounding regions occur in 5% of patients with NF1. The two major types of NF1 deletions encompass 1.4-Mb and 1.2-Mb, respectively, and have breakpoints in the NF1 low-copy repeats or in the JJAZ gene and its pseudogene. Deletions larger than 1.4-Mb are rare, and only seven cases have been reported so far. Here, we describe a 26-year-old NF1 patient with an “atypical” NF1 deletion of 2-Mb. In contrast to the 1.4-Mb deletions, which preferentially occur by interchromosomal recombination during maternal meiosis, the deletion described here occurred intrachromosomally on the paternal chromosome. The centromeric deletion breakpoint lies in an L1-element located 1.3-Mb proximal to the NF1 gene. The telomeric deletion boundary is located in a single copy segment between an AT-rich segment and an AluSx-element in intron 15 of the JJAZ1 gene. Structural analysis implies that non-B DNA conformations at the breakpoints destabilized the duplex DNA and caused double-strand breaks. Although the breakpoints of this 2-Mb deletion are not recurrent, it is conspicuous that one breakpoint is located in the JJAZ1 gene. Paralogous recombination between the JJAZ1 gene and its pseudogene causes the recurrent 1.2 Mb deletions. The genomic architecture of the NF1 gene region, influenced by paralogous sequences such as the JJAZ1 gene and its pseudogene, seems also to stimulate the occurrence of non-recurrent deletions mediated by non-homologous end joining. Patient 442 described here suffers from a very high burden of subdermal neurofibromas. Magnetic resonance imaging of the whole body revealed numerous internal tumors, mainly plexiform neurofibromas and spinal tumors. This demonstrates the value of whole-body MRI scanning in determining the total tumor load, which is an important aspect in genotype/phenotype correlations with regard to large NF1 deletions.