Evaluation of somatic mutations in tibial pseudarthrosis samples in neurofibromatosis type 1

Abstract

Background Tibial pseudarthrosis is associated with neurofibromatosis type 1 (NF1) and there is wide clinical variability of the tibial dysplasia in NF1, suggesting the possibility of genetic modifiers. Double inactivation of NF1 is postulated to be necessary for the development of tibial pseudarthrosis, but tissue or cell of origin of the ‘second hit’ mutation remains unclear. Methods Exome sequencing of different sections of surgically resected NF1 tibial pseudarthrosis tissue was performed and compared to germline (peripheral blood). Results A germline NF1 splice site mutation (c.61-2A>T, p.L21 M68del) was identified from DNA extracted from peripheral blood. Exome sequencing of DNA extracted from tissue removed during surgery of the tibial pseudarthrosis showed a somatic mutation of NF1 (c.3574G>T, p.E1192*) in the normal germline allele. Further analysis of different regions of the tibial pseudarthrosis sample showed enrichment of the somatic mutation in the soft tissue within the pseudarthrosis site and absence of the somatic mutation in cortical bone. In addition, a germline variant in PTPN11 (c.1658C>T, p.T553M), a gene involved in the RAS signal transduction pathway was identified, although the clinical significance is unknown. Conclusions Given that the NF1 somatic mutation was primarily detected in the proliferative soft tissue at the pseudarthrosis site, it is likely that the second hit occurred in mesenchymal progenitors from the periosteum. These results are consistent with a defect of differentiation, which may explain why the mutation is found in proliferative cells and not within cortical bone tissue, as the latter by definition contains mostly mature differentiated osteoblasts and osteocytes.

Identification of One <em>BOCR</em> Mutation and Five <em>NF1</em> Mutations in Male Patients with Neurofibromatosis Type 1 and Congenital Pseudarthrosis of the Tibia

“…Two of them were "de novo" mutations, and the others originated from their parents. Similar to previous studies [8,11,12,16], our results further confirmed that CPT always existed in the NF1 families and each patient with both NF1 and CPT was found harboring mutations in the NF1 gene, suggesting these mutations were closely associated with the complex phenotypes. In addition, the age of patients sustaining tibial fracture was found to be distinctly different.…”

“…Although the molecular pathogenesis of NF1-related tibial pseudarthrosis was poorly understood, it may partially be explained by genetic modifiers [12]. In the current study, we found a rare missense variant (c.455C>T/p.P152L) in the BCOR gene completely co-segregated with the CPT phenotype in the family A1.…”

“…Moreover, a wide clinical variability of CPT in the same NF1 family occurs, suggesting the possibility of additional genetic causes and/or modifiers. A variant in the PTPN11 gene (c.1658C>T/ p.T553M, rs148176616), also involved in the RAS signal transduction pathway, was identified in a patient with NF1 and CPT, along with one NF1 mutation [12]. Although the above studies have considerably advanced our understanding of the pathogenic mechanisms of patients with CPT and NF1, the genetic architecture of these complex phenotypes has not yet been completely clarified.…”

Neurofibromatosis type 1‐related pseudarthrosis: Beyond the pseudarthrosis site

“…For the first time, it was shown that the somatic NF1 mutation in NF1‐related PA cases is not confined to the PA site only, but is also present throughout the periosteum of the affected bone. This is also the first direct evidence that the periosteum is involved in the pathogenesis of NF1‐related PA, which was previously hypothesized (Cho et al, ; S. M. Lee et al, ; Sakamoto et al, ; Sant et al, ). The exact role of the periosteal cells is still unclear.…”

“…PA tissue was shown to contain biallelic NF1 inactivation in approximately 65% of the published cases (S. M. Lee et al, 2012;Paria et al, 2014;Sakamoto et al, 2007;Sant et al, 2015;Stevenson et al, 2006). However, some were examined using only microsatellite marker analyses of NF1, this being suitable to detect copy number variations within NF1 but not point mutations or small insertions and deletions.…”