Hypermorphic and hypomorphic AARS alleles in patients with CMT2N expand clinical and molecular heterogeneities.

Aminoacyl-tRNA synthetases (ARSs) are ubiquitously expressed enzymes implicated in several dominant and recessive disease phenotypes. The canonical function of ARSs is to couple an amino acid to a cognate transfer RNA (tRNA). We identified three novel disease-associated missense mutations in the alanyl-tRNA synthetase (AARS) gene in three families with dominant axonal Charcot-Marie-Tooth (CMT) disease. Two mutations (p.Arg326Trp and p.Glu337Lys) are located near a recurrent pathologic change in AARS, p.Arg329His. The third (p.Ser627Leu) is in the editing domain of the protein in which hitherto only mutations associated with recessive encephalopathies have been described. Yeast complementation assays demonstrated that two mutations (p.Ser627Leu and p.Arg326Trp) represent loss-of-function alleles, while the third (p.Glu337Lys) represents a hypermorphic allele. Further, aminoacylation assays confirmed that the third mutation (p.Glu337Lys) increases tRNA charging velocity. To test the effect of each mutation in the context of a vertebrate nervous system, we developed a zebrafish assay. Remarkably, all three mutations caused a pathological phenotype of neural abnormalities when expressed in zebrafish, while expression of the human wild-type messenger RNA (mRNA) did not. Our data indicate that not only functional null or hypomorphic alleles, but also hypermorphic AARS alleles can cause dominantly inherited axonal CMT disease.

Type 1 papillary renal cell carcinoma in a patient with schwannomatosis: Mosaic versus loss of SMARCB1 expression in respectively schwannoma and renal tumor cells.

In schwannomatosis, germline SMARCB1 or LZTR1 mutations predispose to the development of multiple benign schwannomas. Besides these, other tumors may occur in schwannomatosis patients. We present a 45-year-old male patient who developed multiple schwannomas and in addition a malignant type 1 papillary renal cell carcinoma (pRCC1). We identified a duplication of exon 7 of SMARCB1 on chromosome 22 in the constitutional DNA of the patient (c.796-2246_986 + 5250dup7686), resulting in the generation of a premature stop codon in the second exon 7 copy (p.Glu330*). The mutant SMARCB1 allele proved to be retained in three schwannomas and in the pRCC1 of the patient. Loss of heterozygosity analysis demonstrated partial loss of the wild-type SMARCB1 allele containing chromosome 22, suggesting loss of that chromosome in only a subset of tumor cells, in all four tumors. Immunohistochemical staining with a SMARCB1 antibody revealed a mosaic SMARCB1 expression pattern in the three benign schwannomas, but absence of expression in the malignant tumor cells of the pRCC1. To our knowledge, this difference in SMARCB1 protein expression has not been reported before. We conclude that a germline SMARCB1 mutation may predispose to the development of pRCC1, thereby further widening the spectrum of tumors that can develop in the context of schwannomatosis.

Premature termination of SMARCB1 translation may be followed by reinitiation in schwannomatosis-associated schwannomas, but results in absence of SMARCB1 expression in rhabdoid tumors.

In schwannomatosis, germline SMARCB1 mutations predispose to the development of multiple schwannomas, but not vestibular schwannomas. Many of these are missense or splice-site mutations or in-frame deletions, which are presumed to result in the synthesis of altered SMARCB1 proteins. However, also nonsense and frameshift mutations, which are characteristic for rhabdoid tumors and are predicted to result in the absence of SMARCB1 protein via nonsense-mediated mRNA decay, have been reported in schwannomatosis patients. We investigated the consequences of four of the latter mutations, i.e. c.30delC, c.34C>T, c.38delA, and c.46A>T, all in SMARCB1-exon 1. We could demonstrate for the c.30delC and c.34C>T mutations that the respective mRNAs were still present in the schwannomas of the patients. We hypothesized that these were prevented from degradation by translation reinitiation at the AUG codon encoding methionine at position 27 of the SMARCB1 protein. To test this, we expressed the mutations in MON cells, rhabdoid cells without endogenous SMARCB1 protein, and found that all four resulted in synthesis of the N-terminally truncated protein. Mutation of the reinitiation methionine codon into a valine codon prevented synthesis of the truncated protein, thereby confirming its identity. Immunohistochemistry with a SMARCB1 antibody revealed a mosaic staining pattern in schwannomas of the patients with the c.30delC and c.34C>T mutations. Our findings support the concept that, in contrast to the complete absence of SMARCB1 expression in rhabdoid tumors, altered SMARCB1 proteins with modified activity and reduced (mosaic) expression are formed in the schwannomas of schwannomatosis patients with a germline SMARCB1 mutation.

Germline SMARCB1 mutation predisposes to multiple meningiomas and schwannomas with preferential location of cranial meningiomas at the falx cerebri.

Schwannomatosis is a rare hereditary cancer syndrome in which patients develop multiple non-vestibular schwannomas. The chromatin remodelling gene SMARCB1 (also known as INI1, hSNF5, and BAF47) has been identified as a schwannomatosis predisposing gene, being involved in a subset of sporadic and familial cases. Recent studies have shown that SMARCB1 may also be involved in the development of multiple meningiomas. Previously, we demonstrated that the SMARCB1 exon 2 missense mutation c.143 C > T segregates with the presence of meningiomas in five members of a large family with multiple meningiomas and schwannomas. We extended our genetic analyses by screening 44 additional at-risk family members and identified 13 new carriers. Eleven of these were subjected to magnetic resonance imaging (MRI) of brain and spine. In addition, we analyzed four meningiomas and two schwannomas from family members for the presence of schwannomatosis-specific changes. We found in each tumor retention of the SMARCB1 exon 2 mutation, acquisition of an independent neurofibromatosis type 2 (NF2) gene mutation, and loss of heterozygosity at SMARCB1 and NF2 by loss of the wild-type copy of both genes. The MRI scans revealed one or more falx meningiomas in seven of 11 (64%) newly identified SMARCB1 mutation carriers. We conclude that the SMARCB1 exon 2 missense mutation in this family predisposes to the development of meningiomas as well as schwannomas, occurring via the same genetic pathways, and that this mutation preferentially induces cranial meningiomas located at the falx cerebri.

Fibroblasts derived from chronic diabetic ulcers differ in their response to stimulation with EGF, IGF-I, bFGF and PDGF-AB compared to controls.

Patients with diabetes mellitus experience impaired wound healing, often resulting in chronic foot ulcers. Healing can be accelerated by application of growth factors like platelet-derived growth factor (PDGF). We investigated the mitogenic responses, measured by 3[H]thymidine incorporation, of fibroblasts cultured from diabetic ulcers, non-diabetic ulcers, and non-lesional diabetic and age-matched controls, to recombinant human PDGF-AB, epidermal growth factor (EGF), basic fibroblast growth factor (bFGF) and insulin-like growth factor (IGF-I). We determined the optimal concentration of these factors and investigated which single factor, or combination of factors, added simultaneously or sequentially, induced the highest mitogenic response. For single growth factor additions, in all fibroblast populations significant differences in mitogenic response to different growth factors were observed, with PDGF-AB consistently inducing the highest response and IGF-I the lowest (p < 0.043). IGF-I produced only a 1.7-fold stimulation over control in diabetic ulcer fibroblasts, versus 2.95-fold for chronic ulcer, 3.2-fold for non-lesional (p = 0.007) and 5-fold for age-matched fibroblasts (p = 0.007). The highest mitogenic response induced by EGF was significantly less for chronic ulcer fibroblasts compared with age-matched and nonlesional controls (p < 0.03), chronic ulcer fibroblasts also needed significantly more EGF to reach this optimal stimulus (p < 0.02 versus age-matched and non-lesional controls). The simultaneous addition of FGF-IGF-I, PDGF-IGF-I and FGF-PDGF to diabetic ulcer fibroblasts always produced a higher stimulatory response than sequential additions (p < or = 0.05). Also the addition of bFGF, PDGF-AB and EGF prior to IGF-I induced a higher 3[H]thymidine uptake in all fibroblasts compared to the combination of each in reverse order. Significant differences were observed when comparing the combinations of growth factors with the highest stimulatory responses (PDGF-IGF-I, FGF-PDGF and EGF-PDGF added simultaneously) to a double dose of PDGF, with the highest mean rank for the combination PDGF-IGF-I (p = 0.018). In conclusion, combinations such as PDGF-AB and IGF-I may be more useful than PDGF-AB alone for application in chronic diabetic wounds.

SMARCB1 involvement in the development of leiomyoma in a patient with schwannomatosis.

Germline SMARCB1 mutations predispose in schwannomatosis patients to the development of multiple benign schwannomas and, in some cases, meningiomas. Here, we report on a 34-year-old female patient who developed multiple schwannomas at various locations and in addition a leiomyoma of the cervix uteri. She carried a c.362+1G>A mutation that inactivates the donor splice site of exon 3. This mutation caused the schwannomatosis phenotype in this patient and was also demonstrated to be present in her affected mother. The leiomyoma displayed the genetic features that are characteristic for germline SMARCB1 mutation-associated tumors. The mutant allele retained in the tumor, whereas the wild-type allele was lost by loss of heterozygosity. Furthermore, the loss of heterozygosity involved net loss of chromosome 22. An NF2 mutation was not found. However, quantitative polymerase chain reaction suggested that both NF2 copies were lost in the tumor. Immunostaining with a SMARCB1 antibody revealed the mosaic expression pattern that is typical for schwannomatosis-associated tumors. To our knowledge, this is the first reported case of leiomyoma associated with a germline SMARCB1 mutation. As such, it widens the spectrum of benign tumors associated with a germline SMARCB1 mutation.