Broadening the spectrum of SMARCB1-associated malignant tumors: a case of uterine leiomyosarcoma in a patient with schwannomatosis

ERN GENTURIS clinical practice guidelines for the diagnosis, treatment, management and surveillance of people with schwannomatosis

A Guideline Group (GG) was convened from multiple specialties and patients to develop the first comprehensive schwannomatosis guideline. The GG undertook thorough literature review and wrote recommendations for treatment and surveillance. A modified Delphi process was used to gain approval for recommendations which were further altered for maximal consensus. Schwannomatosis is a tumour predisposition syndrome leading to development of multiple benign nerve-sheath non-intra-cutaneous schwannomas that infrequently affect the vestibulocochlear nerves. Two definitive genes (SMARCB1/LZTR1) have been identified on chromosome 22q centromeric to NF2 that cause schwannoma development by a 3-event, 4-hit mechanism leading to complete inactivation of each gene plus NF2. These genes together account for 70-85% of familial schwannomatosis and 30-40% of isolated cases in which there is considerable overlap with mosaic NF2. Craniospinal MRI is generally recommended from symptomatic diagnosis or from age 12-14 if molecularly confirmed in asymptomatic individuals whose relative has schwannomas. Whole-body MRI may also be deployed and can alternate with craniospinal MRI. Ultrasound scans are useful in limbs where typical pain is not associated with palpable lumps. Malignant-Peripheral-Nerve-Sheath-Tumour-MPNST should be suspected in anyone with rapidly growing tumours and/or functional loss especially with SMARCB1-related schwannomatosis. Pain (often intractable to medication) is the most frequent symptom. Surgical removal, the most effective treatment, must be balanced against potential loss of function of adjacent nerves. Assessment of patients' psychosocial needs should be assessed annually as well as review of pain/pain medication. Genetic diagnosis and counselling should be guided ideally by both blood and tumour molecular testing.

Pathogenic noncoding variants in the neurofibromatosis and schwannomatosis predisposition genes

Neurofibromatosis type 1 (NF1), type 2 (NF2), and schwannomatosis are a group of autosomal dominant disorders that predispose to the development of nerve sheath tumors. Pathogenic variants (PVs) that cause NF1 and NF2 are located in the NF1 and NF2 loci, respectively. To date, most variants associated with schwannomatosis have been identified in the SMARCB1 and LZTR1 genes, and a missense variant in the DGCR8 gene was recently reported to predispose to schwannomas. In spite of the high detection rate for PVs in NF1 and NF2 (over 90% of non-mosaic germline variants can be identified by routine genetic screening) underlying PVs for a proportion of clinical cases remain undetected. A higher proportion of non-NF2 schwannomatosis cases have no detected PV, with PVs currently only identified in around 70%-86% of familial cases and 30%-40% of non-NF2 sporadic schwannomatosis cases. A number of variants of uncertain significance have been observed for each disorder, many of them located in noncoding, regulatory, or intergenic regions. Here we summarize noncoding variants in this group of genes and discuss their established or potential role in the pathogenesis of NF1, NF2, and schwannomatosis.

Multiple primary malignancies associated with a germline SMARCB1 pathogenic variant

A 51-year old presented with a 6-month history of increasing pelvic/lower back pain with nocturnal waking and episodes of anorexia and vomiting. Examination revealed right torticollis and Horner's syndrome, and a large abdominal mass arising from the pelvis. Magnetic resonance and positron emission tomography imaging revealed (A) a 14 cm heterogeneous enhancing mass, abutting the left kidney with standardised uptake value max = 2.9, (B) a large heterogeneous enhancing pelvic mass (C) mesenteric adenopathy standardised uptake value max = 10.3 and (D) 6 cm right lung apex mass standardised uptake value max = 4.3. Computerised tomography-guided biopsy of lesion A was reported as neurofibroma with occasional atypia, lesion B a benign uterine leiomyoma and lesion C follicular lymphoma world health organisation Grade 2. Although she had been given the diagnosis of Neurofibromatosis Type-1 (NF1) 25-years previously following removal of an intradural extramedullary schwannoma she had no cutaneous stigmata of NF1. Genetic analysis of blood lymphocyte DNA identified a pathogenic variant in SMARCB1 confirming a diagnosis of schwannomatosis. Following 6-months chemotherapy for lymphoma, surgery was performed to remove lesion A. Histology revealed a malignant peripheral nerve sheath tumour with areas of low and high-grade change. An incidental, well-differentiated small bowel neuroendocrine carcinoma was also excised. Close surveillance continues with no recurrence after 6 years. This case study describes a novel finding of three separate synchronous primary malignancies in a patient with schwannomatosis and a proven SMARCB1 pathogenic variant.

Malignant progression of a peripheral nerve sheath tumor in the setting of rhabdoid tumor predisposition syndrome

Malignant progression of a benign or low-grade tumor in individuals with germline alteration of SMARCB1 gene is not well characterized. In a family in which two carrier children had germline SMARCB1 mutations and atypical teratoid rhabdoid tumor, we report malignant progression of a nerve sheath tumor over a 7-year period in an affected adult family member. Prompt identification of the germline SMARCB1 alteration and the resultant rhabdoid tumor predisposition syndrome can help guide genetic counseling and surveillance in affected family members.

Germline variants in SMARCB1 and other members of the BAF chromatin-remodeling complex across human disease entities: a meta-analysis

Germline variants that affect function are found in seven genes of the BAF chromatin-remodeling complex. They are linked to a broad range of diseases that, according to the gene affected, range from non-syndromic or syndromic neurodevelopmental disorders to low-grade tumors and malignancies. In the current meta-analysis, we evaluate genetic and clinical data from more than 400 families and 577 patients affected by BAF germline alterations. We focus on SMARCB1, including 43 unpublished patients from the EU-RHAB registry and our institution. For this gene, we further demonstrate whole gene as well as exon deletions and truncating variants to be associated with malignancy and early-onset disease. In contrast, non-truncating variants are associated with non-malignant disorders, such as Coffin-Siris syndrome or late-onset tumors like schwannoma or meningioma (p < 0.0001). SMARCB1 germline variants are distributed across the gene with variants in exons 1, 2, 8, and 9 being associated with low-grade entities, and single-nucleotide variants or indels outside of exon 9 that appear in patients with malignancies (p < 0.001). We attribute variants in specific BAF genes to certain disease entities. Finally, single-nucleotide variants and indels are sometimes detected in the healthy relatives of tumor patients, while Coffin-Siris syndrome and Nicolaides-Baraitser syndrome generally seem to appear de novo. Our findings add further information on the genotype-phenotype association of germline variants detected in genes of the BAF complex. Functional studies are urgently needed for a deeper understanding of BAF-related disorders and may take advantage from the comprehensive information gathered in this article.

The molecular pathogenesis of schwannomatosis, a paradigm for the co-involvement of multiple tumour suppressor genes in tumorigenesis

Schwannomatosis is characterized by the predisposition to develop multiple schwannomas and, less commonly, meningiomas. Despite the clinical overlap with neurofibromatosis type 2 (NF2), schwannomatosis is not caused by germline NF2 gene mutations. Instead, germline mutations of either the SMARCB1 or LZTR1 tumour suppressor genes have been identified in 86% of familial and 40% of sporadic schwannomatosis patients. In contrast to patients with rhabdoid tumours, which are due to complete loss-of-function SMARCB1 mutations, individuals with schwannomatosis harbour predominantly hypomorphic SMARCB1 mutations which give rise to the synthesis of mutant proteins with residual function that do not cause rhabdoid tumours. Although biallelic mutations of SMARCB1 or LZTR1 have been detected in the tumours of patients with schwannomatosis, the classical two-hit model of tumorigenesis is insufficient to account for schwannoma growth, since NF2 is also frequently inactivated in these tumours. Consequently, tumorigenesis in schwannomatosis must involve the mutation of at least two different tumour suppressor genes, an occurrence frequently mediated by loss of heterozygosity of large parts of chromosome 22q harbouring not only SMARCB1 and LZTR1 but also NF2. Thus, schwannomatosis is paradigmatic for a tumour predisposition syndrome caused by the concomitant mutational inactivation of two or more tumour suppressor genes. This review provides an overview of current models of tumorigenesis and mutational patterns underlying schwannomatosis that will ultimately help to explain the complex clinical presentation of this rare disease.