Determination of the mutant allele frequency in patients with neurofibromatosis type 2 and somatic mosaicism by means of deep sequencing

Mosaicism in Tumor Suppressor Gene Syndromes: Prevalence, Diagnostic Strategies, and Transmission Risk

A mosaic state arises when pathogenic variants are acquired in certain cell lineages during postzygotic development, and mosaic individuals may present with a generalized or localized phenotype. Here, we review the current state of knowledge regarding mosaicism for eight common tumor suppressor genes-NF1, NF2, TSC1, TSC2, PTEN, VHL, RB1, and TP53-and their related genetic syndromes/entities. We compare and discuss approaches for comprehensive diagnostic genetic testing, the spectrum of variant allele frequency, and disease severity. We also review affected individuals who have no mutation identified after conventional genetic analysis, as well as genotype-phenotype correlations and transmission risk for each tumor suppressor gene in full heterozygous and mosaic patients. This review provides new insight into similarities as well as marked differences regarding the appreciation of mosaicism in these tumor suppressor syndromes.

Targeted next-generation sequencing for differential diagnosis of neurofibromatosis type 2, schwannomatosis, and meningiomatosis

Background

Clinical overlap between neurofibromatosis type 2 (NF2), schwannomatosis, and meningiomatosis can make clinical diagnosis difficult. Hence, molecular investigation of germline and tumor tissues may improve the diagnosis.

Methods

We present the targeted next-generation sequencing (NGS) of NF2, SMARCB1, LZTR1, SMARCE1, and SUFU tumor suppressor genes, using an amplicon-based approach. We analyzed blood DNA from a cohort of 196 patients, including patients with NF2 (N = 79), schwannomatosis (N = 40), meningiomatosis (N = 12), and no clearly established diagnosis (N = 65). Matched tumor DNA was analyzed when available. Forty-seven NF2-/SMARCB1-negative schwannomatosis patients and 27 NF2-negative meningiomatosis patients were also evaluated.

Results

A NF2 variant was found in 41/79 (52%) NF2 patients. SMARCB1 or LZTR1 variants were identified in 5/40 (12.5%) and 13/40 (∼32%) patients in the schwannomatosis cohort. Potentially pathogenic variants were found in 12/65 (18.5%) patients with no clearly established diagnosis. A LZTR1 variant was identified in 16/47 (34%) NF2/SMARCB1-negative schwannomatosis patients. A SMARCE1 variant was found in 3/39 (∼8%) meningiomatosis patients. No SUFU variant was found in the cohort. NGS was an effective and sensitive method to detect mutant alleles in blood or tumor DNA of mosaic NF2 patients. Interestingly, we identified a 4-hit mechanism resulting in the complete NF2 loss-of-function combined with SMARCB1 and LZTR1 haploinsufficiency in two-thirds of tumors from NF2 patients.

Conclusions

Simultaneous investigation of NF2, SMARCB1, LZTR1, and SMARCE1 is a key element in the differential diagnosis of NF2, schwannomatosis, and meningiomatosis. The targeted NGS strategy is suitable for the identification of NF2 mosaicism in blood and for the investigation of tumors from these patients.

A Full Phenotype of Paraganglioma Linked to a Germline SDHB Mosaic Mutation

CONTEXT

Heterozygous germline pathogenic variants found in succinate dehydrogenase (SDH) complex genes predispose to hereditary paraganglioma (PGL) syndromes. No mosaicism has yet been reported in this setting.

DESIGN AND PARTICIPANT

We describe the clinical history of a case of SDH complex, subunit B (SDHB) mosaicism. A 24-year-old woman who developed a cardiogenic shock during dental surgery was diagnosed with a functional para-aortic PGL, which produced predominantly norepinephrine and its metabolites. The tumor was removed and showed a loss of SDHB expression by immunohistochemistry. Four years after initial laparotomy, the patient had a rapid cardiac decompensation during her second pregnancy, despite negative imaging 10 months before. Two recurrent functional PGLs were found and surgically removed. Initial genetic analysis performed by Sanger sequencing did not reveal any germline pathogenic variant in SDHB, VHL, SDHD, SDHC, SDHAF2, RET, MAX, and TMEM127. Next-generation sequencing performed on tumor- and blood-extracted DNAs highlighted the presence of a mosaic rare variant in SDHB (c.557G>A, p.Cys186Tyr) with an allelic ratio of 15% in the blood DNA.

CONCLUSIONS

We report the full clinical description of a proband with SDHB mosaicism associated with a functional, recurrent PGL. This case strengthens the necessity to complete the genetic analysis with methodologies able to identify germline mosaicism, especially in the case of early disease onset.

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.