The SMARCB1 gene is associated with autosomal dominant rhabdoid tumor predisposition syndrome 1 (RTPS1) (MedGen UID: 322892) and schwannomatosis (MedGen UID: 234775).
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The SMARCB1 gene is a tumor suppressor that encodes a subunit of the SWI/SNF complex, which regulates gene expression through chromatin remodeling.
SMARCB1—rhabdoid tumor predisposition syndrome
MedGen UID: 322892
Rhabdoid tumors are rare, aggressive childhood cancers that most often develop in the kidney (called a malignant rhabdoid tumor [MRT]) and central nervous system (called an atypical/rhabdoid tumor [AT/RT]). These lesions can occur spontaneously or as part of hereditary rhabdoid tumor predisposition syndrome (RTPS) due to pathogenic variants in the SMARCB1 gene. In comparison to sporadic isolated rhabdoid tumors, RTPS is associated with an increased risk of developing multiple tumors at younger ages.
Rhabdoid tumors are very rare. Of all brain tumors diagnosed in children, approximately 1.5%–2.1% are AT/RT (PMID: 25886974) and MRTs account for 0.9%–2.0% of all renal cancers (PMID: 25886974). Most cases of rhabdoid tumors are isolated and non-syndromic, though an estimated 30%–35% of cases are due to RTPS (PMID: 22434719).
The SMARCB1 gene is also associated with schwannomatosis. Schwannomas are benign peripheral nerve sheath tumors, which present primarily in adulthood (PMD: 25494491). Schwannomatosis is a rare form of neurofibromatosis characterized by the development of multiple spinal, peripheral, and cranial-nerve schwannomas. Malignant transformation is rare. Some may also develop meningiomas (generally benign tumors of meningeal tissue). Life expectancy is normal; however, there can be significant morbidity depending upon the location of the schwannomas. The absence of vestibular schwannomas has traditionally distinguished schwannomatosis from neurofibromatosis type 2, however, there is evidence to suggest possible phenotypic overlap (PMID: 24933152, 19898272, 21208904, 17357086, 26848914, 22105938).
While an individual with a SMARCB1 pathogenic variant will not necessarily develop cancer in his/her lifetime, the risk for cancer is higher than that of the general population. The same SMARCB1 pathogenic variant can present differently, even among individuals within the same family. Lifetime cancer risks associated with pathogenic variants in SMARCB1 is currently unclear.
SMARCB1 is a core component of the BAF (hSWI/SNF) complex. This ATP-dependent chromatin-remodeling complex plays important roles in cell proliferation and differentiation, cellular antiviral activities, and inhibition of tumor formation ( UniProtKB – Q12824 (SNF5_HUMAN), Accessed July 2016). If there is a pathogenic variant in this gene that prevents it from functioning normally, the risk of developing certain types of cancers may be increased.
Pathogenic variants in SMARCB1 have autosomal dominant inheritance. This means that an individual with a pathogenic variant has a 50% chance of passing the condition on to their offspring. With this result, it is now possible to identify at-risk relatives who can pursue testing for this specific familial variant. Most cases occur spontaneously (i.e., an individual with a pathogenic variant has parents who do not have it) (PMID: 25060813, 25494491, 21108436).
Due to the rarity of RTPS, there is currently no clear consensus regarding a screening and surveillance protocol. However, it has been suggested that those with a pathogenic SMARCB1 variant consider the following surveillance regimen (PMID: 21210147):
Although no studies are currently available to validate the benefit of the proposed surveillance protocols, the aggressive nature of RTPS-related tumors suggests that screening may be beneficial (PMID: 21210147). Full medical management and surveillance guidelines are anticipated as more individuals are diagnosed.
There is currently no clear consensus regarding a screening and surveillance protocol for individuals with hereditary schwannomatosis. However, it has been suggested that the following management and surveillance be considered (PMID: 23701098, 23595180):
An individual’s cancer risk and medical management are not determined by genetic test results alone. Overall cancer risk assessment incorporates additional factors, including personal medical history, family history, and any available genetic information that may result in a personalized plan for cancer prevention and surveillance.
Knowing if a pathogenic variant in SMARCB1 is present is advantageous. At-risk relatives can be identified, enabling pursuit of a diagnostic evaluation. Further, the available information regarding hereditary cancer susceptibility genes is constantly evolving and more clinically relevant data regarding SMARCB1 are likely to become available in the near future. Awareness of this cancer predisposition encourages patients and their providers to inform at-risk family members, to consider implementing proposed screening protocols, and to be vigilant in maintaining close and regular contact with their local genetics clinic in anticipation of new information.
Review date: July 2016
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Our sequence analysis covers clinically important regions of each gene, including coding exons, +/- 10 base pairs of adjacent intronic sequence, and select noncoding variants. Our assay provides a Q30 quality-adjusted mean coverage depth of 350x (50x minimum, or supplemented with additional analysis). Variants classified as pathogenic or likely pathogenic are confirmed with orthogonal methods, except individual variants that have high quality scores and previously validated in at least ten unrelated samples.
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