The TSC2 gene is associated with autosomal dominant tuberous sclerosis complex (TSC) (MedGen UID: 348170).
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Pathogenic variants in the TSC2 gene are associated with ~66% of clinical cases of tuberous sclerosis complex.
The TSC2 gene encodes a protein called tuberin. Within cells, tuberin interacts with a protein called hamartin, which is produced from the TSC1 gene. Nutrient-sensing is central to essential cellular functions like cell growth, division, and survival. Tuberin inhibits nutrient-sensing pathways and could result in unrestrained or unorganized cell growth when not properly regulated.
TSC2 Tuberous Sclerosis
MedGen UID: 348170
Tuberous sclerosis complex (TSC) is a highly variable, multisystemic neurocutaneous condition. Features include numerous, benign hamartomatous tumors of the skin, brain, kidneys, lungs, heart, and other organs as well as epilepsy, mild-to-severe neurodevelopmental, and behavioral impairments (PMID: 26564073).
Brain: Most affected individuals develop benign brain tumors that have malignant potential; specifically, cortical tubers, subependymal nodules, and subependymal giant cell astrocytomas (SEGAs) (PMID: 27447981, 26564073). Over 90% of affected individuals have cortical tubers of varying size and number (PMID: 27797139). These lesions are hamartomas composed of disorganized neuronal and glial “elements” (PMID: 26564073). Subependymal nodules are TSC-related benign brain tumors that develop in the subependymal region of the ventricles (PMID: 26564073). SEGAs often develop from calcified subependymal nodules and are observed in 5-15% of affected individuals (PMID: 26564073, 9568761). While most develop in childhood, SEGAs may also present neonatally (PMID: 26564073). They may be unilateral or bilateral, and typically extend into the lateral ventricles potentially causing secondary obstructive hydrocephalus (PMID: 26564073).
The primary neurologic features of TSC are epilepsy, intellectual disability, and autism spectrum disorder (PMID: 26564073, 12622312, 16043792). The severity of neurologic symptoms has been suggested to correlate with the number and size of the cortical lesions (PMID: 26564073, 27797139). Approximately 80-90% of affected individuals have epilepsy (PMID: 27797139, 26564073). Those with multiple, large cortical tubers appear to have earlier onset and more severe seizures (PMID: 27797139). Infantile spasms are particularly common during the first year of life while generalized tonic-clonic and complex partial seizures are mostly seen in older individuals (PMID: 26564073).
At least half of those with TSC have intellectual disability, ranging from mild to severe (PMID: 26564073, 16043792). Intellectual disability appears to strongly correlate with severe, uncontrolled seizures (PMID: 26564073, 12622312, 16043792). In addition, affected individuals with multiple, large cerebral lesions seem to have a greater risk of both uncontrolled seizures and intellectual disability (PMID: 26564073, 27797139, 16043792).
Individuals with TSC often manifest a variety of neuropsychiatric symptoms, collectively known as tuberous sclerosis-associated neuropsychiatric disorders (TAND) (PMID: 26706013, 27267556). These include psychosocial, behavioral, psychiatric, and learning difficulties manifesting with aggression, sleep disturbance, attention deficit disorder, and frank psychosis (PMID: 26564073). In addition, autism spectrum disorders (ASD) are observed in approximately 25% of individuals with TSC, and in half of those with both TSC and intellectual disability (PMID: 26564073). The severity of ASD ranges from mild to severe.
Kidneys: TSC-related renal lesions include epithelial cysts, oncocytomas, and angiomyolipomas (AMLs) and develop in approximately 70-80% of affected individuals (PMID: 27447981, 26564073). AMLs are benign renal lesions that may be unilateral of bilateral (PMID: 27447981, 26564073). Their prevalence and size increase with age (PMID: 26564073). While small AMLs are typically asymptomatic, large tumors may cause life-threatening renal disease with signs and symptoms including hematuria, flank or abdominal pain, retroperitoneal hemorrhage, renal failure, and arterial hypertension (PMID: 27447981, 26564073). While uncommon, renal cancers associated with TSC include malignant angiomyolipoma and renal cell carcinoma. The risk of renal malignancy in individuals with TSC is approximately 2-5% (PMID: 21182496).
Eyes: Unilateral or bilateral retinal hamartomas or retinal astrocytic hamartomas are observed in approximately 50% of individuals with TSC (PMID: 2747981). Retinal hamartomas generally remain stable and rarely affect vision; however, some can develop subretinal fluid and rarely cause retinal detachment (PMID: 27447981, 26564073). Hypopigmented hamartomas of the iris and ciliary epithelium have also been described as well as eyelid angiofibromas; however these are much less common compared to retinal findings (PMID: 2747981, 26564073). Obstructive hydrocephalus secondary to subependymal giant cell astrocytomas (SEGAs) can occur and may produce papilloedema (PMID: 2747981).
Lungs: Lymphangioleiomyomatosis (LAM) develops primarily in adult women with TSC (PMID: 26564073). While many with LAM are asymptomatic, some develop symptoms including dyspnea with exertion, cough, hemoptysis, and spontaneous pneumothorax, which can trigger acute pulmonary dysfunction (PMID: 26564073). Estrogen and cigarette smoking have been suggested to exacerbate LAM (PMID: 26564073). Approximately 80% of women with TSC have evidence of cystic lung disease by 40 years of age (PMID: 27267556).
Heart: Two-thirds of newborns with TSC are diagnosed with one or more benign cardiac rhabdomyomas (PMID: 26564073). If present, these are often identified on prenatal ultrasound and are largest in the neonatal period. Over time, they typically regress and spontaneously resolve (PMID: 26564073). Most do not cause any symptoms; however, in some cases, cardiac rhabdomyomas can cause cerebral thromboembolism (PMID: 26564073).
Skin: The hallmark skin findings in TSC include hypomelanotic macules (also known as “ash leaf” spots), facial angiofibromas, cephalic plaque, shagreen patches, and periungual fibromas. While these skin findings are hamartomas and may increase in number over time, they do not appear to have malignant potential (PMID: 26564073). Hypomelanotic macules are observed in over 90% of affected individuals and are the single most consistent physical finding among affected individuals (PMID: 26564073). These lesions are often present in early infancy; however, they may be difficult to identify without an examination using ultraviolet light (PMID: 26564073). Clusters of smaller hypomelanotic macules are also observed and are sometimes called “confetti macules” (PMID: 26564073).
Facial angiofibromas develop in approximately 75% of individuals with TSC (PMID: 26564073). In young children, these lesions appear as flat, erythematous macules. They later develop into small, raised, hardened structures due to underlying fibrous overgrowth (PMID: 26564073). Sun exposure appears to be a precipitating factor influencing the development of facial angiofibromas among affected individuals (PMID: 27267556) A fibrous cephalic plaque (often called a forehead plaque) may develop on the forehead (PMID: 26564073). Shagreen patches are also seen over the lower back in approximately half of individuals with TSC, usually during the first decade of life. Shagreen patches are large, raised lesions that have a texture similar to an orange peel (PMID: 26564073). Up to 80% of adults with TSC have one or more periungual fibromas (PMID: 26564073).
There is also preliminary evidence suggesting an association between pathogenic variants in TSC2 and rhabdomyosarcoma, and this gene is therefore available as a “preliminary-evidence” gene on Invitae’s Sarcoma Panel (PMID: 11920808, 8466239). Preliminary-evidence genes are selected from an extensive review of the literature and expert recommendations, but the association between the gene and the specific condition has not been completely established. This uncertainty may be resolved as new information becomes available and therefore clinicians may continue to order these preliminary-evidence genes.
TSC2 is a tumor suppressor gene that encodes the tuberin protein which, together with hamartin form the tuberous sclerosis complex, a key inhibitory component of the mTOR pathway. This pathway plays a role in the regulation of cell growth and metabolism (PMID: 26564073, 27797139, 27447981, National Library of Medicine. Genetics Home Reference. TSC2 gene. https://ghr.nlm.nih.gov/gene/TSC2. Accessed December 2016, NCBI. Gene. Gene ID: 7249 https://www.ncbi.nlm.nih.gov/gene/7249. Accessed December 2016).
In addition to TSC2, the TSC1 gene is also associated with TSC. Individuals with pathogenic variants in TSC2 are considered as a group to have an overall more severe clinical presentation, characterized by a greater number of tubers, earlier age at seizure onset, and higher prevalence of intellectual disability compared to TSC1 (PMID: 26706013). However, because TSC has a highly variable clinical presentation even among affected individuals within the same family, it is difficult to predict clinical severity based solely on the underlying genetic cause (PMID: 26706013).
TSC2 has 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. While approximately one third of cases are inherited from a parent, the majority occur spontaneously (an individual with a pathogenic variant has parents who do not have it; PMID: 20301399, 26564073).
Due to the complexity of TSC, affected individuals are best served by seeking care through a multidisciplinary clinic with experience treating this condition. Early diagnosis allows immediate implementation of surveillance and management guidelines (PMID: 24053983, 26564073):
mTOR inhibitors such as rapamycin have been shown to significantly reduce epilepsy severity and tumor volume in SEGAs, angiomyolipomas, facial angiofibromas, and lymphangioleiomyomatosis (PMID: 27797139). While this treatment appears effective, mTORs are associated with significant side effects including acne, amenorrhea, abnormal wound healing, and increased risk of infections among others (PMID: 27797139). In addition, tumors will regrow and epilepsy will return in most cases if therapy is discontinued (PMID: 27797139). Therefore, mTOR inhibitors require chronic treatment indefinitely (PMID: 27797139).
Conventional seizure treatments are insufficient in at least one third of affected individuals (PMID: 27267556, 27797139, 16043792). Treatment with mTOR inhibitors is challenging as epilepsy often begins in infancy, and the safety and efficacy of this therapy in infants is not yet established (PMID: 27797139). Therefore, TSC-related epilepsy treatment typically utilizes a variety of therapeutics including antiepileptic drugs, steroids, surgery (e.g., resection of cortical tubers and SEGAs), vagus nerve stimulation, and a ketogenic diet (PMID: 27797139). Vigabatrin appears to be highly effective for infantile spasms in children with TSC (PMID: 27267556). Ultimately, the treatment decision-making process should include a thorough discussion of the risks, complications, side effects, cost, length of treatment, and potential impact on TSC-associated comorbidities (PMID: 24053983).
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 as well as available genetic information that may result in a personalized plan for cancer prevention and surveillance.
It is advantageous to know if a pathogenic variant in TSC2 is present. At-risk relatives can be identified, allowing pursuit of a diagnostic evaluation. In addition, the available information regarding TSC2 is constantly evolving and more clinically relevant data is likely to become available in the near future. Awareness of this cancer predisposition allows patients and their providers to be vigilant in maintaining close and regular contact with their local genetics clinic in anticipation of new information, inform at-risk family members, and diligently follow condition-specific screening protocols.
Review date: February 2017
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