Article ID: 2023-0011-IR
Tuberous sclerosis complex (TSC) is an autosomal dominant inherited disease characterized by systemic hamartomas, neuropsychiatric symptoms known as TAND (TSC-associated neuropsychiatric disorders), and vitiligo. These symptoms are attributed to the constant activation of mechanistic target of rapamycin complex 1 (mTORC1) caused by genetic mutations in the causative genes TSC1 or TSC2. The elucidation of the pathogenesis of this disease and advances in diagnostic technologies have led to dramatic changes in the diagnosis and treatment of TSC. Diagnostic criteria have been created at a global level, and mTORC1 inhibitors have emerged as therapeutic agents for this disease. Previously, the treatment strategy was limited to symptomatic treatments such as surgery. Inhibitors of mTORC1 are effective against all symptoms of TSC, but they also have systemic side effects. Therefore, the need for a cross-disciplinary, collaborative medical care system has increased, resulting in the establishment of a practice structure known as the “TSC Board.” Furthermore, to reduce the side effects of systemic administration of mTORC1 inhibitors, a topical formulation of mTORC1 inhibitor was developed in Japan for the treatment of skin lesions caused by TSC. This report summarizes the pathogenesis and current status of TSC and the contribution of the Neurocutaneous Syndrome Policy Research Group to the policies of the Ministry of Health, Labor, and Welfare with respect to this rare, intractable disease.
Tuberous sclerosis complex (TSC) is an autosomal dominant inherited disease characterized by systemic hamartomas, neuropsychiatric symptoms known as TAND (TSC-associated neuropsychiatric disorders), and vitiligo. Following the recent discovery of two causative genes (TSC1 and TSC2) in the 1990s, the pathogenesis of TSC was elucidated in the 2000s. Essentially, mutations in the TSC1 or TSC2 genes cause disease by constantly activating mTORC1 (mechanistic target of rapamycin complex 1), which is downstream of the TSC1 and TSC2 complex. This information has led to significant advances in the treatment of TSC.
This report summarizes the pathogenesis and current status of TSC, including clinical symptoms, latest treatments, and the development of a novel cross-disciplinary collaborative medical care system. The report also discusses the contribution of the Neurocutaneous Syndrome Policy Research Group for rare intractable disease of the Ministry of Health, Labor, and Welfare (MHLW) to MHLW policy.
TSC is a disease that was first reported in the 1800s. Rayer first illustrated facial angiofibromas of TSC in 1835,1 and von Recklinghausen described TSC in the case of a child with cardiac tumors (myomata) and many scleroses in the brain in 1862.2 Bourneville described gross pathology in the central nervous system of three individuals with seizures and learning disabilities in 1880, and he used the term “tuberous sclerosis of the cerebral convolutions” to describe the brain pathology. This is how the disease came to be called TSC. The manifestation of kidney tumors was first described in 1881.3 Bourneville and Brissaud described individuals with seizures, learning disabilities, cardiac murmurs, small tumors covering the lateral walls of the ventricles on the brain (subependymal nodules; SEN), and small tumors in the kidneys at autopsy, and they proposed an association between the central nervous system and renal manifestations of TSC.4 In 1908, Vogt’s triad of seizures, mental retardation, and adenoma sebaceum, which later became known as facial angiofibromas, was proposed.5 The dominant inheritance of TSC and its high mutation rate had already been shown in 1935 by Gunther and Penrose.6 However, little progress was made in the following 50 years. In 1967, Lagos and Gomez reported that 38% of 69 individuals in a family had average intelligence,7 contrary to the triad of Vogt, which was most commonly used at that time. Based on that report, new diagnostic criteria, known as “Gomez’s criteria,” were proposed8 to decline Vogt’s triad.
In the 1990s, there was significant progress in research on TSC and the technologies used to diagnose this disorder. The discovery of two responsible genes, TSC19 and TSC2,10 which encode hamartin and tuberin, respectively, improved the diagnostic rates of TSC. With this advancement, the frequency of TSC cases increased from 1 in 150,000 in 1956 to 1 in 7000 in 1997. Thereafter, the first International TSC Consensus Conference was held in Washington DC in 1998, and the diagnostic criteria, the so-called “Roach’s criteria,” were developed.11 The Japanese clinical diagnostic criteria and guidelines for TSC were created in 2008 based on Roach’s criteria described above.12
In the 2000s, the pathogenesis of tuberous sclerosis was elucidated; that is, TSC1 (hamartin) and TSC2 (tuberin) act in the PI3K-Akt-mTOR pathway, thereby causing constant activation of mTORC1.13,14,15,16,17 Therefore, mTORC1 inhibitors, such as sirolimus and everolimus, have become therapeutic agents. As a result, sirolimus and everolimus were used to treat TSC hamartomatous lesions.18,19,20,21,22 Simultaneously, genetic testing for TSC was launched in the 2000s and is now used for confirmation of a clinical diagnosis of TSC.23 This significant progress in research, diagnostic technologies, and innovative therapeutic agents for TSC prompted the second International TSC Consensus Conference in Washington DC in 2012 to modify the diagnostic criteria from 1998. The most significant change in the new diagnostic criteria is the addition of genetic criteria. In the new diagnostic criteria, the demonstration of a pathogenic mutation in TSC1 or TSC2 in normal tissue is considered sufficient for diagnosis, independent of clinical manifestations.23 Based on the diagnostic criteria and guidelines ratified in the second International TSC Consensus Conference, the Japanese TSC diagnostic criteria and guidelines have been revised again.24
In the late 2000s, mTORC1 inhibitors began to be used for the treatment of nontumorigenic conditions, such as epilepsy and autism, as well as for hamartomatous lesions.25,26,27,28,29 In addition, topical sirolimus was developed for the treatment of skin lesions in TSC with the aim of reducing the side effects of systemic administration of sirolimus and everolimus.30,31,32,33,34 Based on the above, the diagnostic criteria set in 2012 were updated again. In these two revisions, a new concept known as TSC-Associated Neuropsychiatric Disorders (TAND) was established.
Currently, 0.2% sirolimus gel is approved in Japan and the USA for the treatment of cutaneous lesions in TSC patients. In Japan, sirolimus is approved for pulmonary lymphangioleiomyomatosis (LAM) with/without TSC, and everolimus, a derivative of sirolimus, is approved as a therapeutic drug for TSC itself. In France, everolimus is approved for epilepsy in TSC, renal angiomyolipoma (rAML), and subependymal giant cell astrocytoma (SEGA).
Unlike surgical treatment, the mTORC1 inhibitors everolimus and sirolimus are effective against all lesions of TSC, not only the target lesion.35 By nature, TSC is a disease with a wide range of symptoms throughout the body and requires the involvement of specialists from many medical fields. With the advent of mTORC1 inhibitors, the diagnosis and treatment of TSC now requires more specialist involvement. Against this background, a new consultation system involving cross-disciplinary collaborative medical care (with specialists from many medical fields) is essential to provide optimal medical care to patients. For this reason, many hospitals have established cross-disciplinary collaborative medical care systems, known as “TSC Boards “or “TSC Clinics.”
Before the 1980s, incidence rates for TSC were estimated to be between 1 in 100,000 and 1 in 200,000.36,37 Because of significant progress in research, including the identification of causative genes, elucidation of pathophysiology, and diagnostic technologies such as the development of magnetic resonance imaging (MRI), incidence rates for TSC are increasing. The contemporary incidence rate is estimated to range between 1 in 6000 and 1 in 10,000 for live births to about 1 in 20,000 for the general population.38,39 As a result, the number of TSC patients without neural symptoms is increasing.40
TSC is caused by genetic mutations in either TSC19 or TSC2,10 which encode hamartin and tuberin, respectively. The hamartin–tuberin (TSC1, TSC2, and TBC1D7) complex downregulates mTORC1.14,16 The constant activation of mTORC1 that results from the genetic mutation of TSC1 or TSC2 is associated with protein synthesis in response to S6 K activation and 4E-BP1 inhibition and as a result of glycolysis, angiogenesis, lipid synthesis, suppression of autophagy, mitochondrial biogenesis, and inflammation/immune regulation. mTORC1 itself is a serine-threonine kinase and is composed of mTOR, RAPTOR, LST8, PRAS40, and DEPTOR.
Extracellular growth factors bind to receptor-type tyrosine kinase (PTK) on the plasma membrane and turn on the RTK-PI3K-PDK1-Akt signaling pathway. Increased intracellular energy status (ATP/AMP levels) suppresses AMP-activated protein kinase (AMPK). Activation of AKT or suppression of AMPK results in dephosphorylation of TSC2 and suppression of the TSC complex, respectively. Suppression of the TSC complex converts Reb-GDP to Reb-GTP and activates mTORC1.
The low molecular weight GTPases RagA/RagB and RagC/RagD constitute Rag dimers and are present in the lysosomal membrane by binding to the Ragulator complex (Lamtor1-5 consisting of five proteins) anchored in the lysosomal membrane. Ragulator acts as a GDP-GTP exchange factor (GEF; guanine nucleotide exchange factor). In the presence of amino acids, Ragulator exchanges RagA-GDP for GTP to form a dimer (RagA/B GTP-RagC/D GDP), which binds to mTORC1, anchors mTORC1 to the lysosomal membrane, and then activates it. However, in the absence of amino acids, it becomes a RagA/B GDP-RagC/D GTP dimer, which cannot bind to mTORC1, causing mTORC1 to leave the lysosomal membrane and become inactivate. The vacuolar-type H-transporting ATPase (V-ATPase) on lysosomal membranes binds to Ragulator and regulates its GEF activity. Furthermore, leucine and arginine activate GATOR2 via Sestrin and CASTOR,41,42 respectively, and GATOR2 activates RagA/B by inhibiting GATOR1.
mTORC1 activated by the processes described above regulates protein synthesis, autophagy, lipid synthesis, mitochondrial biogenesis, angiogenesis, and lysosomal biogenesis via its substrates p70-S6 K and 4E-BP, ULK1 and ATG13, SREBP1, PGC1a, hypoxia-inducible factor-1a, and TFEB (Fig. 1). Promotion of protein synthesis and suppression of autophagy and angiogenesis then causes TSC-related various hamartomas, which appear throughout the body, including rAML facial angiofibroma (fAF), SEN/SEGA, and LAM. Furthermore, autophagy through the Ulk1 complex (Ulk1/2, Atg13, FIP200, and Atg101) causes not only tumorigenesis but also the appearance of hypomelanotic macules, epilepsy, and TAND, although the precise mechanisms remain to be elucidated.
Molecular pathology of tuberous sclerosis complex.
The upstream and downstream cascades of the hamartin–tuberin–TBC 1D7 complex are summarized. Modified “Mammalian target of rapamycin and tuberous sclerosis complex” Mari Wataya-Kaneda. JDS 2015; 79 : 93-100 Fig1.
It was already known in the 1930s that TSC was an autosomal dominant condition.6 In fact, one in three TSC patients develop the disease because of parental inheritance, but two in three cases are sporadic. In the 1990s, two causative genes, TSC1 and TSC2, were identified. The TSC1 gene consists of 23 exons, and the TSC2 gene consists of 41 exons. The TSC1 gene product hamartin has a tuberin-binding domain in the middle, whereas tuberin has a hamartin-binding domain at the N-terminus and a Rheb-GTPase-activating domain at the C-terminus. TSC mutations are diverse, and there are no hotspots; nearly 30% of TSC patients have no identified genetic mutation.23 Loss of heterozygosity (LOH) is known to be more frequent in renal angiomyolipoma, LAM, and SEGA but less frequent in central nervous system lesions such as cortical tubers. Japanese patients with TSC have a high proportion of TSC1.40,43
Regarding the genotype and phenotype, it was previously thought that neuropsychiatric symptoms such as learning disabilities and autism spectrum disorders were common in TSC2.44,45,46 However, a recent report has indicated that there is no clear correlation between TSC2 and any neuropsychiatric symptoms except self-injury.47
Mosaic mutations have recently attracted attention. Mosaic mutations occur when the genetic mutation occurs postfertilization. If the genetic mutation occurs before the early embryo differentiates into somatic and germ lineages, the mutation occurs in both somatic and germ lineages and is inherited by the next generation. However, if the mutation occurs after differentiation into somatic and germ lineages, mosaicism occurs only in the germ lineage or somatic lineage, respectively, and mosaic mutations that occur only in the somatic lineage are not inherited by the next generation. Sporadic LAM is thought to be a somatic mosaic mutation of TSC occurring in kidney and lung cells.48
The clinical manifestations of TSC are characterized by the appearance of various symptoms at different times throughout the body. Moreover, each symptom is highly variable, and its specificity is low.
The first manifestation of TSC is multiple cardiac rhabdomyomas, by which a recent diagnosis of TSC has been made in the fetal period. Cardiac rhabdomyomas frequently occur in the fetal, neonatal, and infantile stages, but most regress spontaneously.
At the 2012 International TSC Clinical Consensus Conference, TSC symptoms related to the central nervous system were divided into those related (i) to brain structures, tubers, and tumors, (ii) epilepsy, and (iii) TAND.49 Cortical dysplasias are congenital abnormalities resulting from the inability of a group of neurons to migrate to the appropriate region of the brain during development and include cortical tubers (Fig. 2A) and cerebral white matter radial migration lines. Cortical dysplasia is thought to be associated with the severity of refractory epilepsy and learning disabilities in TSC. Tumors include SEN (Fig. 2B) and SEGA (Fig. 2C). Histologically, the two lesions are similar. SENs are benign tumors that develop along the wall of the lateral and third ventricles, are observed in 80% of TSC patients, and often appear at birth or prenatally. In contrast, SEGAs are large, growing tumors with diameters of more than 1 cm. They arise from the SEN and occur near the foramen of Monroe in 5%–15% of TSC patients. They may also be detected prenatally or at birth, but it is rare for a new tumor to arise after the age of 20 years. SEGA is a benign, slow-growing tumor but may sometimes block the foramen of Monroe and cause obstructive hydrocephalus.23,50
Representative photos and images of the tuberous sclerosis complex.
(A) Cortical tuber. (B) Subependymal nodules. (C) Subependymal giant cell astrocytoma. (D) Renal angiomyolipomas. (E) Lymphangioleiomyomatosis (LAM). (F) Hypomelanotic macules. (G) Facial angiofibromas. (H) Shagreen patches. (I) Ungual fibromas. Black arrows indicate grooves. (J) Intraoral fibroma (white arrow) and dental enamel pits.
Epilepsy is present in 84% of patients with TSC; it is recognized at approximately 4–6 months of age, is often the first symptom, produces a variety of seizures, and is often resistant to treatment. Among them, infantile spasms are present in more than 65% of patients with TSC and are mostly associated with mental developmental delay (West syndrome). Generally, when seizures with treatment-resistant generalized convulsions appear at a young age, they are likely to be associated with mental developmental delay.
TAND, which was proposed at the 2012 TSC International Clinical Consensus Conference, is a concept that summarizes neuropsychiatric symptoms such as aggressive behaviors commonly seen in TSC, autism/autism spectrum disorders, attention deficit hyperactivity disorder, and learning disabilities. The TAND checklist, which is publicly available worldwide, is used to assess TAND.
Between 60% and 80% of patients with TSC have renal involvement.40,51,52,53 Renal lesions include rAML (Fig. 2D), cysts, and renal cell carcinoma. Unlike the sporadic form, TSC-rAMLs occur bilaterally and at multiple sites and increase in frequency with age.40,52 TSC-rAMLs rapidly increase in number and size during the teenage years, often peaking in the twenties. rAMLs are often asymptomatic until they rupture. The risk of rupture is increased if the tumor is larger than 4 cm in diameter, rich in vascular components, and especially has an aneurysm with a diameter of at least 5 mm. Rupture of rAML can cause sudden severe lumber/abdominal pain, hematuria, anemia, and hemorrhagic shock and is one of the complications to be considered in adulthood. Renal cysts occur in 20%–50% of cases. Renal cysts may also involve the polycystic kidney gene (PKD1) adjacent to the TSC2 gene; these cysts often develop in childhood and may become severe. Renal cell carcinoma is found in 2%–4% of patients with TSC and tends to occur at a younger age than sporadic renal cell carcinoma.40,54,55
The pulmonary lesions in this syndrome include LAM, multifocal micronodular pneumocyte hyperplasia (MMPH), and clear cell (sugar) tumor of the lung (CCSTL). Although MMPH is common in TSC and does not require specific treatment, it is important to distinguish it from atypical adenomatous hyperplasia (AAH), miliary tuberculosis, and metastatic tumors.56,57,58 Pulmonary high-resolution computed tomography (HRCT) shows MMPH in more than 60% of cases, and, unlike LAM, there is no sex difference. Histologically, staining for cytokeratin and surfactant protein A/B is positive. However, unlike LAM, staining for HMB45 and α-smooth muscle actin (αSMA) is negative. LAM is an interstitial lung disease in which HMB45-positive, αSMA-positive, abnormally differentiated smooth muscle-like cells (LAM cells) infiltrate the lung interstitium and produce multiple cysts in the lung (Fig. 2E). About 30%–40% of female patients with TSC show LAM. LAM usually develops around the age of 30 years, is asymptomatic in the early stages, and is characterized by recurrent pneumothorax and gradually progressive dyspnea. Only thorough HRCT and a pulmonary function test can abnormalities be identified at an early stage. CCSTL is a rare benign mesenchymal tumor that can be histologically classified as a perivascular epithelioid cell tumor (PEComa).
Skin manifestations are important in the diagnosis of TSC because they are present in 98% of patients with TSC and are diagnosed on visual examination. However, the only symptom present at birth is hypomelanotic macules, and the symptoms usually increase with age. Furthermore, skin lesions with TSC are less specific.59
The skin manifestation present at birth is hypomelanotic macules. There may be three or more foliaceous hypomelanotic macules with a long diameter of at least 5 mm, which is one of the major diagnostic criteria (Fig. 2F), and many small confetti-like hypomelanotic macules, which is one of the minor diagnostic criteria. Facial angiofibromas (fAFs) appear as dilated red blood vessels on the face from early childhood, becoming raised and increasing in number from school age (Fig. 2G). They usually become prominent in the teenage years, but the degree of the lesions vary. fAFs in childhood have high diagnostic value, but fAFs that develop after puberty need to be differentiated from multiple endocrine neoplasia type 1 (MEN1) and Birt–Hogg–Dubé syndrome (BHD). Fibrous plaques on the head also occur on the mandible and forehead. These first appear as reddish purple to brown plaques on the face in infancy and become more prominent with age. Shagreen patches occur in 25% of TSC patients under 5 years of age and in 50% of patients over 5 years of age. Shagreen patches with typical pavement surfaces appear asymmetrically on the lumbosacral and femoral areas (Fig. 2H) and may also appear as scattered small verrucous nodules in childhood. In rare cases, shagreen patches may become giant collagen hamartomas. Ungual fibromas increase after puberty (Fig. 2I). It is a late-onset skin manifestation that gradually increases and is found in 88% of TSC patients aged 30 years and older. Ungual fibromas appear under, on, or around the nail, initially as pits, grooves, or red comets (small bleeding spots under the nail), sometimes lasting for years. Gingival and oral fibromas are also characteristic symptoms of TSC (Fig. 2J). Gingival hyperplasia occurs as a side effect of antiepileptic drugs such as phenytoin, and gingival papules are also seen in MEN1, BHD, and Cowden syndrome, so they must be differentiated. Multiple tooth enamel pits are also features of TSC (Fig. 2J). Other skin manifestations characteristic of TSC include folliculocystic and collagen hamartoma, multiple skin tags (molluscum fibrosum pendulum skin tags), and miliary soft fibroma (gooseflesh).
Other manifestations include multiple nodular hypertrophies of the retina in approximately 50% of TSC patients. In addition, AMLs, cysts, and hemangiomas are also common in intra-abdominal organs such as the liver, pancreas, ovaries, and uterus. Occasionally, multiple fibromatous polyps of the rectum, fibroids and PEComa of the uterus, and narrowing of the lumen of the medium-sized arterial wall may be observed.
Because TSC presents with a variety of symptoms, diagnosis is generally based on diagnostic criteria. As mentioned in the history of TSC, the new diagnostic criteria and guidelines (new guidelines)23,49 approved at the second TSC Clinical Consensus Conference in 2012, which are a revision of Roach’s criteria ratified at the first TSC Clinical Consensus Conference, are often used to determine diagnosis and treatment strategies. The diagnostic criteria in Japan are also based on these new diagnostic criteria24 and were jointly published by three academic societies: the Neurocutaneous Syndrome Policy Research Group for rare intractable disease of MHLW, the Japanese Tuberous Sclerosis Complex Society, and the Japanese Dermatological Association. Subsequently, the new diagnostic criteria were slightly revised in 2021, and the Japanese diagnostic criteria treatment guidelines are also being prepared for revision. In addition to the TSC diagnostic criteria, in Japan, various academic societies have recently established relevant diagnostic criteria and treatment guidelines.
Conventional treatment of TSC has included surgical procedures such as surgery and transarterial embolization (TAE) for rAML, dermabrasion, excision, and skin grafting, and laser therapy for fAFs. Among skin lesions, treatment of fAFs is essential because they cause bleeding, pain, functional impairment, and cosmetic problems, thereby reducing the quality of life. However, surgical treatment is painful and requires anesthesia. In infants and patients with mental developmental disorders who cannot be treated with local anesthesia, surgical treatment requires general anesthesia. In particular, given that skin grafting requires postoperative rest, it is difficult to use skin grafting for patients with developmental disorders and infants who cannot maintain postoperative rest.
As mentioned above, TSC is a disease in which mTORC1 is constantly activated because of abnormalities in the TSC19 and TSC210 genes, resulting in various symptoms throughout the body.15,16,17,60 Therefore, inhibition of mTORC1 is expected to be effective for all symptoms of this disease. In fact, mTORC1 inhibitors are increasingly being used as new nonsurgical treatments. In Japan, sirolimus/rapamycin (trade name: Rapalimus) was approved for LAM in 2014. In addition, everolimus (trade name: Afinitor), a derivative of rapamycin, was approved for rAML and SEGA in TSC in 2012, and the approval was expanded to TSC itself in 2019. Everolimus is also approved in France as an antiepileptic drug for this disease. Systemic administration of mTORC1 inhibitors for rAML and pulmonary LAM is effective against skin lesions.20,61 Although mTORC1 inhibitors are effective during drug administration, their effect is transient, and lesions recur when administration is discontinued.62 Therefore, long-term administration is needed, and side effects are a concern with systemic administration. Therefore, to reduce the side effects of systemic administration, sirolimus gel (trade name: Rapalimus gel 0.2%), a topical mTORC1 inhibitor, was developed in Japan for the treatment of skin lesions in TSC30,63,64,65 and is currently approved and marketed in Japan and the USA.
Topical sirolimus formulation was developed in Japan as a world-first initiative and was supported by the Japan Agency for Medical Research and Development and MHLW. This is an example of best practice collaboration between industry, academia, and government.
The world’s first clinical trial of topical sirolimus formulation for TSC skin lesions was conducted at Osaka University Hospital in Japan as an investigator-initiated randomized double-blind clinical trial (Phase II) from December 2013 to March 2015.33 The trial showed that the active drug group significantly reduced tumors relative to the placebo group, that children responded better than adults, and that 0.2% was the optimal concentration. After the Phase II clinical trial was completed in March 2015, a Phase III trial was conducted by a pharmaceutical company from December 2015 to October 2016,34 followed by a long-term trial.66 These trials confirmed the safety and efficacy of 0.2% sirolimus gel. From this study, the improvement rate in facial angiofibroma at 12 weeks of topical application, which was the primary endpoint, was significantly higher in the sirolimus group than in the placebo group (P < 0.0001), confirming the effectiveness of topical sirolimus.34
Tuberous sclerosis is a systemic disease that causes variable lesions in many organs throughout life. Therefore, diagnosis and treatment require the cooperation of specialists from many medical departments. In particular, systemic administration of mTORC1 inhibitors relieves all symptoms but also causes systemic side effects. As a result, the therapeutic use of mTORC1 inhibitors has increased the need for cross-disciplinary collaborative medical care. In recent years, many hospitals have established TSC Boards or TSC Clinics to fill this role.
TSC is a rare intractable disease with a prevalence ranging from 1 in 6000 to 1 in 10,000. The Neurocutaneous Syndrome Policy Research Group for rare intractable disease of MHLW is conducting TSC frequency surveys, developing diagnostic criteria and clinical guidelines, and creating severity classifications and individual clinical survey forms for the designation of intractable diseases. In addition, this group provides disease information to patients at the Japan Intractable Disease Information Center, where this group is responsible for disease descriptions, disease outlines, and diagnostic criteria for TSC.
Mari Wataya-Kaneda has received consulting fees from Novartis, honoraria from Novartis and Nobelpharma, and is the holder of an endowed professorship funded by Nobelpharma.
