Introduction
Tuberous sclerosis complex (TSC) is an often underdiagnosed and misunderstood disease affecting more than one million patients worldwide. Disruptions in the TSC axis lead to cellular abnormalities that result in abnormal development and postpartum cellular growth. TSC affects every organ system and is often thought of as a tumor predisposition syndrome, although the lesions often seem to share characteristics of more benign lesions, and in some ways a dysplastic process. There has been an attribution to “malignant degeneration” of TSC renal lesions, although this seems to be more of a historical footnote rather than a well-studied phenomenon. There is also confusion regarding the true risk of fat-poor renal lesions being malignant. This chapter will address these issues.
Genetics of tuberous sclerosis complex renal disease
TSC is an autosomal dominant genetic disorder that has a birth incidence of around 1:5800. , Proper diagnosis can be certain if the International Guidelines are followed, but diagnosis can be missed if one relies on the Vogt’s triad for TSC (facial angiofibromas, developmental delay, and intractable epilepsy) because less than 40% of affected patients have these classic features. Approximately half of the patients demonstrate cognitive impairment, autism, or behavioral disorders.
There are two gene loci associated with TSC: TSC1 , located on chromosome 9, and TSC2 , located on chromosome 16. The identification of the TSC2 gene location was assisted because of an observation in a family with autosomal dominant polycystic kidney disease caused by a balanced translocation in the PKD1 gene. A child in this family had autosomal dominant polycystic kidney disease and TSC, which helped in the positional cloning of the TSC2 gene.
TSC may occur by the loss of expression of the nonmutant allele. Both TSC and autosomal dominant polycystic kidney disease are phenotypically expressed because of a second-hit, or somatic mutation mechanism. The kidney disease associated with the PKD1 and the TSC2 loci account for a majority of their respective diseases, and both exhibit a more severe phenotype compared with the disease associated with the PKD2 and TSC1 loci. This association with more severe disease may have a molecular underpinning. The PKD1 and TSC2 loci are immediately adjacent, in a tail-to-tail orientation, on chromosome 16p. The proximity of the genes may be important because the PKD1 gene contains an intronic sequence with unique structural properties , that would predispose to mutation because this tract interferes with deoxyribonucleic acid (DNA) replication and leads to double-strand breaks and an array of somatic mutational effects. This predisposition to DNA double-strand breaks is synergized by the renal microenvironment, which inhibits DNA damage recognition. , This renal microenvironmental predisposition to disease may also help explain the multifocal and bilateral nature of the TSC cystic disease and the angiomyolipomata.
Tuberous sclerosis complex and renal function
Premature impairment of glomerular filtration rate (GFR) is reported in up to 40% of patients with TSC. , This reduction in function occurs in the absence of overt bleeding from angiomyolipomata or interventions, suggesting an intrinsic renal disease, and underscores the need to preserve kidney function by treating hypertension aggressively and avoiding surgical intervention when treating angiomyolipomata preemptively to prevent hemorrhage. Renal function should be assessed at the time of diagnosis and on an annual basis using blood tests to estimate GFR using creatinine , or cystatin C equations. Renal function in patients with TSC is of critical importance because many of the drugs commonly used to treat epilepsy in patients with TSC are renally cleared.
Biology of tuberous sclerosis complex renal disease
Angiomyolipomata
The cell giving rise to the angiomyolipomata, categorized a perivascular epithelial cell tumor (PEComa), has been unknown until recently. Vascular associations with TSC, including aneurysms in the angiomyolipomata, aorta, and brain, along with immunohistochemical staining reveal that angiomyolipomata may arise from vascular mural cells. This origin helps explain the angiomyolipomata propensity to hemorrhage and the proclivity of the cells to home to lung, leading to lymphangioleiomyomatosis. ,
Although the typical TSC-associated angiomyolipoma contains fat, these lesions can also contain spindle cells, epithelioid cells, or a mix of both that express smooth muscle actin and melanocyte markers, such as gp100, a splice variant of Pmel17, and even melanin A ( Fig. 27.1 ). Expression of these melanocyte-associated genes results from MitF family transcription factor activity. This increased MITF transcription factor activity has caused confusion between TSC-associated PEComas and those caused by translocations involving TFE3 or TFEB, such as more aggressive renal cell carcinomas (RCCs) and malignant PEComas. ,
Because approximately one-third of TSC-associated angiomyolipomata have fat-poor components ( Fig. 27.2 ), and because at least half of the patients affected with TSC will have cystic disease, it is common for some patients to have a solid mass that is associated with cystic components. These findings should raise concern for RCC in the general population but should not raise the same level of concern in the population with TSC, because RCC is actually very rare in the population of TSC patients. Such lesions can be serially measured and assessed for growth characteristics that can help sort the fat-poor angiomyolipoma from the malignancy. Current research focuses on noninvasive approaches to help better delineate malignancy from fat-poor angiomyolipoma.
Cystic disease: Intersection of cilial cystogenic and oncogenic signaling pathways
TSC proteins regulate cell growth and proliferation, which are important for organogenesis, organ maintenance, and malignancy. The mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway integrates intra- and extracellular environmental information to properly regulate metabolism, protein translation, growth, proliferation, autophagy, and survival. The TSC2 protein is reported to interact with cleaved C-terminal tail of polycystin-1 (PC-1) to control the mTORC1 pathway. AKT phosphorylation of TSC2 causes its retention at the cell membrane for this regulation of mTORC1. This phosphorylation step is inhibited by the uncleaved, membrane bound C-terminal tail of PC-1. Without this phosphorylation, TSC2 complexes with TSC1 to downregulate mTORC1 activity.
The mTORC1 activation also may involve a nuance involving the PC-1 in explaining the cystogenesis that could help explain why the PKD1/TSC2 contiguous gene syndrome has such a severe phenotype ( Fig. 27.3 ). mTORC1 activity also negatively regulates the biogenesis of PC-1 and proper trafficking of the PC-1/2 complex to cilia. PC-1 is located on the cilia of principal cells, but it is also found on other cell membranes, including intercalated cells, and is strongly expressed on extracellular vesicles. Genetic interaction studies have revealed that PC-1 downregulation by mTORC1 leads to cystogenesis in Tsc1 mutants. These findings may explain the severe renal manifestations of the PKD1/TSC2 contiguous gene syndrome.
Renal cell carcinoma
Although RCC has been recognized as part of TSC renal disease for many years, more in-depth analysis of this rare phenomenon is lacking. The literature does contain cases in both children and young adults with what is described as RCC . These patients are often reported to have multiple and even bilateral tumors. Histologically there are three main types. Previously reported TSC-associated RCCs have been classified as chromophobe-type or described resembling chromophobe RCC. Guo et al. described this chromophobe morphology in almost 60% of the 57 lesions from eight of the 18 patients (44%) with TSC. They based this designation on the basis of lesions having eosinophilic cytoplasm, nuclear membrane irregularity, and perinuclear halos. They also noted extensive PAX8 nuclear staining that excludes the possibility that the chromophobe-like morphology could represent oncocytoma-like angiomyolipomata, a variant composed of polygonal cells with deeply eosinophilic cytoplasm that mimics oncocytoma or eosinophilic variant of chromophobe RCC. The chromophobe-like RCCs were described as being CK7 positive; however, unlike a prototypical chromophobe RCC that has immunoreactivity for CD117, only one of the six evaluated chromophobe-like morphology tumors in the Guo et al. series had limited staining for CD117.
The second TSC-associated RCC’s morphology is the renal angiomyo-adenomatous tumor (RAT). This pattern consists of prominent smooth muscle proliferation with clear neoplastic cells that form predominantly tubules and nests with rare papillae. The Guo et al. series identified this pattern in 39% of patients accounting for 30% of total RCCs. This RAT-like pattern of TSC-associated RCC is reported to be immunohistochemically similar to the RAT and clear cell-papillary RCC spectrum, because they all strongly stain with both CK7 and CA9.
The last TSC-associated tumor histologic morphology is one of a distinct granular eosinophilic-macrocystic morphology, and this pattern represented 11% of all the TSC-associated RCCs in the Guo et al. series, but this accounted for one-third of the patients. Schreiner and colleagues identified this as a distinct histologic pattern of RCC in TSC patients. These lesions have neoplastic appearing cells with voluminous granular eosinophilic cytoplasm and frequently have macrocystic architecture with lining epithelial cells showing a “hobnail” pattern that is typical in TSC cysts. The solid areas of these lesions very closely resemble atypical epithelioid angiomyolipomata, but also exhibit strong nuclear PAX8 staining, supporting the classification as a renal carcinoma. These lesions could also resemble the Xp11/TFE3 translocation RCC in large part because they have abundant eosinophilic cytoplasm. For these three different patterns, an important missing element is long-term follow-up regarding the patient outcomes. Possibly the histology may be caused by the intrinsic intercalated cell plasticity, and it may turn out that the appearance in these TSC RCC lesions may actually look far more aggressive than they actually are in clinical practice. Embryologically, the collecting duct cells arise from the ureteric bud outpouching of the Wolffian duct and give rise to three types of intercalated cells. There are functional relationships between intercalated cell mTORC1, H1-adenosine triphosphatase, and Wnt signaling pathways that may make TSC renal epithelium uniquely poised to proliferate and develop the appearance of, or actual, malignancy.
Clinical aspects and treatment of tuberous sclerosis complex renal disease
Chronic kidney disease
Some 41% of patients with TSC have an estimated GFR of less than 60 mL/min/1.73 m 2 (chronic kidney disease [CKD] stage 3 or less) by their mid-50s compared with 3% of the general population. , However, this percentage may be much higher in those patients with a significant renal angiomyolipomata burden. The risk of end-stage renal failure necessitating renal replacement therapy is reported to be 4% in one study of adults. Patients with reduced renal function experience a high morbidity and mortality from premature cardiovascular disease, with a significant rate of death before developing end-stage renal failure.
Acute kidney injury from renal hemorrhage, loss of normal renal parenchyma following embolization or surgery, hypertension, replacement of normal renal cells with angiomyolipomata or cysts and possibly haploinsufficiency causing mTOR overactivation resulting in premature loss of nephrocytes are thought to contribute to premature loss of GFR. Although TSC patients without identifiable renal tissue on magnetic resonance imaging (MRI) can still have a normal GFR, they most commonly have less renal reserve because of multiple angiomyolipomata or cysts. In this patient group with reduced renal function, special care needs to be taken to prescribe and alter drug doses appropriately for GFR.
Preemptive angiomyolipomata embolization can reduce the risk and consequences of severe hemorrhage but may result in renal impairment. Impaired kidney function was identified in 29% of patients who had undergone embolization compared with 10% for those who had not, although these results may be confounded by the high burden of angiomyolipomata in those who subsequently needed embolization. The use of mTORC1 inhibitors now as the first line of therapy may improve long-term outcome of renal function provided that the mTOR inhibitors do not have an adverse effect. Early work suggests that the use of mTORC1 inhibitors do not interfere with renal function. ,
Hypertension
Hypertension is more common in patients with renal TSC than in the general population. , We suggest hypertension should be aggressively treated in line with standard recommended targets: 140/80 mm Hg or lower for adults and appropriate age-adjusted targets for children.
Initial guidelines discouraged concurrent use of angiotensin-converting enzyme (ACE) inhibitors in those TSC patients taking mTOR inhibitors because of a possible increase incidence of angioedema. But ACE inhibitors and angiotensin 2 blockers are useful in this patient group. ACE inhibitors may exhibit suppression effects on angiomyolipomata cells and cysts. Angioedema has not been reported in the TSC population as a drug limiting problem; the literature currently advocates the concurrent use of ACE inhibitors and mTOR inhibitors with caution.
Nephrolithiasis
Nephrolithiasis is common in TSC patients because of their renal manifestations and side effects of some anticonvulsant therapies. Topiramate is an effective anticonvulsant for some forms of TSC-associated epilepsy. The drug enhances gamma aminobutyric acid–activated chloride channels and inhibits excitatory neurotransmission to reduce seizure activity. Topiramate also inhibits subtypes II and IV carbonic anhydrase, and thus reduces renal citrate excretion. This reduced citrate excretion increases the risk of nephrolithiasis. The ketogenic diet can also significantly improve seizure control for some patients with TSC, but is associated with hypercalciuria, hypocitraturia, and decreased uric acid solubility caused by the low urine pH. All these factors synergize to increase the nephrolithiasis risk of patients on this treatment.
Significant renal cystic disease can alter acid secretion, causing citrate reclamation and result in hypocitraturia. Identifying nephrolithiasis in a developmentally delayed patient can be challenging, but understanding the risk factors can help guide imaging and diagnosis. Medical therapy for nephrolithiasis in this patient population is relatively straightforward and includes adequate hydration and citrate supplementation when required.
Angiomyolipomata
A major complication of angiomyolipomata is life-threatening hemorrhage, historically reported to occur in 25% to 50% of patients, , which more recently has been found to average 30% in a larger population-based study of patients not having active surveillance. Angiomyolipomata secrete vasoactive cytokines and can stimulate a robust blood supply. The blood vessels formed enlarge with angiomyolipoma growth but are poorly supported by adventitia and develop aneurysms. Aneurysms larger than 5 mm are at high risk of rupturing.
Patients with TSC2 mutations seem to develop angiomyolipomata at a younger mean age compared with patients who have TSC1 mutations (13 vs. 24 years), and more often need intervention (27% vs. 13%). Female gender has been associated with an increased incidence of adverse outcomes from angiomyolipomata based on data from a case series and because of the fact that two-thirds of subjects enrolled in the EXIST-2 (Everolimus for angiomyolipoma associated with tuberous sclerosis complex or lymphangioleiomyomatosis) study were women. However, angiomyolipomata prevalence is not statistically different between males and females in the 2216 Tuberous Sclerosis Registry to Increase Awareness subjects; a relationship between outcome and gender is under investigation in this cohort.
Historically there is an association between the angiomyolipoma size and hemorrhage, with those 30 mm in diameter and still enlarging at greatest risk of bleeding. These data are the basis for the recommendation in the International Guidelines that angiomyolipomata that are 30 mm in diameter and enlarging should be treated preemptively to prevent hemorrhage using an mTORC1 inhibitor.
A proactive monitoring program and preemptive embolization may reduce the high risk of bleeding; but there is still a significant renal morbidity and mortality. In the Eijkemans et al. series of 351 patients, 117 underwent embolization of which 57 needed two or more embolization procedures. Sixteen of these patients required a nephrectomy, seven needed dialysis, and seven went on to transplantation. Of the 29 deaths in this series, nine were caused by renal causes.
With the advent of mTOR inhibitors replacing embolization as first choice for preemptive therapy, hemorrhage has been markedly reduced, being just 5% of 2216 subjects in one large study. A series of phase 2 and phase 3 studies has shown that mTOR inhibitor therapy is highly effective at stabilizing or shrinking angiomyolipomata in both adults , and children in the short term and in preventing bleeding and preserving renal function in the longer term. , ,
Angiomyolipomata are almost always benign lesions, though non-TSC varieties can be aggressive. Although angiomyolipomata can stop enlarging when the kidney loses its growth and repair potential in mid-adulthood, at least half continue to grow and need eventual intervention. Angiomyolipomata most commonly affect the kidney, but can be found in the liver, local lymphatics, or elsewhere in the abdomen. It is not known if these represent local spread or if they arise in situ. Angiomyolipomata in patients with known TSC showing malignant behavior have been rarely reported, and aggressive angiomyolipomata appear to be associated with mutations in the MITF / FTE genes, not the TSC genes. The MITF / FTE genes have a significant link to malignancy.
Current guidelines for surveillance and treatment of angiomyolipoma in people with TSC are straightforward. Monitoring growth of lesions using MRI, and an mTORC1 inhibitor is the recommended first line of therapy. mTORC1 inhibitors are approved and effective therapy for TSC angiomyolipomata, , and patients are best followed periodically by TSC centers that follow the guidelines for surveillance and management.
Recognizable patterns of tuberous sclerosis complex-associated renal cystic disease
TSC renal cystic disease is detected in approximately 50% of patients by conventional MRI, and it is associated with mutations in either the TSC1 or TSC2 genes. The TSC renal cysts range in size from the glomerulocystic disease to a polycystic renal phenotype associated with the TSC2 / PKD1 contiguous gene syndrome. To better communicate about TSC renal cystic disease, five basic patterns of cystic disease have been described ( Fig. 27.4 ).
Tuberous sclerosis complex polycystic kidney disease
This manifestation involves the contiguous deletion of a portion of the adjacent TSC2 and PKD1 genes on chromosome 16p13, and accounts for about 2% of TSC patients. There are rare cases that appear identical to this contiguous gene syndrome that are linked to TSC1 . Renal cysts in this form of TSC arise from all nephron segments. There is a high frequency of mosaic cases of the polycystic variety, and most have cystic disease in utero or shortly after birth, but quickly develop significant disease by 2 months of age. Hypertension usually develops in the first 2 weeks of life but can be delayed by several months. The hypertension is best treated with ACE inhibitors or angiotensin receptor blockers. Because of the renal parenchyma disruption, some of these children develop a urinary concentrating defect, and this may further drive cystogenesis by increasing antidiuretic hormone secretion. There are also practical considerations. Because the kidney mass can be so large, balance can be affected, so the ambulation developmental milestone can be delayed.
Tuberous sclerosis complex cortical cystic kidney disease
Another recognizable pattern of TSC cystic renal disease is that which is limited to the cortex and columns of Bertin. This cystic disease occurs early on and the cysts are remarkably uniform in size, usually about 2 to 4 mm in diameter. This imaging pattern may suggest glomerulocystic disease, or even dilatation of other tubular segments, and the usual number of cysts is most often less than two dozen.
Tuberous sclerosis complex multicystic kidney disease
Cysts can also be distributed throughout the cortical and medullary tissue and exhibit variable sizes. This pattern is also associated with either TSC1 or TSC2 mutations and with significant CKD. It can also resemble the polycystic variety, but genetically is different than TSC2 / PKD1 contiguous gene syndrome.
Tuberous sclerosis complex cortical microcystic kidney disease
Cortical microcystic disease can be subtle and is detected by careful inspection of the abdominal MRI. The renal cortex will exhibit an increased water signal before overt discrete cortical cysts develop. Eventually, the cortex may develop an increased echotexture similar to that which occurs in the medullary pyramids in patients with autosomal recessive polycystic renal disease. Identification of this disease pattern is important because affected patients have a more rapid decline in renal function and develop CKD stage 2 or 3 in their late teens or early 20s. The renal pyramids are spared and the urinary concentration capacity is preserved. Sometimes there is evidence of tubular proteinuria accompanying azotemia, but hypertension is not common until significant CKD develops.
Tuberous sclerosis complex focal cystic kidney disease
Focal cystic disease is thought to be the result of a somatic mutation that occurs during branching of the ureteric bud, such that there is significant cystogenesis in a localized renal pyramid. The developmental timing can be such that a small child can have an isolated renal pyramid with significant cystic disease, although the other pyramids are structurally normal. If the mutation occurs after a critical developmental period, phenotypic expression only will occur after acute kidney injury. Such a renal injury has been postulated to constitute a ‘third hit’ in autosomal dominant polycystic kidney disease that results in rapid cyst formation in adult research animals. TSC focal cystic kidney disease also appears to follow this same temporal sequence. Risk factors for acute kidney injury in TSC patients include anticonvulsant and nonsteroidal antiinflammatory medications, and rhabdomyolysis and hypoxia induced by status epilepticus.
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