M. Priscilla Short

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterized by the widespread development of distinctive tumors termed hamartomas. TSC-determining loci have been mapped to chromosomes 9q34 (TSC1) and 16p13 (TSC2). The TSC1 gene was identified from a 900-kilobase region containing at least 30 genes. The 8.6-kilobase TSC1 transcript is widely expressed and encodes a protein of 130 kilodaltons (hamartin) that has homology to a putative yeast protein of unknown function. Thirty-two distinct mutations were identified in TSC1, 30 of which were truncating, and a single mutation (2105delAAAG) was seen in six apparently unrelated patients. In one of these six, a somatic mutation in the wild-type allele was found in a TSC-associated renal carcinoma, which suggests that hamartin acts as a tumor suppressor.

Tuberous sclerosis (TSC) is an autosomal dominant disorder characterized by seizures, mental retardation, and hamartomatous lesions. Although hamartomas can occur in almost any organ, they are most common in the brain, kidney, heart, and skin. Allelic loss or loss of heterozygosity (LOH) in TSC lesions has previously been reported on chromosomes 16p13 and 9q34, the locations of the TSC2 and TSC1 genes, respectively, suggesting that the TSC genes act as tumor-suppressor genes. In our study, 87 lesions from 47 TSC patients were analyzed for LOH in the TSC1 and TSC2 chromosomal regions. Three findings resulted from this analysis. First, we confirmed that the TSC1 critical region is distal to D9S149. Second, we found LOH more frequently on chromosome 16p13 than on 9q34. Of the 28 patients with angiomyolipomas or rhabdomyomas, 16p13 LOH was detected in lesions from 12 (57%) of 21 informative patients, while 9q34 LOH was detected in lesions from only 1 patient (4%). This could indicate that TSC2 tumors are more likely than TSC1 tumors to require surgical resection or that TSC2 is more common than TSC1 in our patient population. It is also possible that small regions of 9q34 LOH were missed. Lastly, LOH was found in 56% of renal angiomyolipomas and cardiac rhabdomyormas but in only 4% of TSC brain lesions. This suggests that brain lesions can result from different pathogenic mechanisms than kidney and heart lesions.

Angiomyolipomas (AMLs) are renal tumors that occur both sporadically and in association with tuberous sclerosis (TSC). TSC is an autosomal dominant disorder characterized by hamartomatous lesions in multiple organs. Two TSC loci are recognized: TSC1 on 9q34 and TSC2 on 16p13. Loss of heterozygosity (LOH) at the TSC1 and TSC2 loci in lesions from TSC patients has recently been reported. Lesions that are not associated with TSC have not been previously examined for LOH at the TSC loci. We analyzed 29 renal angiomyolipomas from patients without a history of TSC. Three tumors demonstrated LOH on 16p13. This is the first report indicating that mutations in TSC2 occur in tumors of patients who do not have TSC. We also found LOH on 16p13 in 5 of 8 TSC-associated AMLs. Two of these tumors were from a single patient and demonstrated different regions of LOH. These findings support the hypothesis that the TSC2 gene functions as a tumor suppressor.