Clinical and molecular insights into lymphangioleiomyomatosis.
Lymphangioleiomyomatosis (LAM) is a rare disease of women that is characterized by a proliferation of abnormal smooth muscle-like cells (LAM cells), which leads to cystic lung lesions, lymphatic abnormalities, and abdominal tumors (e.g., angiomyolipomas). LAM occurs sporadically or in association with tuberous sclerosis complex, an autosomal dominant syndrome characterized by hamartoma-like tumor growths. The tumor suppressor genes TSC1 and TSC2 have been implicated in the etiology of LAM, as mutations and loss of heterozygosity (LOH) in TSC2 have been detected in LAM cells. TSC1 encodes hamartin, with a postulated role in actin cytoskeleton reorganization. TSC2 encodes tuberin, a protein with roles in cell growth and proliferation, transcriptional activation, and endocytosis. LAM cells, as defined by TSC2 LOH, have been detected in blood and body fluids, and can metastasize. LAM presents insidiously with progressive breathlessness, or dramatically with recurrent pneumothorax, chylothorax, or sudden abdominal hemorrhage. CT scans show numerous thin-walled cysts throughout the lungs, abdominal angiomyolipomas, and lymphangioleiomyomas. Pulmonary function tests show reduced flow rates (FEV1) and diffusion capacity (DL(CO)). Twenty per cent of patients have positive bronchodilator responses. Exercise testing shows gas-exchange abnormalities, ventilatory limitation, and hypoxemia that may occur with near-normal lung function. Progression of disease is best assessed by measurements of DL(CO) and FEV1. In the proper clinical setting, LAM may be diagnosed by a thoraco-abdominal CT scan. Tissue biopsy with special stains (HMB-45) should be reserved for cases with atypical presentations. There is no effective treatment for LAM, but on-going therapeutic trials with rapamycin appear promising.