mTOR forms two physically and functionally distinct multiprotein complexes: the rapamycin-sensitive mTORC1 and the rapamycin-insensitive mTORC2. MTORC1 consists of five proteins including Raptor that positively regulate mTOR activity. MTORC2 consists of six proteins including mTOR and Rictor, which defines the activation level of mTORC2 and modulates the assembly of the actin cytoskeleton through Rho GTPases, and Rac1 is required for mTOR activation. In TSC2-null and human LAM cells, Rho GTPase activity is required for cell adhesion, motility, proliferation and survival. Loss of TSC1/TSC2 in LAM induces uncontrolled LAM cell growth and increases LAM cell viability. Upregulation of STAT1 and STAT3 and autophagy are known mediators of LAM cell viability and survival. LAM cells behave, in many ways, like metastatic tumor cells. LAM cells appear to arise from an extrapulmonary source and migrate to the lung. Increased LAM cell migration and invasiveness is rescued by TSC2 re-expression. The cellular and molecular mechanisms of neoplastic transformation and lung parenchymal destruction by LAM cells remain unknown. Lung remodeling may be mediated by an imbalance between matrix degrading metalloproteinases (MMPs) and their endogenous inhibitors TIMPs. The invasive cell phenotype in LAM is associated with TIMP-3 downregulation and TSC2-dependent upregulation of MMPs.Clinical and histopathological evidence demonstrate the lymphatic involvement in LAM. The prevailing hypothesis is that LAM lesions secrete the lymphangiogenic factor VEGF-D, recruit lymphatic endothelial cells (LECs) that form lymphatic vessels and induce lung cysts. VEGF-D serum levels are increased in LAM compared to other cystic lung diseases, including pulmonary Langerhans cell histiocytosis, emphysema, Sjögrens syndrome, or Birt–Hogg–Dubé syndrome.
The consequences of vessel and airway obstruction include chylous fluid accumulations, hemoptysis, airflow obstruction and pneumothorax. The typical disease course displays progressive dyspnea on exertion, spaced by recurrent pneumothoraces and in some patients, chylous pleural effusions or ascites.Most people have dyspnea on exertion with daily activities by 10 years after symptom onset. Many patients require supplemental oxygen over that interval. Genetics
LAM occurs in two settings: in the disease tuberous sclerosis complex (TSC-LAM) and in a sporadic form, in women who do not have TSC (sporadic LAM). In both settings, genetic evidence indicates that LAM is caused by inactivating or “loss of function” mutations in the TSC1 or TSC2 genes, which were cloned in 1997 and 1993 respectively. The TSC1 gene is located on the long arm of chromosome 9 (9q34) and the TSC2 gene is located on the short arm of chromosome 16 (16p13). TSC-LAM occurs in women who have germline mutations in either the TSC1 or the TSC2 gene.Sporadic LAM is primarily associated with somatic TSC2 gene mutations. Germline and somatic mutations in LAM include many types of mutations spread across the genes, with no clear “hot spots,” including missense changes, in-frame deletions and nonsense mutations. Because of the large size of the genes (together they have more than 60 exons) and because mutations can be located virtually anywhere within the genes, mutation detection is often challenging.On a cellular basis, LAM cells carry bi-allelic inactivation of the TSC2 genes, consistent with the “two-hit” tumor suppressor gene model.