Elastin defects in the lungs of avian and murine models of homocysteinemia

Homocysteinemia in animals is associated with disruption of the elastic fiber component of the extracellular matrix, resulting in vascular complications. The authors have utilized both avian and murine models to investigate the effects of homocysteinemia on lung development and repair following injury. Days old chicks were fed a diet containing 2% methionine for 3 weeks. Pregnant mice were given 2% methionine in the diet and feeding continued for up to 6 weeks after birth. The lungs were removed and examined for defects in elastin fiber formation. Methionine levels were elevated 20-fold in the serum from chicks receiving the methionine and 10-fold in pregnant mice. The elastic fibers in the parabronchi and air capillaries of chicks receiving methionine were thin and clearly disrupted. In the 2% methionine neonatal pups, normal lung development was prevented and the alveoli were significantly enlarged. However, after the pups reached 10 days of age the 2% methionine lungs did not differ histologically from the normal controls. Fetal mice reflected the same serum methionine levels as the dams fed the 2% methionine diet, yet after birth the serum levels of the neonates returned to control levels within 3 days. The authors found that the high serum methionine levels of the dams were not transferred to the milk, allowing the pups to reverse the histopathology observed early and then develop normally. The ability of the lung to replace elastin following elastase injury was not different in mice raised on the 2% methionine diet compared to controls. The studies show that continuous exposure of the developing lung to high circulating levels of methionine/homocysteine can result in major disruptions of elastic fibers and lung architecture. However, young mammals such as the mouse are protected from extended lung pathology because toxic levels of methionine are not transferred through the mothers milk.