Menkes disease is an X-linked recessive disorder of copper metabolism. It is caused by a mutation in the copper-transporting P-type ATPase (ATP7A) gene, which regulates copper's absorption from the intestines, its transport through the blood-brain and blood-cerebrospinal barriers and its movement from the cytosol to the cells' Golgi compartments, where it is incorporated into copper-dependent enzymes. Typically, Menkes disease presents in early infancy and is characterized by sparse, kinky, colorless hair and numerous physical and mental disorders. Conventional treatments of subcutaneous copper injections are often ineffective, especially if the ATP7A mutation is severe. In 2011, researchers led by Stephen G. Kaler, M.D., of the National Institutes of Health, Bethesda, Md., showed that copper injections combined with brain-directed gene therapy are very effective in ameliorating Menkes disease in mice (Donsante et al. 2011). This finding may lead to novel therapies for the human disease.
As their model of Menkes disease, the Kaler team used the C57BL/6-Atp7aMo-br/J (002566) mouse, also known as the mottled-brindled or "mo-br" mouse. The mouse has a six-base pair deletion in the Atp7a gene, which is orthologous to the human ATP7A gene. Hemizygous males exhibit many of the phenotypic characteristics of human Menkes disease, including tremors, general inactivity, very low copper levels in the liver and brain, low copper enzyme activity and early death. To test the efficacy of brain-directed gene therapy in the mo-br mouse, the Kaler team injected the lateral ventricle of one group of newborn mice with a transgene containing an adeno-associated virus serotype 5 (AAV5) harboring a human ATP7A complementary DNA (cDNA), a second group with copper chloride, and a third group with both the virus and copper chloride. They found that either individual therapy slightly extends lifespan to less than weaning age (21 days) and modestly improves neurotransmitter ratios. In contrast, the two therapies combined are much more efficacious: median survival increases to 43 days, 86% of the mice survive to 21 days, 37% survive beyond 100 days and 22% survive for at least 300 days.
The combined treatment increases brain copper levels, enhances the activity of dopamine-β-hydroxylase (a copper-dependent enzyme) and significantly mitigates brain pathology. These results suggest that the two therapies have different effects: the copper chloride injections increase the total copper available to the brain, and the gene therapy improves brain cells' abilities to use copper.
The Kaler team is the first to show that gene therapy can markedly alleviate Menkes disease in mice and may do the same in people. Additionally, their finding that the AAV5 vector that delivers the ATP7A transgene selectively localizes to the choroid plexus epithelial cells, which form the blood-brain-cerebrospinal fluid (blood-brain) barrier, indicates that these cells play a major role in the central nervous system's copper homeostasis and suggests that these specialized cells may be important therapeutic targets for other inherited neurometabolic diseases.