Disorders of Copper Transport

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Menkes disease (MD) is a X-linked genetic disorder of copper transport characterized by low brain copper that results in infantile neurodegeneration and premature death. MD is caused by mutations in a copper transporter gene, ATP7A. Current treatment is limited to subcutaneous copper injections and is especially effective in patients with mutant alleles that do not completely abrogate ATP7A function, and who are identified near birth (N Engl J Med 2008 358:605-614). For patients with complete loss-of-function mutations, copper delivered peripherally does not efficiently cross the blood-brain barrier. Thus, alternative treatment approaches are needed. In a murine model of classical MD (atp7amo-br) which cannot be rescued by intraperitoneal or intravenous copper, we are studying two brain-directed therapies: 1) early intracerebroventricular injection of copper chloride, and 2) early intracerebroventricular injection of a recombinant adeno-associated virus serotype 5 (AAV5) vector expressing a reduced size human ATP7A, rsATP7A-AAV. We developed a genotyping assay that identifies mutant pups before clinical signs, and administered 50 ng copper chloride to affected mice by intracerebroventricular injection within 3 days of birth. The dose was extrapolated from a maximum tolerated dose in adult rats that we determined previously (Molec Genet Metab 2007 91:30-36). Treatment modestly extended survival in mutant males and increased mean brain cortex copper levels in mutant mice . In an alternative approach, we are studying the expression and effects of rsATP7A-AAV delivered by intracerebroventricular injection. The transgene product begins at methionine 461 of the Menkes ATP7A and includes the fifth and sixth copper-binding domains. In mice receiving the AAV vector, we documented preliminary evidence of improved brain catechol ratios reflecting enhanced activity of dopamine-beta-hydroxylase, a copper-dependent enzyme. Our preliminary data indicate that intracerebroventricular delivery of copper and rsATP7A-AAV results in distribution throughout the atp7a<mo-br> brain. Whereas both treatments appeared to improve catecholamine biosynthesis in mutant brains, the effect of copper was more dramatic in this very small initial sample. Dose refinements, or combination therapy, may be required to optimize biochemical and clinical outcomes in this mouse model. Brain-directed therapies may ultimately provide a useful approach for treatment of MD patients with severe mutations.