ATP7A gene addition to the choroid plexus results in long-term rescue of the lethal copper transport defect in a Menkes disease mouse model

Abstract

Menkes disease is a lethal infantile neurodegenerative disorder of copper metabolism caused by mutations in a P-type ATPase, ATP7A. Currently available treatment (daily subcutaneous copper injections) is not entirely effective in the majority of affected individuals. The mottled-brindled (mo-br) mouse recapitulates the Menkes phenotype, including abnormal copper transport to the brain owing to mutation in the murine homolog, Atp7a, and dies by 14 days of age. We documented that mo-br mice on C57BL/6 background were not rescued by peripheral copper administration, and used this model to evaluate brain-directed therapies. Neonatal mo-br mice received lateral ventricle injections of either adeno-associated virus serotype 5 (AAV5) harboring a reduced-size human ATP7A (rsATP7A) complementary DNA (cDNA), copper chloride, or both. AAV5-rsATP7A showed selective transduction of choroid plexus epithelia and AAV5-rsATP7A plus copper combination treatment rescued mo-br mice; 86% survived to weaning (21 days), median survival increased to 43 days, 37% lived beyond 100 days, and 22% survived to the study end point (300 days). This synergistic treatment effect correlated with increased brain copper levels, enhanced activity of dopamine-β-hydroxylase, a copper-dependent enzyme, and correction of brain pathology. Our findings provide the first definitive evidence that gene therapy may have clinical utility in the treatment of Menkes disease.

Figure 1

Characterization of C57BL/6-Atp7a^mo-br mouse model of Menkes disease. (a) Mutant (gray coat color) and wild-type (black coat color) 12-day pups with a heterozygote female mother (mottled coat color). (b) Conservation among eukaryotic species of amino acid residues 799 (alanine) and 800 (leucine), which are deleted in Atp7a^mo-br. (c) On copper-deficient solid media, yeast transformation indicated partial complementation of the S. cerevisiae copper transport mutant, ccc2δ, by Atp7a^mo-br. Plating pattern (clockwise from 12 O'clock) of the S. cerevisiae copper transport mutant ccc2δ transformed with: wild-type human ATP7A (sections 1 and 2); an empty vector (sections 3 and 4); rsATP7A used as the adeno-associated virus serotype 5 (AAV5) transgene (sections 5 and 6); Atp7a^mo-br (sections 7 and 8); and wild-type mouse Atp7a (sections 9 and 10). The Atp7a^mo-br allele showed faint growth consistent with partial complementation. The rsATP7A allele restored growth nearly as well as either the wild type mouse or the wild type human copper ATPase. (d) Yeast complementation growth assay. Growth of various ATP7A alleles expressed in ccc2δ, and cultured in copper-deficient media. The wild-type ATP7A allele complemented ccc2δ efficiently, as did the rsAP7A used as the AAV5 transgene in brain-directed treatments of mo-br mice. The rsAP7A showed copper transport capacity ~80% compared to wild type in this growth assay. In contrast, the Atp7a^mo-br allele, which lacks two highly conserved amino acids, showed <15% copper transport function compared to wild type. Transformation with an ATP7A allele harboring deletion of exons 20–23 (del ex20-23), included as a negative control, did not complement ccc2δ. Mock-transformed ccc2δ also failed to grow, as expected. (e) Intracellular localization of Venus-tagged Atp7a (upper row) and Atp7a^mo-br (lower row) following transfection of human embryonic kidney-293 (HEK-293T) cells. Both alleles show trans-Golgi localization as noted by overlap of the ATP7A signal with TGN46, a trans-Golgi marker (merge + DAPI column). DAPI, 4',6 diamidino-2-phenylindole, dihydrochloride nuclear counterstain. (f) Kaplan–Meier survival curve indicating failure of intraperitoneal (i.p.) copper chloride (dose = 10 µg/g body weight on postnatal day 7) to rescue mo-br males.

Figure 2

Brain-directed treatment strategy. (a) Elements of the adeno-associated virus serotype 5 (AAV5) construct. Flanked by inverted terminal repeat (ITR) motifs, the AAV5 construct includes a cytomegalovirus (CMV) enhancer, chicken β-actin (CBA) promotor, and complementary DNA (cDNA)for a reduced-size version (3.1 kb) of ATP7A (rsATP7A) beginning with amino acid residue 461, necessary due to AAV packaging size limit (~5 kb). The sequence for a 6x-His tag (CACCACCACCACCACCAC) was inserted immediately inside the ATG for methionine 461. The rabbit β-globin polyadenylation (polyA) signal terminates transcription. The entire AAV5-rsATP7A construct size was 4.9 kb. (b) Model of ATP7A with the shaded area indicating the segment removed, including the first four of six amino-terminal copper-binding sites, to generate rsATP7A. The red asterisk denotes location of the mo-br mutation (deletion of two conserved amino acids). (c) Kaplan–Meier survival curve indicating slight lengthening of lifespan with each individual brain-directed treatment. (d) Kaplan–Meier survival curve indicating synergy of intracerebroventricular AAV5+Cu therapy (lavender) in rescue of the mo-br mouse (P = 0.00001 compared to untreated mo-br mutants). Replacement of rsATP7A with GFP (green circles) eliminated the survival benefit. (e) Brain copper levels at 12 days of age, by treatment category. Only AAV5+Cu combination-treated mo-br mice showed significantly higher copper levels in comparison to untreated mo-br mice (P < 0.03). (f) Brain catechol ratios at 12 days of age, by treatment category. Only AAV5+Cu combination-treated mo-br mice showed significantly lower ratios, indicative of improved dopamine-β-hydroxylase activity (P < 0.01). (g) Brain cytochrome c oxidase activity at 12 days of age, by treatment category. All treatment groups showed significantly increased activity compared to untreated mo-br mice.

Figure 3

Brain pathology findings. (a) Twelve-day cerebral cortex and hippocampus in wild type, untreated mo-br, and combination-treated mo-br mice. Untreated mutants showed abnormal neurons with pyknotic nuclei (yellow arrows), which were not obvious in wild type or AAV5+Cu mo-br mice. Bielschowsky silver stain indicated minimal axonal development in untreated 12-day mo-br mice compared to wild type, whereas AAV5+Cu combination treatment was associated with early developing axons (arrows). Electron micrographs of untreated 12-day mo-br brain cortex showed palely stained mitochondria (yellow arrows) and swollen dendrites and reduced neurofilament density (blue arrows). The latter ultrastructural abnormality was also noted occasionally in the AAV5+Cu-treated 12-day mo-br mice. Scale bars indicate 25 µm in hematoxylin and eosin (H&E) panels, 200 µm in silver-stained slides, and 500 nm in electron micrographs. The images shown are representative of each treatment group based on pathologic analysis of at least four mice in each category. (b) Quantitation of abnormal pyknotic hippocampal neurons. Untreated mo-br mice show a markedly higher percentage of abnormal hippocampal neurons compared to wild type and AAV5+Cu combination-treated mo-br mice. Error bars reflect standard error of the mean. AAV5, adeno-associated virus serotype 5.

Figure 4

Selective transduction of choroid plexus epithelia. (a) Transduction of choroid plexus epithelia by AAV5-GFP. Low power (Scale bar = 160 µm) and (c) high power (Scale bar = 40 µm) views of the lateral ventricle and choroid plexus (arrow) from a 12-day-old wild type mouse brain stained with anti-GFP, to define the transduction pattern of AAV5 in these experiments. The same transduction pattern was evident in AAV5-GFP+Cu treated mo-br mice. (b) Reverse transcriptase (RT)-PCR and PCR confirm selective choroid plexus transduction by AAV5-rsATP7A. RT-PCR in mice 2,060 and 2,061 shows amplification of the 166 bp expected transgene product (yellow arrow) only in choroid plexus RNA samples. The transgene product was also detected in the choroid plexus RNA specimen from 2,060 without reverse transcription (lane 2,060 –RT).

Figure 5

Long-term growth and neurobehavioral testing. (a) Weight gain in combination-treated mo-br mutants paralleled that for wild-type littermates (gray squares). (b) Wire hang and (c) rotarod test results. Tests were performed weekly beginning at 25 days of age, with three trials per time point, and used a 60 seconds maximum. AAV5, adeno-associated virus serotype 5.

Figure 6

Neuropathology in 300-day wild type and AAV5+Cu-treated mo-br mice. (a) Luxol fast blue staining for myelin indicated the expected dense myelination of the corpus callosum (arrows) in sagittal brain sections from 300-day wild type and AAV5+Cu-treated mutants. Bar = 1 mm. (b) At 300 days of age, electron micrographs of brain cortex showed minimal differences between an AAV5+Cu-treated mo-br mutant and a normal littermate control. The combination-treated brains contained some dendrites with reduced neurofilament density and less organized architecture (arrows), although 300-day wild-type brain also showed occasional dendrites with similar abnormalities (arrow). Three animal pairs were examined, with representative results shown. Bar = 500 nm. (c) Cerebellar histopathology in wild type and 300-day AAV5+Cu-treated mo-br animals indicated subtle differences in the Purkinje cell layer of the treated mutants. While many normal Purkinje neurons (yellow arrows) were visible in mo-br mice, some regions of the Purkinje cell layer (boxed areas) showed few or no Purkinje neurons compared to wild type (boxed areas). These differences may be relevant to mo-br rotarod performance. Bar = 50 µm. AAV5, adeno-associated virus serotype 5.