In gene therapy, different types of vectors are used to incorporate and transfer specific genes to target cells. Gene therapy vectors can be constructed on the basis of viral or non-viral molecular structures, but viral vectors are frequently used due to their high transduction efficiency. The type of vector determines the duration, specificity and inducibility of gene expression. Adeno-associated virus (AAV) is a non-enveloped virus that can be modified to deliver genes to target cells. Simply put, AAV is a protein shell with the ability to contain a small, single-stranded DNA genome of approximately 4.8 kilobases (kb). AAV vectors are particularly interesting because they lack any viral genes and has thus far been one of the safest strategies for gene therapy.

Wilson disease is an autosomal-recessive disease caused by mutations in the ATP7B gene encoding the copper ATPase ATP7B. Dysfunction of this protein leads to organ copper accumulation particularly in the liver and brain and the disease typically manifests at late childhood or in young adults with either hepatic, neurologic or psychiatric symptoms or a combination. The disease is fatal without treatment and the long-term outcome of Wilson disease depends largely on adherence to therapy. Gene therapy has the theoretical potential to become a cornerstone in the treatment Wilson disease.

One of the challenges with AAV-vector based gene therapy is the limited amount of genetic information that can be packed into the virus vector, which in turn limits the size of the transfected protein. In the case of Wilson disease, the ATP7B gene is too large for optimal AAV vector delivery. Other issues are the risk of overexpression of ATP7B potentially leading to copper deficiency and vector induced hepatitis as has been seen in hemophilia trials. While the reported cases have been mild and unharmful to the patients, the inflammation caused damage to the transfected cells and rendered the treatment ineffective.

In Juli 2019, Murillo O et. al published the study “Liver Expression of a MiniATP7B Gene Results in Long-Term Restoration of Copper Homeostasis in a Wilson Disease Model in Mice” in Hepatology2

In this interesting study, Morillo et al. were able to restore copper homeostasis in a mouse model of Wilson disease using AAV-based gene therapy. The authors report a significant reduction in hepatic copper in treated Wilson disease mice not only in liver, but also in brain and kidneys. By utilizing a truncated version of the ATP7B protein with only 2 copper binding domains instead of the normal 6, they seemingly overcame the problem of limited gene size. Furthermore, the study showed that copper restoration only needed to be partial with a transduction of 7%-10% of hepatocytes was needed to prevented liver damage while a higher percentage (17%-23%) was needed to restore ceruloplasmin levels to equal those of WT animals. This is promising news as it lowers the risk of post treatment copper deficiency.

At the time of writing at least two companies are developing AAV-vector based therapies for Wilson disease. French based Vivet therapeutics, who is behind the Murillo paper, and US based Ultragenyx. While there are still many questions and obstacles before we have a standardized gene therapy for Wilson disease, the paper from Morillo et al. represents a key step forward.

Thomas Damgaard Sandahl & Peter Ott
Medical Department of Hepatology and Gastroenterology
Aarhus University Hospital
Palle Juul-Jensens Boulevard 99, 8200 Aarhus
Thomsand@rm.dk

References:
1) Samelson-Jones BJ, Arruda VR. Protein-engineered coagulation factors for hemophilia gene therapy. Mol Ther Methods Clin Dev. (2019) 12:184–201. 10.1016/j.omtm.2018.12.007

2) Murillo O, et al. Liver Expression of a MiniATP7B Gene Results in Long-Term Restoration of Copper Homeostasis in a Wilson Disease Model in Mice. Hepatology. 2019 Jul;70(1):108-126. doi: 10.1002/hep.30535. Epub 2019 Mar 20.