Challenges in progressing cell therapies to the clinic for Huntington's disease: A review of the progress made with pluripotent stem cell derived medium spiny neurons.

Huntington's disease (HD) is a hereditary, neurodegenerative disorder characterized by a triad of symptoms: motor, cognitive and psychiatric. HD is caused by a genetic mutation, expansion of the CAG repeat in the huntingtin gene, which results in loss of medium spiny neurons (MSNs) of the striatum. Cell replacement therapy (CRT) has emerged as a possible therapy for HD, aiming to replace those cells lost to the disease process and alleviate its symptoms. Initial pre-clinical studies used primary fetal striatal cells to provide proof-of-principal that CRT can bring about functional recovery on some behavioral tasks following transplantation into HD models. Alternative donor cell sources are required if CRT is to become a viable therapeutic option and human pluripotent stem cell (hPSC) sources, which have undergone differentiation toward the MSNs lost to the disease process, have proved to be strong candidates. The focus of this chapter is to review work conducted on the functional assessment of animals following transplantation of hPSC-derived MSNs. We discuss different ways that graft function has been assessed, and the results that have been achieved to date. In addition, this chapter presents and discusses challenges that remain in this field.