Written by Dr Hannah Shorrock Edited by Dr. Larissa Nitschke

Pastor and colleagues identify FDA-approved small molecules that selectively reduce the toxic polyglutamine-expanded protein in SCA6.

Selectively targeting disease-causing genes without disrupting cellular functions is essential for successful therapy development. In spinocerebellar ataxia type 6 (SCA6), achieving this selectivity is particularly complicated as the disease-causing gene produces two proteins that contain an expanded polyglutamine tract. In this study, Pastor and colleagues identified several Food and Drug Administration (FDA) approved small molecules that selectively reduce the levels of one of these polyglutamine-containing proteins without affecting the levels of the other protein, which is essential for normal brain function. By using drugs already approved by the United States Food and Drug Administration to treat other diseases, referred to as FDA-approved drugs, the team hopes to reduce the time frame for pre-clinical therapy development.

SCA6 is an autosomal dominant ataxia that causes progressive impairment of movement and coordination. This is due to the dysfunction and death of brain cells, including Purkinje neurons in the cerebellum. SCA6 is caused by a CAG repeat expansion in the CACNA1A gene. CACNA1A encodes two proteins: the a1A subunit, the main pore-forming subunit of the P/Q type voltage-gated calcium ion channel, as well as a transcription factor named a1ACT.

The a1A subunit is essential for life. Its function is less affected by the presence of the expanded polyglutamine tract than that of a1ACT. The transcription factor, a1ACT, controls the expression of various genes involved in the development of Purkinje cells. Expressing a1ACT protein containing an expanded polyglutamine tract in mice causes cerebellar atrophy and ataxia. While reducing levels of the a1A subunit may have little effect on SCA6 disease but impact normal brain cell function, reducing levels of a1ACT may improve disease in SCA6. Therefore, Pastor and colleagues decided to test the hypothesis that selectively reducing levels of the a1ACT protein without affecting levels of the a1A protein may be a viable therapeutic approach for SCA6.

In this study, about 1120 FDA-approved drugs were screened for their ability to interfere with the production of the a1ACT protein. Proteins are produced in a process called translation where RNA is converted into a protein sequence. a1A and a1ACT proteins are produced by different forms of translation. The a1A protein is produced through normal translation. However, the a1ACT protein is produced by a special form of translation that uses different cellular machinery. This special type of translation is called IRES-mediated translation. By interfering with this special form of translation, the group sought to prevent the production of the toxic a1ACT protein containing the expanded polyglutamine tract without affecting the production of the a1A protein.

To screen the FDA-approved drugs, the group developed a system to easily measure the levels of a luminescent reporter protein produced by IRES-mediated translation. This would be compared to a different luminescent protein produced by normal translation. Using this screening system, ten drugs were found to interfere with the IRES-mediated translation without affecting normal translation.

These ten drugs were then tested in a cell culture system where both a1A and a1ACT were expressed. Four of these drugs were found to reduce levels of a1ACT without affecting a1A protein levels. These drugs have previously been approved for the use in various diseases. This includes chemotherapy agents for leukemia and lymphoma and drugs to suppress abnormal rhythms of the heart. Because these drugs have such diverse uses, it will be important to understand how these drugs selectively reduce levels of a1ACT compared to a1A if they are to be used to treat SCA6.

It is still unclear whether these drugs rescue cells from the toxic effects of the polyglutamine tract in a1ACT and can improve the phenotypes of animal models of SCA6. In previous studies, this group has shown that small RNAs called microRNAs can selectively interfere with the translation of a1ACT. A specific microRNA reduced levels of a1ACT without altering levels of a1A. In an animal model of SCA6, treatment with this microRNA protected mice from ataxia, motor deficits, and Purkinje cell degeneration caused by a1ACT containing the expanded polyglutamine tract. This suggests that the drugs identified in this study also have the potential to protect against SCA6 disease. Future studies will be necessary to investigate the therapeutic potential of these drugs. However, the fact that levels of the toxic a1ACT can be reduced without changing levels of a1A provides hope for the future of this selective approach to therapy development.

Key Terms

Ataxia: A loss of muscular control, leading to abnormal walking, speech changes and irregular eye movements.

Cerebellum: A primary area of pathology in the spinocerebellar ataxias. This brain region sits toward the back of the skull and, though small in stature, contains the majority of the nerve cells (neurons) in the central nervous system. Contains the circuits that fine-tune our movements, giving us the ability to move with precision.

Translation: The process after transcription where RNA is converted into a protein sequence.

Ion channel: A protein complex which allows movement of electrically charged molecules (ions) in and out of cell membranes, like through in nerve cells.

Protein: A molecule determined by a specific sequence of DNA. These molecules have a specific function in cells, tissues, and organs.

Purkinje Neurons: A type of neuron in the cerebellum. They are some of the largest cells in the brain. They help regulate fine movement.  Purkinje cell loss/pathology is a common feature in cerebellar ataxia.

Gene: A unit of heredity made up of DNA that fully or partially controls the development of specific traits.

Autosomal Dominant Inheritance: A condition that requires only one copy of a mutated gene to manifest.

MicroRNA (miRNA): Small single-stranded noncoding RNAs that regulate gene expression.

United States Food and Drug Administration (FDA): The United States Food and Drug Administration is a federal agency responsible for protecting public health by regulating drugs and biologics as well as food, cosmetics, medical devices, tobacco products and radiation emitting electronics. For a new drug, therapeutic protein, vaccine, cellular therapy or blood/blood product to be marketed in the US, it must be proven safe and effective to FDA’s satisfaction and be manufactured according to federal standards. The FDA ensures this by reviewing the testing performed by the manufacturers. FDA-approval means that the FDA has determined that the benefits of the product outweigh the known risks for the intended use.

Conflict of Interest Statement

The author and editor declare no conflict of interest.

Citation of Article Reviewed

Pastor, P. D. H., Du, X., Fazal, S., Davies, A. N. & Gomez, C. M. Targeting the CACNA1A IRES as a Treatment for Spinocerebellar Ataxia Type 6. Cerebellum 17, 72-77, doi:10.1007/s12311-018-0917-6 (2018).

Additional References

Miyazaki, Y., Du, X., Muramatsu, S. & Gomez, C. M. An miRNA-mediated therapy for SCA6 blocks IRES-driven translation of the CACNA1A second cistron. Sci Transl Med 8, 347ra394, doi:10.1126/scitranslmed.aaf5660 (2016).

Du, X. et al. Second cistron in CACNA1A gene encodes a transcription factor mediating cerebellar development and SCA6. Cell 154, 118-133, doi:10.1016/j.cell.2013.05.059 (2013).