New ‘Checkpoint’ Model Has Potential for Screening Therapeutics for CMT

The proteins NME3 and CSNK1G3 can help increase production of proteins lacking in genetic diseases with an underlying nonsense mutation – such as Charcot-Marie-Tooth type 2 disease – leading to a reduction in symptoms, a new study shows.

The study “Aminoglycoside-mediated promotion of translation readthrough occurs through a non-stochastic mechanism that competes with translation termination” was published in the journal Human Molecular Genetics.

Proteins are the molecules that carry out most of the functions within a cell. The DNA sequence of a gene has all the instructions to make a protein, including the signals to start and stop.

More than 2,500 genetic disorders have been identified that occur due to the introduction of a premature stop sequence, called a codon, in the gene-reading frame. This so-called nonsense mutation results in the production of an incomplete protein, which often is unable to carry out its original function.

Translation is the process that results in the production of a protein. There are certain small molecule compounds that have been identified as helping promote translation of genes that have a nonsense mutation. These compounds make the translational machinery less sensitive to the premature stop codon, which will result in higher levels of the protein and can work to reduce a patient’s symptoms.

In order to further identify such small compounds, researchers from the University of Bradford and King’s College London set out to screen compounds and their activity in regulating the readthrough of the sequence used to make the protein.

Researchers first developed a new model to determine compounds that could be useful for treatment of genetic disorders, which has not been shown before.

Talat Nasim, PhD, lead researcher, explained in a press release: “Our model acts like a simple roadside checkpoint. Compounds which can fulfil certain criteria will enable some proteins to get past the pre-mature stop signal. Even a small amount of the right proteins would be enough to give some therapeutic protection, and that would lessen the symptoms of the disorder.”

This led the team to develop and conduct a screen for chemical and siRNA compounds that could regulate readthrough. siRNAs are compounds that act to reduce expression of target genes.

Researchers determined that siRNAs that target casein kinase 1 gamma 3 (CSNK1G3) and nucleotide diphosphate kinase 3 (NME3) genes negatively regulate readthrough. Thus, the proteins CSNK1G3 and NME3 potentially promote readthrough. In this manner, they were able to identify two compounds that could be targeted for drug development.

Next, they screened existing FDA-approved drugs collected by the National Institute of Neurological Disease Syndrome (NINDS) in order to determine if they could be used to target nonsense mutations. None of the previously approved drugs were shown to aid the readthrough process.

The researchers also looked into a compound called PTC124, aka Ataluren or Translarna, which was shown to help patients with Duchenne muscular dystrophy who had nonsense mutations. While it is available in the U.K., it was rejected by the FDA. It previously was thought that PTC124 acts by increasing readthrough of the mutated dystrophin, however, this study shows that it does not work in this manner, but may work in a different way.

The researchers concluded: “Whilst this study identifies kinases such as CS3K1G and NME3 as potential regulators, the bioactive NINDS compounds and PTC124 do not promote readthrough.”