PMP22 Gene Editing by CRISPR CaS9 Shows Reduction in PMP22 Expression in CMT1A Mice
"We are already envisioning the possibilities that gene therapy holds for our community of 2.8 million people worldwide living with CMT." — John Svaren, PhD, Chair, CMTA Scientific Advisory Board
Scientists are working on ways to directly target the gene defects that cause the various subtypes of Charcot Marie Tooth disease. They are using CRISPR CaS9 to perform genomic surgery in order to repair these gene defects. Ji-Su Lee, Jae Y Lee, Dong W Song, et al, demonstrated that a reduction in PMP22 gene expression, the gene responsible for causing CMT Type 1A, can be achieved in mice through the use of CRISPR CaS9. Their findings were reported in Nucleic Acids Research, Volume 48, Issue 1, 10 January 2020, Pages 130-140.
Charcot Marie Tooth disease Type 1A, or what is referred to as CMT1A, is the most common subtype of CMT. According to the Charcot-Marie-Tooth Association, CMT1 represents approximately 55% of all CMT cases, and 1A represents approximately 66% of all CMT1 cases. CMT1A is understood to be caused by a duplication of the Peripheral Myelin Protein 22 (PMP22) gene. This gene encodes Schwann cells to initiate the production of peripheral nerve myelin. This gene duplication is referred to as an over-expression. This over-expression disrupts the tightly regulated peripheral nerve myelination process, thereby causing an over-production of peripheral myelin. This over-production impairs the peripheral nerve’s normal function. There have been previous studies and trials to reduce this over-expression.
There was previously a large multi-continent clinical study to assess the effectiveness of high-dose Ascorbic Acid (Vitamin C) to reduce PMP22 expression, after there were some promising signs in a CMT1A mouse model. However, these lab findings did not translate well into the clinic setting as demonstrated by skin biopsy of treated patients in the study. Another previous study assessing progesterone antagonist as a means by which to reduce PMP22 expression also found that it was not a viable treatment. Both of these explored therapies can influence other cells and other biological processes. To eliminate these potential issues, why not target the specific gene? As it turns out, scientists have been working on ways to do exactly that.
Recently, CRISPR CaS9 has been shown to have potential as a use in developing treatments and even cures in genetic disorders, including CMT. CRISPR CaS9 is a gene editing technology that allows scientists to target and edit specific genes, thereby potentially avoiding issues arising from therapies that could inadvertently affect off-target cells and processes. Ji-Su Lee, Jae Y Lee, Dong W Song, et al, demonstrated that, by using CRISPR CaS9 to target the over-expressed PMP22 gene in a CMT1A mouse model, a reduction in overall PMP22 expression can be achieved.
The authors successfully demonstrated that, via targeted PMP22 TATA-box editing by CRISPR CaS9, PMP22 over-expression can be reduced, and their findings support that a reduction in PMP22 in CMT1A in the mouse model improves peripheral nerve conduction characteristics (peripheral nerve conduction characteristics are significantly impaired in CMT1A, causing a motor and sensory disease phenotype), and improves the overall disease severity caused by the over-expression of PMP22.
The authors caution that one must be careful to not reduce PMP22 expression to the point of creating an under-expression that would be associated with/cause HNPP. HNPP is another subtype of CMT, and it is understood to be the genetic opposite of CMT1A insofar as HNPP is understood to be caused by a deletion of PMP22. Normally, there are two copies of the gene. In CMT1A, there is an extra copy; and in HNPP, there is only one copy. To safeguard against inducing an under-expression, the authors suggest that “in contrast to targeting protein coding sequence of PMP22, TATA-box editing may prevent unwanted effects from knocking down PMP22 expression too low.” TATA-box editing refers to editing the non-coding DNA - the TATA-box. Rather than targeting the coding sequence of PMP22, the authors targeted the non-coding sequence of the gene. In so doing, it is thought that a safeguard against unwanted coding transcriptions can be achieved. What does all of this mean for CMTers though?
Researchers have now been able to demonstrate that, by reducing overall expression of PMP22 in CMT1A via direct gene targeting, an improvement in disease symptoms is possible. At the same time, researchers extrapolated that an increase in PMP22 expression can potentially achieve the same results for HNPP as does a reduction in over-expression for CMT1A.
The study authors report that their results have the potential to carry over to the clinic, and they suggest that, because CRISPR CaS9 can directly edit a specific gene, there is a potential for a single dose therapy. While results thus far are only in a mouse model, it is a critical step in developing viable treatments and a potential cure for, in this study, CMT1A. Time will tell if these results will or can translate into potential therapies or a cure for all of the CMT subtypes.
