Stephanie Budhan ‘20
Early-onset Alzheimer’s disease (AD) is dominantly inherited genetic disorder that causes the development of Alzheimer’s Disease in individuals younger than 65 years old. AD is caused by a point mutation in three major genes, one of which is amyloid precursor protein (APP). APP is cleaved to generate beta amyloid which makes up the amyloid plaque found in the brains of AD patients. One specific form of the mutation of APP, which originated in Sweden, causes increased enzymatic cleavage of APP and more rapid plaque build-up in individuals heterozygous for the mutation.
Although there is no cure for AD, the current researchers utilized a revolutionary gene editing tool called the CRISPR/Cas9 system in attempting to treat AD. The Cas9 protein, a protein that cuts DNA, is recognized by and guided to target DNA by a complementary guide RNA (gRNA) sequence. The DNA and gRNA will base pair and cas9 will try to cut the DNA of the genome. Then, the DNA will be repaired by the cell’s repair mechanisms which will lead to insertions or deletions at the targeted site, effectively knocking out gene expression. The researchers hypothesized that knocking out the mutated APP using CRISPR/Cas9 the mutant allele would reduce overproduction of plaque in diseased mice.
The researchers pinpointed their target DNA sequences by synthesizing the complementary gRNA recognizing sequences. CRISPR/Cas9 were effective in disrupting the mutated APP gene expression by causing an insertion and a frameshift mutation. However, effectiveness of CRISPR/Cas9 was low, and only caused a 2% mutation rate. This may be due to the high amount of target alleles and the inability of CRISPR/Cas9 to mutate them all. As a result, the researchers were unable to assess reduced plaque build-up in the brains of the mice. Furthermore, it was unclear what amount of cells would have to be targeted to avoid disease development. Regardless, the current study shows proof-of-concept that CRISPR can successfully disrupt mutations associated with AD.
- B. Gyorgy, et. al., CRISPR/Cas9 Mediated Disruption of the Swedish APP Allele as a Therapeutic Approach for Early-Onset Alzheimer’s Disease. American Society of Gene and Cell Therapy 11, 429-440 (2018). doi: https://doi.org/10.1016/j.omtn.2018.03.007
- Image retrieved from: https://en.wikipedia.org/wiki/CRISPR#/media/File:DNA_Repair.png