Down syndrome, caused by an extra copy of chromosome 21 (trisomy 21), is the most common genetic reason for intellectual disability. Because of this, it remains a major focus for prenatal and preimplantation diagnosis. However, efforts to directly remove the extra chromosome from human cells have been very limited.
In a groundbreaking study, scientists have shown that CRISPR-Cas9, a powerful gene-editing tool, can be used to eliminate the extra chromosome from cells affected by trisomy 21. By designing the CRISPR system to specifically target unique DNA sequences (allele-specific sites) on the extra chromosome, researchers were able to “cut” and remove it from human induced pluripotent stem cells (iPSCs) and fibroblasts (skin cells).
Unlike earlier approaches that were not specific to which chromosome copy was targeted, this new method precisely identifies and removes only the extra chromosome. The researchers developed a detailed strategy to extract exact target sequences for Cas9, ensuring that only the supernumerary chromosome is affected while leaving the normal copies untouched.To increase the success rate of chromosome removal, the team temporarily reduced the activity of certain genes involved in DNA damage repair. This made it easier for the cells to lose the targeted chromosome.
Importantly, after removing the extra chromosome, the corrected cells showed improvements in gene expression and cellular functions that are usually disrupted in Down syndrome.One of the most remarkable findings is that this technique also works in non-dividing, mature cells, not just in stem cells. This opens the possibility for future therapies that could correct trisomy 21 in a wide variety of tissues.The researchers believe that this allele-specific (AS) approach could pave the way for advanced treatments aimed at correcting trisomy-related disorders like Down syndrome at the cellular level.
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