CRISPR-Cas9 Allele-Specific Chromosome Elimination in Human Trisomy 21 Cells: A Critical Review of the Mie University Breakthrough and Its Scientific, Clinical and Ethical Implications

Down syndrome (DS), caused by trisomy of human chromosome 21 (HSA21), is the most prevalent chromosomal disorder worldwide, affecting approximately one in every 700 live births. For more than six decades following the 1959 discovery by Jérôme Lejeune, therapeutic interventions remained purely supportive, unable to address the disorder’s fundamental genetic cause. In February 2025, researchers at the Mie University Graduate School of Medicine published a landmark study in PNAS Nexus demonstrating that allele-specific multiple chromosome cleavage using the CRISPR-Cas9 platform can achieve trisomic rescue—that is, the targeted elimination of the supernumerary chromosome from human induced pluripotent stem cells (iPSCs) and dermal fibroblasts derived from individuals with DS. Through haplotype phasing and the design of guide RNAs exclusive to the M2 allele, the team attained chromosome elimination rates of up to 37.5%. The corrected cells displayed normalized gene expression, enhanced proliferation, reduced reactive oxygen species (ROS), and improved mitochondrial function. This review critically analyses the molecular strategy employed, evaluates the experimental outcomes, and identifies the technical challenges that persist. It also explores the far-reaching ethical, social, and regulatory questions posed by chromosomal-level human genome editing. Finally, we discuss the implications of this breakthrough for regenerative medicine, the treatment of other chromosomal aneuploidies, and the ongoing global debate over neurodiversity versus medical correction.