Researchers have found a way to shut down the extra chromosome that causes the developmental issues seen in Down syndrome — a step forward in possibly treating the disease or preventing it from causing harm early in pregnancy. Currently, people who are diagnosed with Down syndrome have physical as well as developmental and mental deficits. Educating, training, and helping diagnosed individuals to live a full life have progressed significantly in the last 30 years, leading to extended lifespans and helping individuals to hold jobs and become self-sufficient.
Down syndrome, also known as Down's syndrome and trisomy 21, is a condition that results from having an extra copy of the 21st chromosome. When the sex cells of the sperm or egg duplicate themselves, an uneven number of the normal 23 chromosomes may be transferred to the daughter cells, a process called non-disjunction. This ultimately results in an extra copy of chromosome 21 in the sperm or egg, and when combined with the opposite sex cell, it results in three copies of the chromosome.
Testing for Down syndrome can be done by a sampling of the amniotic fluid of the womb, or amniocentesis, or by an ultrasound that can visually pick up features of the fetus without being invasive and risking harm to the fetus. In the UK, elective abortion rates for fetuses found to have Down syndrome hovered around 91 to 93 percent with rates in the U.S. estimated to be between 87 and 98 percent.
Researchers at the University of Massachusetts Medical School thought of an innovative way to shut down the extra chromosome by using a tool that nature has been using for millions of years. Each female has two copies of their sex chromosome, the X chromosome, yet only one is randomly active in any cell at any one time. The gene called XIST, acting through its RNA, is responsible for physical changes to the shutdown X chromosome that can alter its density and limit accessibility to factors that can activate genes on the chromosome. This is also active in blocking expression of genes on the Y chromosome found in male cells that limits the expression of genes that the Y and X chromosome share in common. This prevents excessive expression of genes on the the chromosomes and is a tightly-controlled and well-known mechanism.
The researchers used a new targeted technology called zinc finger nucleases (ZFN), which is being used by the biotechnology company Sangamo BioSciences to target other diseases like Huntington's Disease and is avaliable to research institutions for their experiments. The technique utilizes enzymes to specifically insert a DNA segment into a specific spot on a chromosome, without affecting other genes. The researchers inserted the XIST gene into one copy of chromosome 21 in induced pluripotent stem cells (iPS) from a patient with Down syndrome.
By using a system where they could induce activation and expression of XIST, the researchers were able to effectively shut down the third copy of chromosome 21. This technique will allow researchers in the future to better understand the mechanisms that start at the third copy of chromosome 21 and result in various physical and mental deficits in patients with Down syndrome.
"Finally, the more forward-looking implication of this work is to bring Down's syndrome into the realm of consideration for future gene therapy research. Although development of any clinical gene therapy is a multi-step process, any prospect requires that the first step, functional correction of the underlying genetic defect in living cells, is achievable," the researchers said. "We have demonstrated that this step is no longer insurmountable for chromosomal imbalance in Down's syndrome. Our hope is that for individuals and families living with Down's syndrome, the proof-of-principle demonstrated here initiates multiple new avenues of translational relevance for the 50 years of advances in basic X-chromosome biology."
Source: Jiang J, Jing Y, Cost G. Translating dosage compensation to trisomy 21. Nature Genetics. 2013.