Human Artificial Chromosomes

The importance of centromeres was discovered by Australian scientist Andy Choo from the Murdoch Childrens Research Institute in Melbourne, Australia while he was studying the genome of a developmentally delayed 5-year-old child. He observed that the tip of chromosome 7 had been broken off in all the cells he studied. Normally, fragmented DNA broken off from chromosomes gets lost or extruded from the cell. Interestingly, he also noticed that this broken off fragment remained in the nucleus and did not get extruded because it had somehow developed a new centromere called a neocentromere. By using this neocentromere, Choo and his colleagues were able to produce an HAC approximately one-hundredth the size of a normal human chromosome.

Earlier attempts to create HACs failed because such artificial chromosomes lacked fully functional centromeres. Without a functional centromere, these early HACs would not properly divide during cell division and thus, would not remain intact or stable for more than a few cell cell divisions. In 1997, research scientists at Case Western Reserve University and Athersys, Inc., (a private company that conducts research into the development of therapeutic and diagnostic products, including research into the stability of chromosome structure and function) announced the creation of the first stable HAC. Functional HAC centromeres were constructed from alpha satellite DNA, a type of highly repetitive DNA found in and surrounding normal chromosomal centromeres. Alpha satellite DNA is difficult to sequence and might not be practical clinically due to regulatory requirements mandating knowledge of the exact sequence of any vector used for gene therapy. Choo's HAC, however, does not have alpha satellite DNA and is therefore more easily sequenced.

Another report of a DNA-based HAC that has been developed came from a joint venture between Chromos Molecular Systems Inc. of Canada and the Biological Research. These HACs might potentially provide scientists with the alternative, low risk vector for gene therapy that researchers pursue. This vector has been shown to be stable, and expresses DNA in a reproducible manner. This method allows geneticists to insert genes into human cells without the risk of disrupting other genes because it is a distinct chromosome itself and does not integrate directly with the human genome.