CRISPR gene editing offers promise for treating defects

A new gene editing technique called CRISPR-Cas9 has turned the biology world upside down and will have an immeasurable effect on genetic and molecular biology research in the coming years.

CRISPR-Cas9 is a word processor for editing and engineering genes.

Genetic engineering has made great strides since its beginnings in 1972 when Paul Berg created the first recombinant DNA.

By the late 1970s a company was producing insulin for diabetics using genetically modified Escherichia coli that contained a synthetic human gene.

In 1990 the National Institutes of Health and the Department of Energy joined with international partners to undertake the Human Genome Project. Its quest was to sequence all 3 billion letters, or base pairs, in the human genome, which is the complete set of DNA in the human body.

In April 2003 researchers successfully completed the Human Genome Project, under budget and more than two years ahead of schedule.

CRISPR-Cas9 technology can repair genes by inserting short sequences to replace defective genes, such as in cystic fibrosis, or to provide immunity to certain existing cancer cells. CRISPR is an acronym for “Clustered Regularly Interspaced Short Palindromic Repeats.” In bacterial DNA there are unusual repeating DNA segments that are separated by noncoding spacers, sometimes mistakenly called “junk DNA.”

Near each CRISPR sequence, there are genes for a variety of CRISPR-associated enzymes (Cas) that constitute a bacterial immune system. Bacteria tackle invading viruses by making Cas enzymes that snip off bits of the invaders’ DNA and stuff them into spacers. Here they act as RNA guides to recognize future invaders.

CRISPR works in nature by creating two small RNA segments called crRNA and trRNA. These two RNAs bind to Cas proteins to form complexes that snip the viral DNA and inactivate it. Researchers use this natural defense system to synthesize guides that search out a specific sequence of DNA bases.

CRISPR-Cas9 is a two-stage system packaged in a single molecule: a programmed, guiding crRNA (gRNA) and the cutting Cas9 enzyme. When the gRNA finds its targeted DNA, it bonds to it so that Cas9 can slice through both strands of the double helix.

In nature, once the DNA is cut, the cell attempts to repair it by ligating it back together. In doing so, it often causes a small insertion or deletion that changes the DNA sequence. Most of these mutations are fatal, but occasionally one of them carries through cell reproduction, rendering successive offspring immune to that virus.

Scientists can take advantage of this by deleting or adding their own sequences at the site of the break.

CRISPR technology is rapidly evolving to add tools to molecular biology and genetics research since it is possible to alter the DNA of any cell and all cells depend on DNA.

This includes the possibility of altering human embryos, which is the subject of much debate.

Meanwhile, watch for a Nobel Prize for researchers Jennifer Doudna and Emmanuelle Charpentier for their work in developing CRISPR.

Richard Brill is a professor of science at Honolulu Community College. His column runs of the first and third Friday of the month. Email questions and comments to brill@hawaii.edu.