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A breakthrough in “chemical surgery” that can correct a type of genetic mutation behind a host of diseases has been unveiled by researchers.
Scientists are hopeful that the approach could offer new ways to understand – and even one day tackle – certain human genetic diseases by correcting mutations in a patient’s body.
There are tens of thousands of human genetic variations associated with disease. While many diseases involve multiple variations, a large proportion, including sickle cell anaemia, arise as a result of just one genetic “misspelling” – known as a “point mutation”.
In other words, there is an error in one of the “rungs” of the DNA double helix – structures that are made of pairs of molecules known as “bases” that come in four forms dubbed A (adenine), T (thymine), C (cytosine) and G (guanine). While A pairs with T, C pairs with G.
About half of the 32,000 known point mutations that are linked to diseases are down to bases that ought to be G instead being A and their corresponding pair being a T instead of a C.
Now scientists say they can fix these errors in a process known as “base editing”, turning A bases back to G and T bases back to C using a modified version of the gene editing tool Crispr–Cas9.
Crispr, or to give it its full name, Crispr-Cas9, allows scientists to precisely target and edit pieces of the genome. Crispr is a guide molecule made of RNA, that allows a specific site of interest on the DNA double helix to be targeted. The RNA molecule is attached to Cas9, a bacterial enzyme that unwinds the DNA and works as a pair of “molecular scissors” to cut it at the exact point required. This allows scientists to cut, paste and delete single letters of genetic code.
The study comes hot on the heels of previous research by the same team that converted C to T and G to A in cells, as well as research by other scientists including a team in China who recently used base-editing techniques to convert G to A in human embryos.
“We are currently using base editing to try to study or validate potential future therapeutic treatments for blood diseases, genetic deafness, genetic blindness … and some neurological disorders as well,” said David Liu, co-author of the new research from the Broad Institute of MIT and Harvard.
Writing in the journal Nature, Liu and colleagues describe how they developed the new Crispr-Cas9 base editing tool.
Within the new machinery, a section of single-stranded genetic material known as RNA directs the tool to the faulty section of DNA, which the Cas9 protein “unwinds”. A specially-developed enzyme within the tool then chemically alters the A base, turning it into a molecule known as inosine, which is “read” as a G by the cell’s machinery.
Crucially, the Cas9 protein used in the tool has been hobbled so that it cannot sever the two strands of DNA, as is typical with gene-editing techniques.
Instead, the editing tool only makes a notch in the opposite strand of DNA near the “problem” base, tricking the cell into replacing the DNA strand around the site.
“That nick prompts the cell to replace the T with a C, because the base opposite the T has been converted to inosine, which pairs with C,” said Liu, adding that the team has shown the approach to work in both the cells of bacteria and of humans.
The technique, Liu adds, has some advantages over traditional Crispr-Cas9 techniques for switching base pairs, not least that it is less prone to problems of random insertions or deletions, was not found to cause unwanted changes to the base pairs, and works well in adult cells. However, he noted that base-editing cannot be used to insert or delete stretches of DNA.
The team also demonstrated applications of the work, including using the new base editing tool to correct a point mutation in cells taken from a patient with a condition known as hereditary haemochromatosis, which causes excessive iron-build up in the body and is currently managed by regularly taking blood from patients.
But, Liu warns, more work will be needed to “cure” diseases. “There are many additional steps beyond simply making the mutation that may be needed to treat [a] disease,” he told the Guardian.
Robin Lovell-Badge, a developmental geneticist from the Francis Crick Institute in London, described the new research as clever and important science.
“Many genetic diseases are due to mutations where a single base pair has been substituted for another,” he said. “This makes these new base editing methods of great value in both basic research to make disease models and, in theory to correct genetic disease – making either non-hereditable or hereditable alterations.”
Darren Griffin, a geneticist at the University of Kent, suggested the research could lead to improvements in cultures for embryos which might improve IVF. But he said, it is less likely to be widely used to fix genetic errors in human embryos destined for birth, since – even without considering ethical issues – embryo screening can already allow doctors to select embryos without harmful mutations.
However, Liu notes that the technology could be used in patients after birth to tackle genetic diseases.
A second study, published in the journal Science, reveals another step forward for gene-editing with a new type of Crispr-based tool that can convert A bases to inosine in RNA.
Since the changes produced using the tool do not occur in the DNA, they only temporarily alter the proteins generated – a development that could not only avoid some of the ethical dilemmas around gene editing, but also offer ways to tackle diseases caused by temporary changes within a cell, such as inflammation. However, further work is needed to tackle the problems of changes being made to unintended targets.
“I am hopeful that as complementary approaches, DNA base editing and RNA base editing will together enable an especially broad set of potential research and therapeutic applications,” said Liu.
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'Chemical surgery' can correct genetic mutations behind many diseases – studyhttps://goo.gl/o2bZAF
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