Artist's impression of double-stranded DNA being opened. In cells, this is the first step in the transcription of a DNA template sequence into mRNA. Credit: Eugene Mymrin/ Moment/ Getty Images.
COVID-19 vaccines may have been the first approved therapies based on messenger ribonucleic acid (mRNA), but they will not be the last. The pandemic has brought into the spotlight a technology that had been under investigation for a decade, and could now be applied to conditions ranging from viral infections to cancer and autoimmune diseases.
Italian laboratories, in particular, are taking advantage of the surge of interest, and investment, in this technology to accelerate research on inherited metabolic disorders. These diseases are caused by faulty genes that impair the expression of proteins with key roles in metabolic pathways, resulting in the lack of a vital protein, or in build-up of toxic by-products. They are often life-threatening or disabling, but they could be cured by replacing the missing or defective protein. That is exactly what scientists expect from messenger RNA. In the organism, its job is to copy the information contained in genes and deliver the blueprint to produce a specific protein to the cell’s machinery. A synthetic mRNA sequence with the right blueprint can be turned into a drug and cause the organism to produce the desired protein.
Research on these disorders is particularly active in Italy, where every newborn is tested for more than 40 of such conditions, to ensure early diagnosis and access to a proper diet or supplements when necessary. “The national screening programme gives scientists a unique opportunity to study different phenotypes of metabolic diseases and their natural history from the very beginning”, says Alberto Burlina, Head of the Division of Inherited Metabolic Diseases at University Hospital in Padova.
An alternative to transplants
Burlina and his team are currently working on methylmalonic acidemia, a rare disorder that prevents the body from processing some amino acids and results in the accumulation of methylmalonic acid in the kidneys and the central nervous system. It can cause kidney failure, brain damage, seizures, coma, and affects between one per 50,000 and 1 per 100,000 live births, with life expectancy spanning from days to years, depending on the severity.
The Padua team was already collaborating with the US biotech company, Moderna, on m-RNA based therapies for methylmalonic acidemia before the company started developing its Covid-19 vaccine. “Moderna created a specific messenger to instruct cells to replace defective enzyme methylmalonyl-CoA mutase, which is the source of the disease,” says Burlina. RNA is not a once-and-for-all cure: patients would need periodic infusions, but this approach has the advantage of bypassing potential risks of gene therapy. “Messenger RNA therapy can be interrupted at any time if needed for safety reasons, and mRNA is rapidly eliminated”, says Paolo Martini, Chief Scientific Officer of Rare Diseases at Moderna Therapeutics in Boston, in the US. “Besides, dosing can be modulated in order to control effectiveness”. The Padua University Hospital helped Moderna validate experiments on animals. “Later this spring mRNA for methylmalonic acidemia will be tested on patients,” says Martini.
Currently, the most serious cases of methylmalonic academia require a liver transplant, says Carlo Dionisi-Vici at Bambino Gesù Pediatric Hospital in Rome, who is also researching new therapies for this disease. “With mRNA therapy, patients wouldn’t need to take immunosuppressant drugs for the rest of their lives like transplanted children do,” he explains. But, current mRNA technology would also share the limits of transplants, that reduce the acid build-up in almost every tissue, but not in the cerebrospinal fluid, and thus are not effective on neurological damage. Similarly, the m-RNA drug delivered in bloodstream goes straight to the liver, but does not get to the brain. Martini says his group is working on options to cross the blood-brain barrier, composed of endothelial cells lining the brain micro blood vessels which only let a few selected molecules reach the brain tissues. A possibility is use nanoparticles armed with molecules that bind to specific receptors on these endothelial cells’ surface, like keys opening locks.
Another concern is the possibility of an immune response to the lipid nanoparticles that envelop mRNA and help deliver it to the cells. In a vaccine, such a reaction is welcome and nanoparticles act as adjuvants to enhance it. But, in a drug that needs repeated administrations, that could reduce the effectiveness in the long term. “We haven’t seen this issue in animals after chronic treatment”, says Martini. “But antibodies directed to polyethylene glycol, a component of nanoparticles, might be an issue and we are testing a slightly different type of nanoparticles that free themselves from polyethylene glycol once injected”.
Methylmalonic acidemia is a model for a number of inborn metabolic disorders that could benefit from mRNA-based therapy. An example is glycogen storage disease type 1B, where a lack of the enzyme glucose-6-phosphate translocase, causes an accumulation of glycogen and fat in the liver and kidneys. It affects one per 100,000 live births, and severe undiagnosed cases result in early childhood death. “Patients suffer from recurrent hypoglycemia episodes, sometimes neutropenia (low white blood cells count) and inflammatory bowel disease,” says Nicola Brunetti-Pierri, at Telethon Institute of Genetics and Medicine in Naples. His group is also collaborating with Moderna to develop a mRNA-based therapy that targets cells in the liver. “We are now about to begin experiments on mice, to understand whether mRNA is able to fix the problem and how long its effect would last” he says.
The next steps
Researchers at the Institute of Genomic Medicine at the Catholic University of the Sacred Heart in Rome are working on a mRNA-based therapy for fragile X syndrome, the most common cause of inherited intellectual disability. “It is due to the deficit of a protein normally expressed by the FMR1 gene in neurons”, explains Elisabetta Tabolacci. “We tried to reactivate the gene, but it’s a difficult task because we lack selective drugs to address the specific target” she says. After reading about research on methylmalonic acidemia, Tabolacci thought of bypassing the impaired gene altogether, and trying to induce synthesis of the protein with mRNA. “We tested the idea in vitro, on neurons from fragile X patients, and it worked. It could reverse neurological damage in adult patients” she says. Now Tabolacci and colleagues are going to test the treatment on mice in collaboration with another research group, but, just like for methylmalonic acidemia, success will ultimately depend on the possibility to reach the central nervous system from the bloodstream. “We could use intraspinal injections, but it would be better to be able to cross the blood-brain barrier.”
Researchers still have a lot of work to do, but the role of mRNA in the rapid development of COVID-19 vaccines highlighted its strong suit: a great versatility that makes it possible to design a new drug with a simple re-write of RNA coded information without changing the production platform and facilities, a feature that is particularly useful when focusing on rare diseases. In addition to Moderna, several US biotech companies such as Translate Bio, Arcturus, Intellia are working on other metabolic disorders, ranging from cystic fibrosis to amyloidosis. “What the pandemic showed us is only the tip of the iceberg”, says Burlina. “mRNA technology has the potential to change the clinical course of a number of diseases, and the next years will be pivotal in the fight against inherited metabolic conditions.”
