Gene Therapy: A Cure for Methylmalonic Acidemia
It takes a village. This African proverb used to mean that a whole community of people are dedicating many of their efforts to reach a common goal, cannot apply more to the scientific community, where researchers are bursting with new ideas, leveraging their arsenals of cutting-edge technologies, and bringing their weapons of knowledge to raise awareness and discover treatments for rare diseases. In particular, doctors and affected families alike are all pouring their efforts into the common goal of finding treatments for methylmalonic acidemia, a very rare and devastating genetic disorder. Well first, what is methylmalonic acidemia?
What is Methylmalonic Acidemia?
Methylmalonic acidemia is a genetic disorder in which the body is unable to break down certain proteins and fats properly. Foods that are in a typical person’s diet, such as chicken, eggs or potatoes are completely off the table. It affects newborn children ( the first year of life), and can range from mild to fatal. Patients often experience vomiting, breathing problems, extreme drowsiness (lethargy), and failure to gain weight and thrive. They may experience complications long-term such as feeding problems, intellectual disability, and kidney and pancreas complications. Even with treatment, the metabolic crises patients face can cause irreversible neurocognitive damage. If the patient isn’t treated for it, this disorder may lead to a coma or death.
*note: this article is only about isolated methylmalonic acidemia
Science behind Methylmalonic acidemia
In a nutshell: Methylmalonic acidemia is caused by changes in one of the five genes: MMUT, MMAA, MMAB, MMADHC, or MCEE. And changes in these genes affect the body’s ability to break proteins and fats down into energy the body can use as fuel.
Deep-dive: About 60% of methylmalonic acidemia cases are caused by the MMUT gene. This gene provides instructions for making an enzyme called methylmalonyl CoA mutase. A mutation in the gene can cause part or all of an enzyme called Methylmalonyl-CoA mutase to be missing. Methylmalonyl-CoA mutase has a partner in crime, called vitamin B12, and they collectively convert the amino acids, fats and cholesterol into a molecule that other enzymes break down into molecules that store energy. Therefore, because this enzyme is missing, the body isn’t able to convert proteins and fats into energy, and as such potentially toxic compounds can accumulate in the body, leading to the disease methylmalonic acidemia.
Methylmalonic acidemia can also be caused by mutations in the genes MMAA, MMAB, or MMADHC. The proteins that these genes encode are required for methylmalonyl CoA mutase to function properly. And because these proteins are affected by mutations in the genes methylmalonyl CoA mutase is impaired again, which causes methylmalonic acidemia. When methylmalonic acidemia is caused by a mutation in the MCEE gene, this disrupts a different enzyme called methylmalonyl CoA epimerase, which also has a role in the breakdown of proteins, fats, and cholesterol.
Here’s an in-depth overview. Amino acids, fats, cholesterol → Propionyl-CoA → D-Methylmalonyl-CoA → L-Methylmalonyl-CoA → Succinyl-CoA. See how methylmalonyl-CoA epimerase, methylmalonyl-CoA mutase, and vitamin B12 are essentially catalysts to break down the proteins and fats, then convert it into succinyl-CoA, which will be further broken down for energy usage? Therefore, if they aren’t functional, the proteins and fats can’t be broken down.
Quick summary:
- There is a mutation in one of the genes MMUT, MMAA, MMAB, MMADHC, or MCEE.
- The mutation of these genes in one way or another causes the enzymes methylmalonyl-CoA mutase or methylmalonyl-CoA epimerase to partially or fully not exist. Remember these genes have the blueprint on how to make these enzymes.
- These enzymes perform key functions: converting proteins and fats into stored energy for later usage.
- Therefore, when these enzymes aren’t functional, the proteins and fats aren’t broken down, and become toxic to the body → which brings about the symptoms of methylmalonic acidemia.
How is methylmalonic acidemia currently treated?
There is no cure for methylmalonic acidemia, but there are ways to manage it. Currently, physicians recommend a low-protein, but a high-calorie diet. This is because since the patient is unable to break down those proteins, ingesting them would further progress the disease. Patients may also receive a liver transplant, because the liver is heavily involved with breaking down proteins and fats. Medication may include antibiotics such as neomycin or metronidazole to kill gut bacteria which produces propionyl-CoA, taking in Vitamin B12 which is essential in the breakdown of proteins and fats (the partner in crime to methylmalonyl-CoA mutase), L-Carnitine to remove propionyl groups and release CoA.
Sometimes, in the case of methylmalonic acidemia, physicians and care-givers believe their current treatments will not offer therapeutic benefit to the patient, and will therefore forgo giving any treatment because it will cause excessive hardship if their life were to be extended. Therefore, caregivers feel not giving treatment is in the best interests of the patient.
Current treatment options all focus on the enzymes along the chain that break down the proteins and fats, or the proteins and fats ingested themselves. This seems like a logical solution. However, these medications are not permanent cures, and they must be taken for life. The average life expectancy of a patient with methylmalonic acidemia is only 20 to 30 years. There HAS to be a better way.
We NEED New Treatment Options
What if we targeted the root cause of the disease itself? The gene that caused the enzyme malfunction — MMUT ~ for 60% of methylmalonic acidemia patients! It seems reasonable that if we are able to edit the gene so that it produces the properly functioning enzyme, then this could be a permanent cure for methylmalonic acidemia, right? Enter gene therapy.
Wait, but what is Gene Therapy?
Gene therapy is a technique to modify a person’s genes to treat or cure disease, by replacing a disease-causing gene with a healthy copy, introducing a new gene that will help the body treat the disease, or inactivating a disease-causing gene.
There are a few gene therapy products currently, but for this article, we’re going to focus on viral vectors and gene editing technology. Viruses are naturally able to deliver genetic material into genes so gene therapy leverages this mechanism to deliver therapeutic genes into human cells. A virus is first modified so it is unable to cause infectious disease, then it can be used as a vector. Think of the modified virus as a car, it transports you to the location you want to go to. Viral vectors are the same, delivering genes to specific human cells in an organ.
Gene editing aims to delete, alter or augment disease-causing genes by introducing breaks in the DNA of cells. First, nuclease enzymes which are taken from bacteria, CUTS the DNA to delete, alter or correct the body’s DNA. Then, once the DNA has been cut, homology-directed repair (HDR) or non-homologous end joining is used. HDR precisely fits the correct DNA into the site the DNA was cut. HDR is super precise, and can avoid any new mutations being introduced at the correction site.
How is this Applicable to Methylmalonic Acidemia?
There have been many studies pioneering new gene therapy approaches to cure MMA, dating back to 2009. However, the first clinical trial is only happening now. It’s called the SUNRISE trial, and it’s currently enrolling patients 3–12 years old and will enroll children who are 6 months to 3 years old once specific safety measures have been established. LogicBio Therapeutics hopes to eliminate the need for invasive liver transplantation, and provide “a safe and durable therapeutic option to treat MMA early enough to make a meaningful difference in patients’ lives.”
Now how does their technology work? It functions on GeneRide technology, which has 4 main components:
- DNA Constructs: A gene encoding the MUT enzyme is encapsulated in an AAV vector, and coupled with homology guides, and 2A peptide to be delivered into the albumin locus (in the liver).
- Homology guides: Precise guiding of gene sequences directs the integration to be placed following a highly expressed gene → this leads to high levels of therapeutic gene expression.
- Transgene: the corrective/therapeutic gene is sensed by the cell’s DNA repair system and is integrated into a site on the patient’s genome called the albumin locus.
- 2A peptide for polycistronic expression: VERY IMPORTANT - allows the transgene to be integrated seamlessly by coupling transcription of the transgene with a highly expressed target gene (called ALB). The ALB gene typically only produces albumin, but with the addition of the transgene, will now produce therapeutic proteins. 2A helps the therapeutic proteins to be expressed at equal levels as albumin, which is now coded for by the ALB gene.
*polycistronic expression: producing 2 distinct proteins from the same mRNA
The site of integration is super important because the location will regulate the transgene expression, such as the levels of therapeutic protein production. This therapy specifically targets the albumin locus, because it’s the highest expressed gene in the liver
Is this method effective?
Following GeneRide treatment, mouse models of MMA significantly improved in weight gain, and most importantly, had significant reductions in plasma levels with methylmalonic acid and disease-relevant biomarkers that typically accumulate in MMA patients.
Amazing, now what? The success of this therapy in mice makes this a very promising treatment, since a single administration can potentially cure a patient. And even more, this is a huge milestone for the MMA community, as it is the only gene therapy treatment that has been brought to the clinical trial stage. Now, we wait for the results of the trial and hope that the results in patients are just as promising.
