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Gene therapy for MLD - What is this?

A few days ago, I read some exciting news on BBC News that gave me no peace, and I wanted to investigate. A 19-month-old boy is the first patient to receive gene therapy in England through NHS, the national health service. The cellular drug to be administered is the most expensive ever approved by the NHS, costing 2.3 million euros. The boy's condition is Metachromatic Leukodystrophy, one of the 'Rare Diseases' as these are called. Many pharmaceutical and biotechnology companies are vigorously researching to develop RNA drugs and cellular therapies to treat rare diseases.

Many questions immediately came to mind, and I researched the disease and therapy and then looked at the research field on rare diseases.

1. What is the boy's disease?

2. What is the gene, and what is the gene defect in MLD?

3. What is the approach to treatment?

4. How does this type of gene therapy work?

5. How is the drug administered, and how does it work in the body?

6. What are rare diseases in research, and how will they be treated?

7. What causes rare diseases?

Let's dive in ......

What is the boy's disease?

The boy’s rare disease is called metachromatic leukodystrophy, abbreviated as MLD. In the white matter of the brain and the peripheral nervous system, which contains the processes of the nerve cells (info: the gray matter has the cell bodies), a harmful accumulation of fat molecules of the class sulfatides happens.

Sulfatides are produced in the brain’s oligodendrocytes and the Schwann cells of the peripheral nervous system to form the myelin sheaths that wrap around nerve processes. The production of sulfatides must be balanced with their degradation so that these molecules do not accumulate.

The degrading enzyme is called arylsulfatase A. Patients with MLD have a deficiency of arylsulfatase A, caused by a defect in the corresponding gene. As a result, sulfatides accumulate, and oligodendrocytes and Schwann cells degenerate. Nerve cells without myelin sheaths transmit excitation very slowly. The consequences are muscle weakness and paralysis, loss of sensory perception, and dementia.

MLD is diagnosed by blood analysis. By definition, MLD is present in patients with symptoms when arylsulfatase A activity in leukocytes is less than 10%. In addition, genotyping can be performed, which means that DNA is analyzed for mutations.

The boy has the most common form of MLD, which begins in children between 15 and 24 months of age. Less common forms start in adolescence or adulthood but are also lethal.

What is the gene, and what is the gene defect in MLD?

The gene defect is a mutation in the ARSA gene on chromosome 22. ARSA codes for the lysosomal enzyme arylsulfatase A. Over the past three decades, many patients have been genetically diagnosed in research studies, and various errors have been found in the gene. This indicates that multiple mutations exist and are inherited in the population. Most mutations result in the inability to form a functional protein.

Info: What are gene defects?

Genes can be defective. This means that the code, which the DNA defines via the nucleotide bases, is changed. Please see the illustration 'Protein made from messenger RNA' in my Blog article 'What is a gene?'. It happens, for example, that a single base is missing or has been exchanged - instead of A-C-G-T-C-C, the code is A-C-T-T-C-C. Such a change is called a mutation. Now imagine that the mutation is in the DNA sequence of a gene that codes for a protein. Then this mutation is also in the gene transcript that is the template for building a protein - so the messenger RNA is also changed. As a result, a different amino acid is incorporated into the polypeptide chain. This results in an altered protein. The protein could be too small or incorrectly shaped, and it is also possible that no protein is formed at all.

What is the approach to treatment?

MLD is treated with gene therapy. At the end of 2020, the European Medicines Agency (EMA) approved the marketing authorization of the drug Libmeldy. Libmeldy was developed by an English biotechnology company called Orchard Therapeutics.

This therapy is based on introducing a functional ARSA gene into the body’s hematopoietic stem cells. These subsequently differentiate into various blood cells, which produce an active enzyme. The enzyme reaches the cells of the organs via the blood. The therapy can be used only once and only by physicians experienced in hematopoietic stem cell transplantation.

The therapy is effective only if it is started before the first symptoms appear. Doctors, therefore, recommend screening of newborns.

How does gene therapy in MLD work?

Libmeldy is produced individually for each patient because the patient's hematopoietic stem cells are used. These are taken from the patient's blood after first stimulating their multiplication in the bone marrow and then their emigration into the blood – a process known as hematopoietic stem cell mobilization. Alternatively, the stem cells can be taken directly from the bone marrow.

A functional ARSA gene is inserted into a lentivirus vector because this has the property of integrating into the host cell’s chromosomes. The vector carries only a part of the lentiviral genome, meaning it does not contain genes required for virus production. The ARSA gene that can be activated in human cells was added to this vector. Finally, genetically modified vector-DNA is combined with virus proteins and reagents and brought into special packaging cells to produce viruses. These cells are kept in a nutrient solution, and viruses are synthesized in large numbers and can be harvested.

Info: A vector is a ring-shaped DNA, also called a plasmid, which occurs naturally in bacteria and is an addition or part of their genome. Plasmids confer specific characteristics by the genes they contain. Researchers have modified plasmids to insert DNA they want to work with. Vectors are, therefore, vehicles for particular DNA-related tasks.

The next step is to combine the stem cells with the viruses. Now, the lentivirus vector containing the ARSA gene integrates into the chromosomal DNA of the stem cells. New viruses cannot be made because they lack the critical genes for multiplication.

Stem cells containing the ARSA gene at an appropriate location in the genome are grown in a particular culture medium so that a sufficient number of cells are available for therapy. This is the drug Libmeldy.

How does the gene therapy drug enter the patient?

An appropriate number of the patient's genetically modified hematopoietic stem cells are transferred to an infusion solution. This is approximately 10 to 20 ml of Libmeldy per infusion bag.

The infusion can be administered after the patient has been appropriately medically prepared for the stem cell transplant.

The cells find their way through the bloodstream into the bone marrow, where they can settle. These give rise to blood cells that express the ARSA gene and supply the body with arylsulfatase A.

What are rare diseases in research, and how would they be treated?

A rare disease, by definition, affects less than 1 in 2,000 people. According to the European Commission, between six and eight thousand rare diseases exist, affecting around 30 million people in the EU. Most often, these result in ambiguous symptoms and are challenging to diagnose. Because they are so rare, doctors often have no experience with them. Rare diseases can be very severe and lead to early death. There are four million patients with a rare disease in Germany.

Through molecular biological and technological achievements and the publication of the human genome in the last two decades (Info: Genome = totality of genetic information encoded in DNA), it is now possible for research and industry to develop suitable therapeutics based on gene, cell and RNA therapies, and biomarkers for diagnostics.

The European Commission and the German Federal Ministry of Research and Education generously support the development of rare disease therapies.

What causes rare diseases?

These diseases result from a genetic defect, i.e. a mutation in the DNA that remains undetected until the gene usually functions. In my blog article „How does a gene make a protein“, I explain that genes code for proteins and that most genes are present in duplicate because one is inherited from the father and the other from the mother. I show a learn box about this topic below.

A functional protein can likely be sufficiently produced when one of the two genes has a regular DNA sequence. Conversely, if both parents inherit a defective gene, the disease that results from the absence of the protein or from the production of a toxic one will develop. Such gene-phenotype occurrences are called recessive. The gene defects hide in the population and are only detected by DNA analysis.


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