What is gene therapy?

Gene therapy is ‘the use of genes as medicine’. It involves the transfer of a therapeutic or working gene copy into specific cells
of an individual in order to repair a faulty gene copy. Thus it may be used to replace a faulty gene, or to introduce a new gene whose function is to cure or to favourably modify the clinical course of a condition.

The scope of this new approach to the treatment of a condition is broad, with potential in the treatment of many genetic conditions, some forms of cancer and certain viral infections such as AIDS.
Gene therapy remains an experimental discipline however and much research remains to be done before this approach to the treatment of certain conditions will realise its full potential.

The majority of clinical gene therapy trials are being conducted in the United States and Europe, with only a modest number initiated in other countries, including Australia. The majority of these trials focus on treating acquired conditions such as cancer.

A form of immune deficiency called adenosine deaminase (ADA) deficiency was the first condition to be treated with a gene therapy approach in humans in the early 1990s. It is also the first condition for which therapeutic gene transfer into stem cells (see later) has been attempted in the clinical arena (Candotti F, 2001).

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A limited glossary relevant to CNS Gene Therapy

Genes, DNA, alleles. Our genetic program is made up of thousand of genes, stretches of DNA, that generally code for different proteins that do particular jobs in the cells in our body. The process of decoding a gene into a protein occurs in two basic stages: 'transcription', where specific mRNA molecules are generated from the DNA stretch concerned; and 'translation', where the specific mRNA molecules are used as a template to make the specific protein. For any particular gene there are usually two copies (sometimes the term 'allele' is used instead of copy), one from the mother and a second passed on from the father. For most genes, only one normal copy is required for normal function.

Chromosomes. The DNA/genetic code in the nucleus is packed into large very long macromolecules called chromosomes, 46 in total , including two sex chromosomes. One half of the chromosomes come from the mother, i.e., there is a maternal chromosome 1, 2, 3, 4, etc and an X (sex) chromosome One half come from the father, i.e., there is a paternal chromosome 1, 2, 3, 4, etc and a sex chromosome, either an X or Y. If the two sex chromosomes are both X, then the individual is female. If there is one maternally derived X and a paternally derived Y, then the individual is male.

Cell, nucleus, cytoplasm. A cell consists of a nucleus, i.e., a membrane bound compartment that contains the cell's DNA (genes, chromosomes). The part of the cell outside the nucleus is called the cytoplasm.

Exons, introns, promoter. The stretch of DNA making up a gene is separated into coding stretches called exons with non-coding stretches of DNA in between the exons called introns. Just before any gene is an area of DNA that is specific for switching on or off the gene known as the 'promoter'.

Wild type, Knockout. Wild type – refers to cells in culture or animal models where the gene under consideration is normal. Knockout – refers to cells in culture or animal models where the gene under consideration is non-functional.

Vector, transgene, transduction. A 'vector' is used to insert a new gene, known as the 'transgene', into a cell. The vector is usually a modified non-disease causing virus. Examples of viral vectors used include lentiviruses (a type of 'retrovirus'), adeno-associated virus, or foamy virus. The process of using a vector to insert a transgene into a cell is called transduction (transfection is sometimes used as an alternative term). Some vectors result in the transgene being incorporated into the cell's DNA. Other vectors do not.

Insertional mutagenesis. Insertional mutagenesis refers to the possibility of a transduced cell becoming cancerous if a transgene is incorporated into the DNA of a cell such as to disrupt the control of a cell division gene.

Methylation, silencing. One way in which the cell can switch off genes in an almost permanent manner is by a process of 'methylation' to the promoter region or other parts of the gene. This switching off is known as 'silencing'. A transgene can also be sometimes turned off or silenced by methylation.

In vitro, in vivo. In vitro – an experiment or process done in the test tube/cell culture. In vivo – an experiment or process done in a living animal or in humans.

Blood brain barrier. The blood brain barrier is a cellular barrier around blood vessels in the brain that provides extra protection to the brain from circulating substances. To do gene therapy in the CNS, a strategy has to be considered to get through the blood brain barrier.

CSF. Cerebrospinal fluid (CSF) is clear fluid that bathes the brain and spinal cord, produced for the most part in four cavities in the brain known as ventricles. About a pint of such fluid is produced on a daily basis.

Neurons, astrocytes, oligodendrocytes, microglia, anterior horn cells. There are four major cell types in the nervous system (note that astrocytes, oligodendrocytes, and microglial cells) are collectively known as glial cells:

  • Neurons – the nerve cells with connecting processes called axons. The communication points between neurons are called synapses.
  • Astrocytes – these are the supporting cells in the brain, assisting the functioning of neurons.
  • Oligodendrocytes – these cells provide the 'wire insulation' to the neuron axons in the form of myelin sheaths.
  • Microglia – these are the CNS white blood cells and are derived from the bone marrow. Their turnover is over a one to two year period.

Note that one important type of neuron in the spinal cord responsible for control of muscles is called the anterior horn cell.

Autosomal recessive, autosomal dominant, X-linked. These terms are used to describe the common modes of inheritance for genetic disorders.

  • Autosomal recessive – where a genetic disorder requires both copies of a gene to be abnormal to cause the disease. Both parents of the affected individual are carriers, i.e., carry one abnormal copy but also have a normal copy so they themselves are not affected.
  • Autosomal dominant – some genetic disorders only need one copy of the gene to be abnormal, i.e., having one normal copy is just not enough. One of the parents is usually affected.
  • X-linked – is where the gene is on the X (sex) chromosome The mother is usually a carrier with only the male children being at risk of having the disorder.

Homozygous/heterozygous. Terminology used in a number of different contexts. One context is: homozygous, where a mistake is present in both copies of a gene; versus heterozygous, where the mistake is present in only one of the two gene copies.

Further glossaries relevant to gene therapy can be found at http://www.genetherapynet.com/glossary-of-gene-therapy-terms.html and http://ghr.nlm.nih.gov/glossary=genetherapy.

Thomas Carroll, 29th October 2011