Gene Therapy, Cell Replacement & Tissue Engineering

Gene Therapy

• Aims to treat or cure genetic abnormalities by replacing faulty genes with healthy ones

• Way of using genes themselves as treatment

• Human genome project revealed location of around 4000 potentially faulty genes

• Currently, gene therapy concentrating on single-gene disorders such as cystic fibrosis, Huntington’s disease, muscular dystrophy & sickle- cell anaemia

• Has potential to correct underlying cause by replacing faulty gene with healthy one

• Cystic Fibrosis (CF)

  CF is most common life-threatening genetic disorder among Australians of European descent

  • Affects lung & pancreas, sometimes liver & reproductive organs

  • Characterised by thick, sticky mucus secreted by mucous glands

    • In lungs, mucus can clog air passages & trap bacteria (more susceptible to infection)

  • Can cause irreversible lung damage & shorten life expectancy

  • Pancreas also affected, preventing secretion of enzymes required for digestion

  Results when individual inherits recessive allele for condition from each parent

  • Identification of Cystic Fibrosis Transmembrane Regulator (CFTR) gene in 1989 was a major step forward in developing a treatment of CF

  • 1991: Scientists successfully corrected faulty CFTR genes in cultured cells by adding normal copies of gene to the culture. First step toward gene therapy for CF

  • Logical choice for gene therapy for 3 main reasons:

    • Single gene disorder

    • Lungs (most severely affected organ) relatively easy to access to provide treatment

    • Disease is slow to progress, lungs of a newborn being virtually normal

  • Would enable gene therapy to begin before significant lung damage started to occur

  1993: First experimental gene therapy CF patient

  • Researchers modified common cold virus to act as vector to carry normal genes to cells in airways of lungs

  • First study was mainly concerned with safety issues of treatment

  • Amount of gene transfer was probably too small to have any real therapeutic benefit & any benefit short lived

  • Trials with alternative methods of gene transfer are continuing

• Huntington’s Disease

  • Single-gene, incurable genetic disorder that researchers believe gene therapy can be used to slow down or prevent its development

  • Caused by a mutation on chromosome 4 called IT15 & symptoms seldom appear before age of 40

  • Mutated form of protein called huntingtin results in nerve cells in brain being damaged, causing physical, mental & emotional changes

  • Unintentional flailing movements of arms & legs, difficulty with voluntary movements of limbs, progressive dementia

    • US: positive research on mice

    • France: experimented with a modified virus to deliver corrective gene into brain cells that boosts natural shield against defective huntingtin protein (rats & primates)

Cell Replacement Therapy

• Stem cells are undifferentiated cells that are capable of repeated mitotic divisions for long periods of time & in right conditions, can differentiate into specialised cells

  • Stem cells are ideal for producing replacement tissues

• Any disorder involving loss of, or injury to, normal cells is a potential candidate for stem cell replacement therapy

• Cell replacement therapy for nervous system has generated most interest due to debilitating nature of neurodegenerative disorders such as Parkinson’s & Alzheimer’s disease

  • Parkinson’s: Pilot studies using embryonic stem cells have been carried out in humans with some success

• Replacement of dying neurons with healthy neuronal tissue

• Transplanted cells not only survived but also grew & established connections with adjacent neurons

• Use of embryonic stem cells is controversial & raises a number of ethical questions

• Researchers currently exploring other sources of cells to help restore patients’ brain function

Tissue Engineering

• Primary objective is to restore healthy tissues or organs for patients & eliminate need for tissue, organ or artificial transplants

• Requires an abundant supply of disease-free cells of specific types

• These cells then need to be induced to grow on a scaffold of natural or synthetic material to produce a three-dimensional tissue

• Tissue engineering scaffolds serve as a template for tissue growth, and need to have high pore sizes that enable cells to grow while at same time allow diffusion of nutrients throughout whole structure

• Biodegradable so they don’t require surgical removal

• Rate of scaffold degrade = rate of tissue formation

• Once scaffold devised, suitable stem cells need to be cultured & cells seeded onto scaffold- cell growth & proliferation

• Cell-covered scaffold implanted into patient at site where tissue is required-

• Material of scaffold begins to degrade

  • Tissues developed: bone, skin, cartilage, adipose tissues

Stem Cells

• Stem cells are used for cell replacement therapy and are increasingly being used for tissue engineering

• There are different sources of stem cells

  • The patient – these will contain the same genes (so unsuitable for treating genetic disorders)

  • Embryonic (virtually always from another individual) – ethical issues arise about their use and supply is limited

• Use of stem cells overcomes a major problem:

  • Specialised cells of the organ from which cells were to be harvested was diseased, so not enough normal cells present to enable successful culture