genetherapy-its problems and prospects-my credit seminar

GENE THERAPY – ITS PROBLEMS AND PROSPECTS- my credit seminar(01-04-2006).....




Ariprasath. K
2004 -13 - 05
CAS-AGB


Introduction:
The gene, as gene is the segment of DNA , DNA is made up of nucleotides with 4 different bases. At the end of Human Genome Project it was found that there are about 3 billion base pair and around 25000 - 35000 genes are present. these genes make a human being unique of his own.

Gene therapy is introducing a normal gene against a defective gene. The defective genes are the cause of various genetic disorders. There are at least 2000 different genetic disorders can be caused by these mutations. The imported ones and the defective gene, and the year of discovery are enlisted here.

Genetic Diseases:
Thalessemia, Alzheimer’s disease, SCID, sickle cell anemia, phelnylketonuria, familial hypercholestrolemia, Duchene muscular dystrophy, cystic fibrosis, Huntington’s syndrome, fragile X syndrome. Among which the Alzheimer’s disease, chronic memory loss which may leads to mental retardation at latter stages (US president Ronald Regan). Fragile X syndrome, where the X chromosome is very fragile and will easily break at the time of microscopic preparation of chromosomes. this is due to presence of highly repetitive sequences at the X chromosome. This will leads to mental retardation, the most severe one next to Down's syndrome. Since it is X linked the male children are the worst victims. So also Huntington’s syndrome where the CAG segment is highly repetitive and the mutant gene is longer than the normal gene.

SCID(Severe Combined Immuno Deficiency Syndrome) :
The most studied one is SCID (Severe Combined Immuno Deficiency Disease) in this disease, the child has the mutated defective gene for ADA (Adnosine Deaminase) synthesis. Since this enzymes is essential in lymphocyte formation , the child will have defective immune system. Just like AIDS the child is open for all kind of infections. The prognosis of the disease is not simply grave , it is certain death with in 1 year. It is popularly known as "Bubble disease" because a boy with this disease was maintained in a big plastic bubble for 14 yrs, who died in 1970s. The treatment for this disease is supplement of ADA , which will cost $100000 per annum. the only alternative is bone marrow transplantation. But was never successful even between closed relatives.

A giant leap has occurred in 1990s when the bone marrow cells were isolated cultured and were introduced with the normal gene and replaced in to the patient. The patients were become normal. But this is not possible in other tissues like brain cells, lung cells which can't be cultured in vitro. So remedy by this protocol for Alzheimer’s disease (Which needs brain transplantation) and cystic fibrosis (Which need lung tissue transplantation) is not possible.

Viral Vectors:
The alternative tool is through vectors. That is virus particles loaded with the normal genes used to deliver the DNA into the target cells. Viruses are obligate intra-cellular parasites, designed through the course of evolution to infect cells, often with great specificity to a particular cell type. They tend to be very efficient at transfecting their own DNA into the host cell, which is expressed to produced new viral particles. By replacing genes that are needed for the replication phase of their life cycle (the non-essential genes) with foreign genes of interest, the recombinant viral vectors can transduce the cell type it would normally infect. To produce such recombinant viral vectors the non-essential genes are provided in trans, either integrated into the genome of the packaging cell line or on a plasmid. As viruses have evolved as parasites, they all elicit a host immune system response to some extent. Though a number of viruses have been developed, interest has centered on four types, are retro viruses, adeno viruses, adeno-associated viruses, herpes simplex viruses. There are also non viral vectors namely, naked DNA, liposomes, and molecular conjugates. In naked DNA the DNA is directly introduced or attached with gold particles and bombarded to the target cells. The liposomes are bilipid layers entrapping a fraction of aqueous liquid. DNA attaches to its surface due to its charge. Molecular Conjugates are proteins of synthetic ligands to which the DNA is attached.


Retro virus as viral vectors
gag-Code for core proteins
pol-Codes reverse transcriptase
env-Codes envelope protein
LTR- Lateral Terminal repeats


Genedicine:
The gene therapy is of two types one is somatic Gene therapy and other one is germ line gene therapy. The first gene therapy product GENEDICINE is commercially released in China by the company Sibiono. The drug is to treat neck and head cancer. the results were promising is 3 times more results than the conventional radiotherapy. The adeno virus is loaded with p53 gene and given intra venous .The P53 code for protein which trigger cell suicide when the cell start to grow out of control. In tumors use to arise due to mutated p53 gene. Drug is commercially available only in china, one dose cost around 3000yuan ($360) 1012 virion particles per week for 8 weeks found effective. the medicine was developed after 14 hrs of hectic research.

when ever the gene therapy is tested it should fulfill some basic criteria, like the gene therapy does not remove of destroy defective gene, it should exactly insert the gene at the place of interest. When ever gene therapy is discussed public ally the press raise the issue of "Super Human" fortunately such traits like intelligence and fertility are poly genic and manipulations of such traits are not possible for a long time.

As conclusion, our knowledge about how human body works is elementary. The recently completed human genome project and developing nano technology will fasten the success journey. There are still miles to go to attain fruitful results. Till then we the public should not simply meddle in areas where we are so ignorant.

References:

Abdel-Wahab, Z., Weltz, C., Hester, D., Pickett, N., Vervaert, C., Barber, J. R., Jolly, D. and Seigler H. F. (1997). A phase I clinical trial of immunotherapy with interferon-gamma gene-modified autologous melanoma cells. Cancer 80: 401-412.

Andreansky, S. S., He, B., Gillespie, G. Y., Soroceanu, L., Market, J., Chou, J., Roizman., B. and Whitley, R. J. (1996). The application of genetically engineered herpes simplex viruses to the treatment of experimental brain tumors. Proceedings of the National Academy of Sciences USA 93: 11313-11318.

Armentano, D., Zabner, J., Sacks, C., Sookdeo, C. C., Smith, M. P., St. George, J. A., Wadsworth, S. C., Smith, A. E. and Gregory, R. J. (1997). Effect of the E4 region on the presistence of transgene expression from adenovirus vectors. Journal of Virology 71: 2408-2416.

Blmer, U., Naldini, L., Kafri, T., Trono, D., Verma, I. M. and Gage, F. H. (1997). Highly efficient and sustained gene transfer in adult neurones with a lentivirus vector. Journal of Virology 71: 6641-6649.

Contassot, E., Ferrand, C., Certoux, J., Reynolds, C. W., Jacob, W., Chiang, Y., Cahn, J., Herv P. and Tiberghien. P. (1988). Retrovirus-mediated transfer of the herpes simplex type 1 thymidine kinase gene in alloreactive T lymphocytes. Human Gene Therapy 9: 73-80.

Ferry, N., Duplessis, O., Houssin, D., Danos, O. and Heard, J. (1991). Retroviral mediated gene transfer into hepatocytes in vivo. Proceedings of the National Academy of Sciences USA 88: 8377-8381.

Fujiwara, T., Grimm, E. A., Mukhopadhyay, T., Cai, D. W., Owen-Schaub, L. B. and Roth, J. A. (1993). A retroviral wild-type p53 expression vector penetrates human lung cancer spheriods and inhibits growth by inducing apootosis. Cancer Research 53: 4129-4133.

Gerard, R. D. and Collen, D. (1997). Adenovirus gene therapy for hypercholesterolemia, thrombosis and restenosis. Cardiovascular Research 35: 451-458.

Glorioso, J. C., DeLuca, N. A. and Fink, D. J (1995). Development and application of herpes simplex virus vectors for human gene therapy. Annual Review of Microbiology 49: 675-710.

Kennedy, P. G. E. (1997). Potential uses of herpes simplex virus (HSV) vectors for gene therapy of neurological disorders. Brain 120: 1245-1259.

Kim, S. H., Yu, S. S., Park, J. S, Robbins, P. D, An, C. S. and Kim, S. (1998). Construction of retroviral vectors with improved safety, gene expression, and versitility. Journal of Virology 72: 994-1004.

Roth, J. A. and Cristiano, R. J. (1997). Gene therapy for cancer: what have we done and where are we going? Journal of the National Cancer Institute 89: 21-38.

Smith, A. E. (1995). Viral vectors in gene therapy. Annual Review of Microbiology 49: 807-838.

Verma, I. M. and Somia, N. (1997). Gene therapy - promises, problems and prospects. Nature 389: 239-242.

Wu, C. H., Wilson, J. M. and Wu., G. Y. (1989). Targeting genes: delivery and persistent expression of a foreign gene driven by mammalian regulatory elements in vivo. Journal of Biological Chemistry. 264: 16985-16987.