Ashanthi’s parents would have been carriers of the disease. Individuals inherit genes in bundles called chromosomes. Out of the 46 chromosomes we receive 23 from the egg cell and 23 from the sperm. When the corresponding genes coming from the two parents are not identical they are called alleles. In the case of Ashanthi,   both adenose deaminase secreting alleles were faulty. This led to a complete  breakdown of the immune system.

Fig.1  How the babies inherit parental chromosomes. The mutated gene is shown in red.

Fig.1. shows how a couple carrying a hereditary disease may transmit it to their children. The mutated gene is marked in red colour in the chromosomes. Dad's chromosome transfer  is in red dotted lines while that from the mom is in blue. The faulty gene is marked in red inside the chromosome.starting from left the 

1st baby is neither a carrierof the disease nor a patient. 

The 2nd and 3rd are carriers while 

The 3rd. is the victim as it has inherited the faulty gene from both sides.

SOMATIC GENE THERAPY

In the nineties, her doctors had several options other than confining her into a bubble. The enzyme ADA could have been injected to her periodically. Blood transfusions could have been given from a healthy donor. A bone marrow transplant from a matching donor would have induced the production of the enzyme.

In September 1990 three doctors,  French Anderson, Michael Biase, and Ken Culver at the
National Institute of Health (US)  drew blood from the tiny hand of the patient.  They have
received the government approval for a unique therapeutic procedure, gene therapy.

Fig.2. Dr. French Anderson and Ashanthi.

Someone  not familiar with genetics is bound to think that this is a defect in a single cell
that has to be corrected. It is far from reality. Ashanthi could boast of trillions of cells
and each and every cell would have had the complement of this defect. The number of genes in a human cell would be close to 20,500. So finding a defective gene even inside a single cell
is much worse than finding a needle in a hay stack. Yet the molecular biologists have
developed needle sharp  tools  to locate genes. These are mainly enzymes obtained from the
lowest ranking organisms that attack human cells such as viruses, bacteria and worms.  It is
possible to replace the defective gene with a healthy one but the insurmountable problem was to find a way to insert the healthy gene into such a vast number of cells.

Viruses are very clever agents in locating specific tissues. The flu virus targets the
throat and the lungs. Hepatitis viruses go to the liver and the human papilloma virus causes
cervical cancer. Once a virus enters a system they infiltrate cells and insert their genes
into the host nucleus.  So the doctors decided to damage the viral genes that would help its
replication and insert the synthetic genes to produce the enzyme adenose deaminase into some of the white blood cells of the patient.  As the doctors had taken the blood out of the body they had no fear of the vectors getting lodged in wrong places. The girl’s new gene produced the correct enzyme  and the girl even attended college the doctors cannot confirm complete success as there was no control and the patient continued other forms of therapy too.

GERM LINE GENE THERAPY

Much easier way to treat hereditary diseases is to tackle the single cell that makes a man or a woman. This can be done by fertilizing the eggs in vitro. (Fertilizing the egg in a Petri dish) The single fertilized cell go on dividing rapidly producing identical cells up to an eight cell stage. Each of those could develop into an individual. The faulty genes could be replaced with corrected ones. The most suitable cell could be selected and planted in the uterus of the mother. The developing foetus may be checked for side effects by testing the foetal fluids or the umbilical cord blood.  The added advantage of this method is that the cure is permanent as it could be passed on to future generations.

Although gene therapy is an important break through in medicine much caution has to be applied. In 2001 when eleven children were given somatic gene therapy in a French hospital it was considered a great success.  A year later two children developed symptoms similar to leukemia. The introduced gene has excited neighboring leukemia inhibitor gene. This has produced extra white blood cells which caused leukemia.

Other than the complexities in the treatment procedures, there are many other ethical, social and religious issues that must be addressed. For example there was a treatment for dwarfism. Although causes for this condition may vary, a pharmaceutical company developed growth hormone called ‘protropin’ which acted positively in many cases.  Hardly a person would object a dwarf using such a drug produced by genetically modified bacteria.  In spite of the strict measures that had taken with the issue of the drug only to needy patients, some basket ball players wanted to use it to gain height.   

Although  gene therapy is not pool proof,  there is lot of scope for the treatment for the estimated 4000 odd hereditary diseases; including all forms of cancer, HIV, arthritis and the common killer coronary thrombosis. One science journalist has described this form of therapy as a 'double edged sword'.