( a ) I n v i vo gen e th erapy

( b ) E x v i vo gen e th erapy

in sert tran sgen ein to lip o so m es o rlip o p lex es, o r v iralv ecto r/ v eh ic le system s

g ro w cells ex vi vo

rem o v e cells

rep lace th em o d ifi ed cells

ad d tran sgen e an dselect m o d ifi ed cells

d eliv er to site o factio n ( lu n g) b y

aero so l sp raye.g.

Gene therapy for cystic fibrosis

1. The possibility of replacing a defective gene with a ‘good’ copy of the gene to overcome the problems caused by the defect is called gene therapy. In this way one is able to deal with the cause not just the symptoms.

2. Gene therapy might also be used to kill abnormal cells, such as cancerous cells, or to inhibit the spread of viruses by preventing DNA replication.

3. In the case of cancer, gene therapy would avoid many of the side-effects associated with the drugs used in chemotherapy.

4. The difficulty with gene therapy in practical terms is that there are difficult criteria to be met before it can be used. The conditions are:

• the defective gene function is characterised;• the normal gene is available in cloned form;• the affected cells are accessible, either the cells can be treated outside the

body (ex vivo) and replaced, or may have to be treated within the patient’s body (in vivo);

• there are suitable vehicles (vectors) for delivery of the gene e.g. viruses, liposomes or artificial chromosomes

• viral vectors – virus have evolved to alter a host cells metabolism by implanting their DNA into the host cells DNA. Altered viruses with therapeutic genes can be given to patient with virus implanting therapeutic gene. ( risky)

• the use of liposomes that can fuse with cell membranes

• human artificial chromosomes - the gene functions normally in its target cells.

The difficulty of meeting the criteria means that gene therapy is still in the early stages of development although progress is being made.

Gene therapy which raises ethical questions in the type of cells that are the targets, i.e. whether somatic cells (body cells) or germ cells are used. If genes within reproductive cells could be altered then the alteration could be passed on to the next generation and effectively alter the species gene pool.