Section 6 Gene diagnosis

Characteristics: Highly specific Highly sensitive Highly directed Even before the occurrence of illness Gene diagnosis is a process which examines the genetic material contained inside a person’s cells to determine if the person has or will develop a certain desease. Most diseases, if not all, are concerning abnormal changes of genes.

Techniques used in gene diagnosis  NA hybridization  PCR  DNA sequencing  Polymorphism analysis

The basic technique for detecting RFLPs involves fragmenting a sample of DNA by a restriction enzyme, which can recognize and cut DNA wherever a specific short sequence occurs, in a process known as a restriction digest. The resulting DNA fragments are then separated by length through agarose gel electrophoresis, and transferred to a membrane via the Southern blot procedure. Hybridization of the membrane to a labeled DNA probe then determines the length of the fragments which are complementary to the probe. A RFLP occurs when the length of a detected fragment varies between individuals. Restriction fragment length polymorphism

× Mst Ⅱ cutting site (GCTNAGG) 5´3´ Gene of normal hemoglobin beta subunit 5´3´ Mutated gene 1.15kb 1.35kb Sickle anemia patient’s genome mutation (GCTNTGG)

正常人 突变携带者 患者 Normal Carrier Patients

PCR single strand conformation polymorphism, PCR-SSCP Single strand conformation polymorphism (SSCP) is defined as conformational difference of single stranded nucleotide sequences of identical length as induced by differences in the sequences under certain experimental conditions. This property allows to distinguish the sequences by native gel electrophoresis, which separates the different conformations.

Normal PCR productmutationed PCR product denaturation Native PAGE NormalCarrierPatients PCR-SSCP

Applications of gene diagnosis Inherited diseases( Phenylketonuria) Diagnosis of cancer Diagnosis of infectious diseases Forensic medicine Abnormal gene expression

Section 7 Gene therapy

Gene therapy is the way to transfer genetic material which exerts the biological function into cells of patients to treat the disease. A normal gene, recombinant DNA, RNA, synthetic oligonucleotides can be used.

1. Therapeutic strategies  gene correction  gene replacement  gene augmentation  gene inactivation  Killing of specific cells

The suicide gene therapy strategy

 Germ line gene therapy Germ cells, i.e., sperm or eggs, are modified by the introduction of functional genes, which are ordinarily integrated into their genomes. Therefore, the change due to therapy would be heritable and would be passed on to later generations. This new approach, theoretically, should be highly effective in counteracting genetic disorders and hereditary diseases. However, many jurisdictions prohibit this for application in human beings, at least for the present, for a variety of technical and ethical reasons.  Somatic gene therapy Therapeutic genes are transferred into the somatic cells of a patient. Any modifications and effects will be restricted to the individual patient only, and will not be inherited by the patient's offspring. 2. Gene therapy may be classified into two types

3. Basic process of gene therapy  Selection of a therapeutic gene wild type gene  Selection of vehicles virus(retrovirus, adenovirus, adeno-associated virus…)  Selection of target cells somatic cells  Gene transfer directly, indirectly

Gene transfer strategies

Gene therapy introduced by virus indirectly

截至 2004 年 6 月底，全世界范围内基因治疗的临床试验方案

Severe combined immunodeficiency, SCID (ADA syndrome) SCID is a genetic disorder in which both B cells and T cells of the adaptive immune system are crippled, due to a defect in adenosine deaminase (ADA) gene, necessary for the breakdown of purines. Lack of ADA causes accumulation of dATP. This metabolite will inhibit the activity of ribonucleotide reductase, the enzyme that reduces ribonucleotides to generate deoxyribonucleotides. The effectiveness of the immune system depends upon lymphocyte proliferation and hence dNTP synthesis. Without functional ribonucleotide reductase, lymphocyte proliferation is inhibited and the immune system is compromised. SCID is a severe form of heritable immunodeficiency. It is also known as the “bubble boy" disease because its victims are extremely vulnerable to infectious diseases.

In 1990, Ashanthi's gene therapy procedure, doctors removed T cells from the child's body, let the cells grow in the laboratory, inserted the missing ADA gene into the cells, and then infused the genetically modified T cells back into the patient's bloodstream. The T cells treated genetically only work for a few months, after which the process must be repeated. As of early 2007, she was still in good health, and she was attending college.