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Genetics

By Dr Rusul M. abid

3. Connective tissue disorders

Mutations in different types of collagen, fibrillin &elastin. Make up the majority of connective tissue disorders.  The clinical features of these disorders vary, depending on the structural function and tissue distribution of the protein which is mutated.  For example, autosomal dominant loss-of-function mutations in the gene encoding elastin cause either supravalvular aortic stenosis, The most commonly involved systems are:  skin (increased or decreased elasticity, poor wound healing)  eyes (myopia, lens dislocation)  blood vessels (vascular fragility)  bones (osteoporosis, skeletal dysplasia)  joints (hypermobility, dislocation, arthropathy)

4. Learning disability, dysmorphism & malformations

Congenital cognitive impairment (also called mental handicap or learning disability) affects about 3% of the population.  There are important ‘environmental & Genetic’ causes of cognitive impairment, including:  teratogen exposure during pregnancy (alcohol,anticonvulsants)  Congenital infections (cytomegalovirus, rubella, toxoplasmosis, syphilis)  premature delivery (intraventricular haemorrhage)  Birth injury (hypoxic encephalopathy).  Genetic disorders that contribute to the etiology of cognitive impairment are (Chromosome disorders & dysmorphic syndromes)

Chromosome disorders Any significant gain or loss of autosomal chromosomal material (aneuploidy) usually results in learning disability and other phenotypic abnormalities, Down’s syndrome is the best known of these disorders, The DNA analysis can identify causative structural chromosome abnormalities in 10–25% of cases of significant learning disability.

Dysmorphic syndromes Almost all dysmorphic syndromes are characterized by the occurrence of cognitive impairment, malformations and a distinctive facial appearance associated with various other clinical features. Making the correct diagnosis is important, as it has implications on immediate patient management, detection of future complications and assessment of recurrence risks in the family. Clinical examination remains the mainstay of diagnosis and the patient often needs to be evaluated by a clinician who specializes in the diagnosis of these syndromes. The differential diagnosis in dysmorphic syndromes is often very wide and this has resulted in computer aided diagnosis becoming an established clinical tool. The clinical diagnosis may then be confirmed by specific genetic investigations, as the genetic basis of a wide range of dysmorphic syndromes has been identified.

5. Familial cancer syndromes

Most cancers are not inherited but occur as the result of an accumulation of somatic mutation. However some families are prone to one or more specific types of cancer affected individuals tend to present with: Tumors at an early age and are more likely to have multiple primary foci of carcinogenesis.

Examples


Hereditary non-polyposis colorectal cancer Hereditary non-polyposis colorectal cancer (HNPCC) is an autosomal dominant disorder with mutations can occur in several different genes encoding proteins involved in DNA mismatch repair that presents with early onset familial colon cancer, particularly affecting the proximal colon. Other cancers, such as endometrial cancer, are often observed in affected families.

Familial breast cancer is an autosomal dominant disorder that is most often due to mutations in genes encoding either BRCA1 or BRCA2. Both these proteins are involved in DNA repair. Individuals who carry a BRCA1 or BRCA2 mutation are at high risk of early-onset breast and ovarian tumours, and require regular screening for both these conditions. Many affected women will do Prophylactic bilateral mastectomy and oophorectomy.

General principles of diagnosis

Diagnosis can be made by a careful clinical history and examination and an awareness and knowledge of rare disease entities. Although DNA-based diagnostic tools are now widely used, it is important to be aware that not all diagnostic genetic tests involve analysis of DNA. For example, a renal ultrasound can detect adult polycystic kidney disease. By definition, all genetic testing (whether it is DNA-based or not) has implications both for the patient and for other members of the family.

1. Clinical history and examination including constructing a family tree

The family tree—or pedigree—is a three-generation family history it may reveal important genetic information of relevance to the presenting complaint, particularly relating to cancer. A pedigree must include details
 from both sides of the family
 any history of pregnancy loss or infant death,
 consanguinity,
 Details of all medical conditions in family members.
 dates of birth and
 Age at death

2. Non DNA-based diagnostic tools:

It may sometimes be more economical or convenient to measure enzyme activity rather than sequencing the coding region of the genes involved.  Haemoglobinopathy as sickle-cell disease can be diagnosed by haemoglobin electrophoresis  Immune deficiencies as hypogammaglobulinaemia can be diagnosed by Ig levels, Complement levels  Inborn errors of metabolism e.g. phenylketonuria can be diagnosed by Enzyme assays, amino acid levels  Endocrine disease e.g. congenital adrenal hyperplasia by Hormone levels, enzyme assays  Renal disease can be diagnosed by e.g. polycystic kidney can be diagnosed by Radiology, renal biopsy

3. DNA-based diagnostic tools

Polymerase chain reaction (PCR) and DNA sequencing: PCR involves amplification of DNA from small quantities of starting material. It is the most important technique in DNA diagnostic analysis. Almost any tissue can be used to extract DNA for PCR analysis, but most commonly, a sample of peripheral blood is used. The ability to determine the exact sequence of a fragment of DNA amplified by PCR is also of critical importance in DNA diagnostics.


Hybridization:
This is a procedure used in the diagnosis of genetic and other
pathologies as well as in the diagnosis of cancer. It is based on the fact that the two DNA strands are not identical but complementary. The test is performed by adding a synthetic, single stranded DNA sequence (called a probe) [that is made complementary to a specific region of DNA under study and is being labeled with a specific dye] to the double stranded DNA from the patient (after making it single stranded by a process called denaturation). If the probe found its complementary region along the patient's DNA, it'll combine (hybridize) to it and starts emitting a color or "fluoresce". This emitted color can be detected using a UV-microscope. This procedure forms the basis of what is known as fluorescent in situ hybridization (FISH).

 Nevertheless, this procedure cannot detect single point mutations or even addition / deletion of 2 or more nucleotide bases. So, the technique used for detection of such smaller defects is usually DNA-based; the most representative and most commonly used one is polymerase chain reaction (PCR) that revolutionalized the diagnostic ability of genetic testing. Most new techniques used nowadays are PCR-based

Gene Therapy

Gene therapy is a form of treatment that involves introducing genetic material into a person’s cells to fight or prevent disease A gene can be delivered to a cell using a carrier known as a “vector.” The most common types of vectors used in gene therapy are viruses. The viruses used in gene therapy are altered to make them safe, with introducing a therapeutic gene in the vector which will be then transferred to the patient.
Although some risks still exist with gene therapy. The technology has been used with some success.  Gene therapy for had been tried for a number of diseases, such as severe combined immunodeficiencies, hemophilia ,Parkinson's disease, cancer and even HIV.

Several approaches to gene therapy are being tested, including:  Replacing a mutated gene that causes disease with a healthy copy of the gene  Inactivating, or “knocking out,” a mutated gene that is functioning improperly  Introducing a new gene into the body to help fight a disease  In general, a gene cannot be directly inserted into a person’s cell. It must be delivered to the cell using a carrier, or vector.

Vector systems can be divided into: 1) Viral vectors 2) Non-viral vectors

 The most common type of vectors is viruses that have been genetically altered to carry normal human DNA . Viruses have evolved a way of encapsulating and delivering their genes to human cells in a pathogenic manner. It has been tried to use this ability by manipulating the viral genome to remove disease-causing genes and insert therapeutic genes.

Target cells such as the patient's liver or lung cells are infected with the vector. The vector then unloads its genetic material containing the therapeutic human gene into the target cell. The generation of a functional protein product from the therapeutic gene restores the target cell to a normal state.

Gene therapy can be split into two categories: 1) Ex vivo, which means exterior (where cells are modified outside the body and then transplanted back in again). In some gene therapy clinical trials, cells from the patient’s blood or bone marrow are removed and grown in the laboratory. The cells are exposed to the virus that is carrying the desired gene. The virus enters the cells and inserts the desired gene into the cells’ DNA. The cells grow in the laboratory and are then returned to the patient by injection into a vein. This type of gene therapy is called ex vivo because the cells are treated outside the body. 2) In vivo, where genes are changed in cells still in the body, This form of gene therapy is called in vivo, because the gene is transferred to cells inside the patient’s body.

Types of Gene Therapy All cells in the human body contain genes, making them potential targets for gene therapy. However, these cells can be divided into two major categories: somatic cells (most cells of the body) or cells of the germline (eggs or sperm). In theory it is possible to transform either somatic cells or germ cells.
1) Gene therapy using germ line cells results in permanent changes that are passed down to subsequent generations. The application of germ line gene therapy is its potential for offering a permanent therapeutic effect for all who inherit the target gene. Successful germ line therapies introduce the possibility of eliminating some diseases from a particular family, and ultimately from the population, forever. However, this also raises controversy. Some view this type of therapy as unnatural, Others have concerns about the technical aspects. They worry that the genetic change propagated by germ line gene therapy may be harmful, with the potential for unforeseen negative effects on future generations.


2) Somatic cell therapy is more conservative and safer approach because it affects only the targeted cells in the patient, and is not passed on to future generations. However, the disadvantage of this type of therapy is that the effects of somatic cell therapy are short-lived. Because the cells of most tissues ultimately die and are replaced by new cells, repeated treatments over the course of the individual's life span are required to maintain the therapeutic effect. Transporting the gene to the target cells or tissue is also problematic. Regardless of these difficulties, however, somatic cell gene therapy is appropriate and acceptable for many disorders, including cystic fibrosis, muscular dystrophy, cancer, and certain infectious diseases, the results of any somatic gene therapy are restricted to the actual patient and are not passed on to his or her children. All gene therapy to date on humans has been directed at somatic cells, whereas germline engineering in humans remains controversial .

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رفعت المحاضرة من قبل: Ali Haider
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