Transcript m5zn_db523f23f00100a

Genetic Testing
Dr Jehad Al-Shuneigat
Genetic testing
is a medical test that identifies changes in
chromosomes, genes, or proteins.
Goal of genetic testing
• To recognize the potential for a genetic condition
(defect) at an early stage.
• The results of a genetic test can confirm or rule out a
suspected genetic condition or help determine a
person’s chance of developing or passing on a
genetic disorder. More than 1,000 genetic tests are
currently in use, and more are being developed.
Several methods can be used for genetic testing:
1. Molecular genetic tests (or gene tests)
study single genes or short lengths of DNA to identify
variations or mutations that lead to a genetic disorder.
2. Biochemical genetic tests
study the amount or activity level of proteins;
abnormalities in either can indicate changes to the DNA
that result in a genetic disorder.
3. Chromosomal genetic tests
Using Chromosome Karyotypes
Chromosomes may differ widely in:
1- Appearance, 2- Size, 3- Staining properties, 4- The
location of the centromere, 5- The relative length of the
two arms on either side of the centromere.
• chromosomes aligned and are most condensed in
metaphase
Types of Genetic tests:
•
•
•
•
Newborn screening
Heterozygote screening
Presymptomatic diagnosis
Prenatal testing
Newborn screening
•
•
•
•
•
•
•
•
Most countries require that newborn infants be tested for
phenylketonuria and galactosemia.
Phenylketonuria (PKU)
Prevalence 1:11,000 births.
is due to an autosomal recessive allele that causes deficiency
in most of times to phenylalanine hydroxylase mental
retardation that normally metabolizes the amino acid
phenylalanine.
When this enzyme is defective, phenylalanine is not
metabolized, and its build up causes brain damage in children.
Galactosemia
Prevalence 1: 30,000- 60,000 births.
Galactosemia is a condition in which the body is unable to
metabolize the simple sugar galactose due to a deficiency of
enzyme the most common galactose-1-phosphate uridyl
transferase, and, if untreated, it cause damage to liver , brain,
kidneys, and eyes.
Testing is done by analysing a drop of the infant’s blood
collected soon after birth.
Heterozygote screening
• Testing to identify carriers of recessive disease, who are
healthy but have the potential to produce children with the
particular disease.
• Carriers will not get the disease, but can pass on the
alteration to their children.
• Testing for Tay-Sachs disease is a successful example of
heterozygote screening. If a man and woman are both
heterozygotes, approximately one in four of their children is
expected to have Tay- Sachs disease.
• Tay-Sachs disease results from deficiency in enzyme called
hexosaminidase A
• The function of Hexosaminidase A is to metabolise a lipid
called GM2 ganglioside in the brain.
• Excessive GM2 ganglioside accumulates in the brain,
causing swelling and neurological symptoms.
Presymptomatic testing
• This test shows which family members are at risk
for a certain genetic condition and to determine
whether they have inherited a disease-causing
allele gene.
• Example, presymptomatic testing is available for
members of families that have an autosomal
dominant form of breast cancer. In this case, early
identification of mutation in BRCA1 & BRCA2 genes
allows for closer surveillance and the early
detection of tumors.
• Another example Huntington disease, an autosomal
dominant disease that leads to slow physical and
mental deterioration in middle age. It is untreatable
conditions.
Breast cancer is the second major cause of cancer death in American
women (lung cancer is first), with an estimated 44,190 lives lost (290
men and 43,900 women) in the USA in 1997.
• While ovarian cancer accounts for fewer deaths than breast cancer,
it still represents 4% of all female cancers.
• For some of the cases of both types of cancer, there is also a clear
genetic link.
• In 1994, two breast cancer susceptibility genes were identified:
BRCA1 on chromosome 17 and BRCA2 on chromosome 13.
• When an individual carries a mutation in either BRCA1 or BRCA2,
they are at an increased risk of being diagnosed with breast or
ovarian cancer at some point in their lives.
• BRCA1 & BRCA2 are tumor suppressor genes and their function is
to participate in repairing damages and breaks in DNA.
• It is though that mutations in BRCA1 or BRCA2 might disable this
mechanism, leading to more errors in DNA replication and ultimately
to cancerous growth.
Prenatal (before birth) testing
• The major purpose of prenatal tests is to provide families
with the information that they need to make choices during
pregnancies and, in some cases, to prepare for the birth of a
child with a genetic condition.
• Common techniques used for prenatal diagnosis include:
• 1- Ultrasonography
• Some genetic conditions can be detected through direct
visualization of the fetus. Such visualization is most
commonly done with ultrasonography—usually referred to
as ultrasound. In this technique, high-frequency sound is
beamed into the uterus; when the sound waves encounter
dense tissue, they bounce back and are transformed into a
picture. The size of the fetus can be determined, as can
genetic conditions such as neural tube defects (defects in
the development of the spinal column and the skull) and
skeletal abnormalities.
• 2- Amniocentesis
• a procedure for obtaining a sample of amniotic fluid from a
pregnant woman.
• Amniotic fluid—the substance that fills the amniotic sac and
surrounds the developing fetus—contains fetal cells that can
be used for genetic testing.
• Procedure:
• First, ultrasonography is used to locate the position of the
fetus in the uterus.
• Second, a long, sterile needle is inserted through the
abdominal wall into the amniotic sac, and a small amount of
amniotic fluid is withdrawn through the needle.
• Fetal cells are separated from the amniotic fluid and placed
in a culture medium that stimulates them to grow and divide.
Genetic tests are then performed on the cultured cells.
Complications with amniocentesis (mostly miscarriage) are
rare.
• Disadvantage with amniocentesis
A- Performed in about the 16th week of a pregnancy. By this stage, abortion
carries a risk of complications and may be stressful for the parents.
B- Cells obtained with amniocentesis must then be cultured before genetic
tests can be performed, requiring more time.
3- Chorionic villus sampling (CVS)
• Chorionic villus sampling (CVS) can be performed earlier (between the
10th and 11th weeks of pregnancy) and collects more fetal tissue, which
eliminates the necessity of culturing the cells.
• In CVS, a catheter—a soft plastic tube—with the use of ultrasound for
guidance, is pushed through the cervix into the uterus. The tip of the tube
is placed into contact with the chorion, the outer layer of the placenta.
Suction is then applied, and a small piece of the chorion is removed.
• Although the chorion is composed of fetal cells, it is a part of the placenta
that is expelled from the uterus after birth; so the removal of a small
sample does not endanger the fetus. The tissue that is removed contains
millions of actively dividing cells that can be used directly in many genetic
tests.
• Chorionic villus sampling has a somewhat higher risk of complication
than that of amniocentesis; it increase the incidence of limb defects in the
fetus when performed earlier than 10 weeks of gestation.
• 4- Maternal blood tests
• Some genetic conditions can be detected by
performing a blood test on the mother
• For instance, α-fetoprotein is a protein normally
produced by the fetus during development and is
present in the fetal blood, the amniotic fluid, and the
mother’s blood during pregnancy.
• Measuring the amount of α-fetoprotein in the mother’s
blood gives an indication of these conditions.
• However, because other factors affect the amount of αfetoprotein in maternal blood, a high or low level by
itself does not necessarily indicate a problem. Thus,
when a blood test indicates that the amount of αfetoprotein is abnormal, follow-up tests (additional αfetoprotein determinations, ultrasound, amniocentesis,
or all three) are usually performed.
• 5- Fetal cell sorting
• Prenatal tests that utilize only maternal blood are
highly desirable because they are non-invasive
and pose no risk to the fetus.
• During pregnancy, a few fetal cells are released
into the mother’s circulatory system, where they
mix and circulate with her blood. Recent
advances have made it possible to separate
fetal cells from a maternal blood sample (a
procedure called fetal cell sorting).
• With the use of lasers and automated cellsorting machines, fetal cells can be detected and
separated from maternal blood cells. The fetal
cells obtained can be cultured for chromosome
analysis or used as a source of fetal DNA for
gene testing.
• 6- Preimplantation genetic diagnosis
• Through in vitro fertilization (IVF) and preimplantation
testing, it is possible to select an embryo without the
disorder for implantation in her uterus.
• The embryos are allowed to divide several times until they
reach the 8 or 16-cell stage. At this point, one cell is
removed from each embryo and tested for the genetic
abnormality. Removing a single cell at this early stage does
not harm the embryo. After determination of which embryos
are free of the disorder, a healthy embryo is selected and
implanted in the woman’s uterus.
• Preimplantation genetic diagnosis requires the ability to
conduct a genetic test on a single cell. Such testing is
possible with the use of the polymerase chain reaction
(PCR) through which minute quantities of DNA can be
amplified (replicated) quickly. After amplification of the cell’s
DNA, the DNA sequence is examined.
• Preimplantation diagnosis is widely used technique in many
medical centres in Jordan.
Human Genome Project
• Begun in 1990 completed in 2003.
Project goals
• identify all the approximately 30,000 genes in
human DNA
• determine the sequences of the 3 billion
chemical base pairs that make up human DNA,
• improve tools for data analysis,
• address the ethical, legal, and social issues
(ELSI) that may arise from the project.
• What we come to know from human genome
• The human genome contains 3164.7 million chemical
nucleotide bases (A, C, T, and G).
• The average gene consists of 3000 bases, but sizes vary
greatly, with the largest known human gene being
dystrophin at 2.4 million bases (a protein located mainly in
muscles used for movement ).
• The total number of genes is estimated at 30,000 —much
lower than previous estimates of 80,000 to 140,000.
• Almost all (99.9%) nucleotide bases are exactly the same in
all people.
• The functions are unknown for over 50% of discovered
genes.
• Less than 2% of the genome codes for proteins.
• Repeated sequences that do not code for proteins ("junk
DNA") make up at least 50% of the human genome.
• Repetitive sequences are thought to have no known
functions.