Download Molecular Biology of Diseases

Monogenic Disorders
Molecular Biology of Diseases
SLIDES 1-3 We can distinguish the following types of diseases on the basis of the genetic
background:
1.
2.
3.
4.
5.
6.
7.
Single-gene disorders
Mitochondrial diseases
Multifactorial diseases
Non-inherited genetic diseases (e.g. most types of cancer)
Chromosomal disorders
Epigenetic diseases
Non-inherited diseases (e.g. infectious diseases, injuries, etc.)
Both environmental and genetic factors have roles in the development of any disease.
1. Single-gene disorders – basic requirements
SLIDES 4-6 Single gene disorders (also called Mendelian or monogenic disorders) are caused
by changes or mutations that occur in the DNA sequence of one gene. A mutation can occur
in the protein coding or the regulatory region of a gene. Genes code for proteins, the
molecules that carry out most of the work, perform most life functions, and even make up the
majority of cellular structures. When a gene is mutated so that its protein product can no
longer carry out its normal function, a disorder can result. There are more than 6,000 known
single-gene disorders, which occur in about 1 out of every 200 births. Single-gene disorders
are inherited in recognizable patterns: autosomal dominant, autosomal recessive, and X-linked
dominant, X-linked recessive, Y-linked.
Cystic fibrosis
Cystic fibrosis is a common autosomal recessive genetic disorder that affects the entire body,
especially the respiratory and digestive systems. Difficulty breathing is the most serious
symptom and results from frequent lung infection that is treated with, though not cured by
antibiotics and other medications. The name cystic fibrosis refers to the characteristic scarring
(fibrosis) and cyst formation within the pancreas. People with cystic fibrosis inherit a
defective gene on chromosome 7 called CFTR (cystic fibrosis transmembrane conductance
regulator). CFTR functions as a CAMP-activated ATP-gated anion channel, increasing the
conductance for certain anions (e.g. Cl-) to flow down their electrochemical gradient.
Structurally, CFTR is a type of gene known as an ABC gene. ATP-driven conformational
changes, - which in other ABC transporter proteins fuel uphill substrate transport across
cellular membranes, - in CFTR open and close a gate to allow transmembrane flow of anions
down their electrochemical gradient. CFTR channels open and close in an ATP-dependent
manner, the open state catalyzing exclusively “downhill” Cl− movement. Essentially, CFTR is
an ion channel that evolved as a 'broken' ABC transporter that leaks when in open
conformation. The CFTR is found in the epithelial cells of many organs including the lung,
liver, pancreas, digestive tract, reproductive tract, and skin. Normally, the protein moves
chloride ions out of an epithelial cell to the covering mucus. This results in an electrical
gradient being formed and in the movement of (positively charged) sodium ions in the same
direction as the chloride via a paracellular pathway (the route between cells). If CFTR protein
doesn't work correctly, that movement is blocked and an abnormally thick sticky mucous is
produced on the outside of the cell. The cells most seriously affected by this are the lung cells.
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This mucous clogs the airways in the lungs, and increases the risk of infection by bacteria.
The thick mucous also blocks ducts in the pancreas, so digestive enzymes can't get into the
intestines. Without these enzymes, the intestines cannot properly digest food. People who
have the disorder often do not get the nutrition they need to grow normally. Finally, cystic
fibrosis affects the sweat glands. Too much salt is lost through sweat, which can disrupt the
delicate balance of minerals in the body. Symptoms of cystic fibrosis can include: coughing or
wheezing, respiratory illnesses (such as pneumonia or bronchitis), weight loss, salty-tasting
skin, and greasy stools. Because the lungs are clogged and repeatedly infected, lung cells
don't last as long as they should. Therefore, most cystic fibrosis patients only live slightly
more than thirty years old. The CFTR gene, found at the q31.2 locus of chromosome 7, is
230,000 base pairs long, and creates a protein that is 1,480 amino acids long. The most
common mutation, F508, is a deletion (Δ) of three nucleotides that results in a loss of the
amino acid phenylalanine (F) at the 508th (508) position on the protein. This mutation
accounts for two-thirds (66-70%) of CF cases worldwide and 90 percent of cases in the
United States; however, there are over 1,500 other mutations that can produce CF. Although
most people have two working copies (alleles) of the CFTR gene, only one is needed to
prevent cystic fibrosis. CF develops when neither allele can produce a functional CFTR
protein. In addition, there is increasing evidence that genetic modifiers besides CFTR modulate the frequency
and severity of the disease. One example is mannan-binding lectin, which is involved in innate immunity by
facilitating phagocytosis of microorganisms. Polymorphism in one or both mannan-binding lectin alleles that
result in lower circulating levels of the protein are associated with a threefold higher risk of end-stage lung
disease, as well as an increased burden. Genetic tests can identify a faulty CFTR gene using a sample of the
patient's blood. Although there is no cure for cystic fibrosis, new treatments are helping people with the disease
live longer than before. Most treatments work by clearing mucous from the lungs and preventing lung infections.
The common treatments include: (1) physical therapy, in which the patient is repeatedly clapped on the back to
free up mucous in the chest; (2) inhaled antibiotics to kill the bacteria that cause lung infections; (3)
bronchodilators that help keep the airways open; (4) pancreatic enzyme replacement therapy to allow proper
food digestion; (4) gene therapy, in which the healthy CFTR gene is inserted into the lung cells of a patient to
correct the defective gene.
Heterozygote advantage and cystic fibrosis The presence of a single CF mutation may influence
survivorship of people affected by diseases involving loss of body fluid, typically due to diarrhea. The most
common of these maladies is cholera, which throughout history has killed many Europeans. Those with cholera
would often die of dehydration due to intestinal water losses. A mouse model of CF was used to study resistance
to cholera, and it was found that heterozygote (carrier) mouse had less secretory diarrhea than normal, noncarrier mice. Thus it appeared for a time that resistance to cholera explained the selective advantage to being a
carrier for CF and why the carrier state was so frequent. Another theory for the prevalence of the CF mutation is
that it provides resistance to tuberculosis. Tuberculosis was responsible for 20% of all European deaths between
1600 and 1900, so even partial protection against the disease could account for the current gene frequency. The
selective pressure for the high gene prevalence of CF mutations is still uncertain. Approximately 1 in 25 persons
of European descent is a carrier of the disease, and 1 in 2500 to 3000 children born is affected by cystic fibrosis.
Sickle cell anemia
Sickle cell anemia (SCA; sickle cell disease; SCD) is an autosomal recessive genetic disorder
that affects the hemoglobin of red blood cells. Normally, red blood cells are round and
flexible so they can travel freely through the narrow blood vessels. The hemoglobin molecule
has two parts: an alpha and a beta. Patients with sickle cell disease have a mutation in a gene
on chromosome 11 that codes for the beta subunit of the hemoglobin protein. In the mutant
protein the hydrophobic amino acid valine takes the place of hydrophilic glutamic acid at the
sixth amino acid position of the hemoglobin beta polypeptide chain. This substitution creates
a hydrophobic spot on the outside of the protein structure that sticks to the hydrophobic region
of an adjacent hemoglobin molecule's beta chain. As a result, hemoglobin molecules don't
form properly (clump together - polymerization), causing red blood cells to be rigid and have
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a concave shape (like a sickle used to cut wheat). These irregularly shaped cells get stuck in
the blood vessels and are unable to transport oxygen effectively, causing pain and damage to
the organs. Unlike normal red blood cells, which can live for 120 days, sickle-shaped cells
live only 10 to 20 days. In the United States, the disease most commonly affects AfricanAmericans. About 1 out of every 500 African-American babies born in the United States has
sickle cell anemia. Sickle cell disease is most common among people from Africa, India, the
Caribbean, the Middle East, and the Mediterranean. The high prevalence of the defective gene
in these regions may be due to the fact that carriers of a mutation in the beta-subunit of
hemoglobin are more resistant to malaria (heterozygote advantage). Malaria is a disease
caused by a parasite that is transmitted to a person when they are bitten by an infected
mosquito.
Phenylketonuria
Phenylketonuria (PKU) is a rare autosomal genetic disorder (a metabolic disorder) that affects
the way the body breaks down protein. PKU is caused by a mutation in a gene on
chromosome 12. The gene codes for a protein called PAH (phenylalanine hydroxylase), an
enzyme in the liver. This enzyme breaks down the amino acid phenylalanine into other
products the body needs. When this gene is mutated, the shape of the PAH enzyme changes
and it is unable to properly break down phenylalanine. Phenylalanine builds up in the blood
and poisons nerve cells (neurons) in the brain. Babies born with PKU usually have no
symptoms at first. But if the disease is left untreated, babies experience severe brain damage.
This damage can cause epilepsy, behavioral problems, and stunt the growth of the baby.
Because PKU must be treated early, babies in every U.S. state are routinely tested for the
disease. A small blood sample is taken from the baby's heel or arm and checked in a
laboratory for high levels of phenylalanine. People who have PKU must eat a protein-free
diet, because nearly all proteins contain phenylalanine. Infants are given a special formula
without phenylalanine. Older children and adults have to avoid protein-rich foods such as
meat, eggs, cheese, and nuts. Norwegian doctor Asbjørn Følling discovered PKU in 1934.
About 1 out of every 15,000 babies in the United States is born with PKU.
SCID
(1) X-linked SCID SCID (severe combined immunodeficiency) is a group of very rare-and
potentially fatal-inherited disorders related to the immune system. The immune system
normally fights off attacks from dangerous bacteria and viruses. People with SCID have a
defect in their immune system that leaves them vulnerable to potentially deadly infections.
There are several types of SCID. The most common form is caused by a mutation in the
SCIDX1 gene located on the X chromosome. This gene encodes a protein called IL2RG
(interleukin-2 receptor). These receptors reside in the plasma membrane of immune cells.
Their job is to allow two types of immune cells - T cells and B cells - to communicate. When
the gene is mutated, the receptors cannot form and are absent from immune cells. As a result,
the immune cells can't communicate with one another about invaders in the environment. Not
enough T and B cells are produced to fight off the infection, and the body is left defenseless.
Another form of SCID is caused by a mutation on chromosome 20 and is characterized by a
deficiency of the enzyme adenosine deaminase (see ADA Deficiency). The most common
form of SCID exhibits an X-linked recessive pattern of inheritance, and is therefore referred
to as X-linked SCID. When a gene is located on the X chromosome, males are more often
affected than females. Males do not have a second X chromosome to compensate for the
defective one. Symptoms usually appear in the first few months of life. Because the immune
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system cannot protect the baby's body, babies with the disorder tend to get one infection after
another. Some of these bacterial infections may be life-threatening, including pneumonia
(lung infection), meningitis (brain infection), and sepsis (blood infection). To make matters
worse, SCID patients often don't respond to the antibiotics used to treat bacterial infections.
They may suffer more frequently from ear infections, sinus infections, a chronic cough, and
rashes on the skin. Early diagnosis of SCID is very important, because without quick
treatment, children with the disease aren't likely to live past age 2. SCID can be identified
before the baby is born by removing and testing cells from the placenta (chorionic villus
sampling or CVS), or by removing and testing a sample of the fluid surrounding the baby
(amniocentesis). The most common screening methods are an immune function test, and a
blood test that detects low white blood cell counts, as well as low levels of immune cells (T
cells and B cells). The most effective treatment is a bone marrow transplant. Unspecialized
stem cells (that will form blood and immune cells) are taken from the bone marrow of a
healthy donor and injected into the SCID patient. Ideally, these new cells will stimulate the
production of the needed immune cells. Transplants done within the first few months of life
are most successful. The tissue must be "matched" to the patient, however, which can limit
the usefulness of this therapy. Siblings make the best donors as their cells likely contain a
similar genetic makeup.
Gene therapy for this disorder would compensate for the faulty gene by injecting healthy
copies of the gene into a patient's bone marrow stem cells. About 1 out of every 100,000
babies is born with SCID. SCID is sometimes called Bubble Boy disease. In the 1970s, a boy
named David Vetter had to live in a plastic bubble for 12 years because of SCID.
(2) ADA (adenosine deaminase) deficiency is one form of SCID. ADA deficiency is very
rare, but very dangerous, because a malfunctioning immune system leaves the body open to
infection from bacteria and viruses. The disease is caused by a mutation in a gene on
chromosome 20. The gene codes for the enzyme adenosine deaminase (ADA). Without this
enzyme, the body is unable to break down a toxic substance called deoxyadenosine. The toxin
builds up and destroys infection-fighting immune cells called T and B lymphocytes. ADA
deficiency is an autosomal recessive disorder. Because ADA deficiency affects the immune
system, people who have the disorder are more susceptible to all kinds of infections,
particularly those of the skin, respiratory system, and gastrointestinal tract. Doctors can
identify ADA deficiency during the mother's pregnancy (1) by taking a tiny sample of tissue
from the amniotic sac where the baby develops (called chorionic villus sampling), or (2) by
looking at enzyme levels in a fetal blood sample taken from the umbilical cord. After the child
is born, doctors can test a sample of his or her blood to see if it contains ADA.
Duchenne Muscular Dystrophy
Duchenne muscular dystrophy (DMD) is a recessive X-linked form of muscular dystrophy,
which results in muscle degeneration, difficulty walking, breathing, and death. The incidence
is 1 in 3,000. Only males are affected, though females can be carriers. The disorder is caused
by a mutation in the dystrophin gene located in humans on the X chromosome. The
dystrophin gene codes for the protein dystrophin, an important structural component within
muscle tissue. Dystrophin provides structural stability to the dystroglycan complex (DGC),
located on the cell membrane. Dystrophin is the longest gene known on DNA level, covering
2.4 megabases (0.08% of the human genome) at locus Xp21. However, it does not encode the
longest protein known in humans. The primary transcript measures about 2,400 kilobases and
takes 16 hours to transcribe; the mature mRNA measures 14 kilobases. The 79 exons code for
a protein of over 3500 amino acid residues.
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