CYSTIC FIBROSIS

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CYSTIC FIBROSIS
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"Woe is the child who tastes salty from a kiss on
the brow, for he is cursed, and soon must die.
-Northern European Folklore
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CYSTIC FIBROSIS

• Cystic Fibrosis (CF) is a lethal inherited
  disease that affects the lungs, digestive system,
  sweat glands, and male fertility
• Its name derives from the fibrous scar tissue
  which develops in the pancreas

Figure 8
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CYSTIC FIBROSIS

• Cystic Fibrosis affects the bodys ability to
  move salt and water in and out of a cell
• This results in the lungs and the pancreas
  secreting a thick mucus
• This mucus blocks passageways and prevents proper
  function

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Hallmarks of CF

• Very salty-tasting skin
• Appetite, but poor growth weight gain
• Coughing, wheezing shortness of breath
• Lung infections, e.g. pneumonia/bronchitis

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Clinical Aspects

• Cystic fibrosis affects the entire body

• Lungs and sinuses
• GI, liver and pancreas
• Endocrine system
• Reproductive system

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Epidemiology

• CF is a rare disease
• Approximately 30,000 in the U.S. people have CF
• Over 10 million Americans are unknowing carriers.
• Around 2,500 children with CF are born each year.

• CF is a disease of Caucasians.

Group Incidence Carriers Delta F508
Caucasians 1 / 3,300 1 / 29 70
Hispanics 1 / 9,000 1 / 46 46
African Americans 1 / 15,300 1 / 60 48
Asian Americans 1 / 32,100 1 / 90 30
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Heritability

• CF is a hereditary disease.
• Unaffected parents can have children with CF.
• Males and females are equally likely to be
  diagnosed.

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Carrier Dad
Carrier Mom
Eggs
Sperm
Baby with CF
Normal carrier
Normal carrier
Normal non-carrier
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Signs and Symptoms

• Thick, viscous mucus secretion in the lungs
• Changes in color and amount of sputum
• chronic coughing
• Wheezing
• Bronchitis
• salty skin or poor growth
• Weight loss (abdominal swelling)

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Molecular and Genetic Basis of Cystic Fibrosis
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Mapping the gene for CF

• Gene linkage studies were able to map the
  mutation to chromosome 7.
• Classical genetics techniques were not able to
  accurately pinpoint the mutated gene.

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Gene Locus
Chromosome 7 Locus 7q31.2

• The CFTR gene
• is 250, 000 bp long
• contains 27 exons
• the protein has 1, 480 amino acids with a
  molecular mass of 168, 138 Da
• nucleotide of 6,500

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Mapping the gene for CF

• 1989 Lap-Chee Tsui, at the Hospital for Sick
  Children in Toronto, clones the CFTR gene.
  Victory tastes sweet.

Chromosome walking and jumping techniques were
used to identify and sequence the 180,000 bp
gene.
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The ?F508 Mutation
A 3 base pair deletion called ?F508 is the most
common mutation causing cystic fibrosis

• The mutation results in the deletion of a single
• amino acid (Phe) at position 508.

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Benefits of ?F508

• The ?F508 mutation most likely occurred
• over 50,000 years ago in Northern Europe.

Individuals with two copies of ?F508 get cystic
fibrosis and often cannot reproduce. Having one
copy of ?F508 reduces water loss during cholera,
greatly increasing the chance of survival.
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The Function of CFTR

• CFTR encodes a 170 kDa, membrane-based
• protein with an active transport function

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Role of CFTR Protein

• The CFTR protein plays a vital role in mucus
  function
• The CFTR protein helps to keep mucus from
  becoming thick

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Role of CFTR Protein

• In CF, the CFTR cannot allow chloride ions to
  move out of the mucus-producing cells
• This means water doesnt leave and it results in
  the mucus becoming thick
• This in turn blocks the passageways, and allow
  bacteria to feed off the mucus, which results in
  more infections

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From Mutation to Disease

• The mutant form of CFTR
• prevents chloride transport,
• causing mucus build-up

Mucus clogs the airways and disrupts the
function of the pancreas intestines.
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CFTR Mutations

• Over 1,000 mutations in
• CFTR have been found.
• ?F508 accounts for just
• 70 of CF cases.

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The Sweat Test

• Measures the concentration of chloride and sodium
  that is excreted in sweat.
• Two reliable positive results on two separate
  days is diagnostic for CF.
• Clinical presentation, family history and patient
  age must be considered to interpret the results.

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5 Classes of CFTR Mutations

• CF Mutations can be classified by the effect they
  
• have on the CFTR protein.

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5 Classes of CFTR Mutations
I Defective Production
II Defective Processing
III Defective Regulation
IV Defective Conductance
V Reduced Amounts
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Genotype Class and Mortality

• Mutation class can affect disease mortality.

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Genotype and Phenotype

• Clinical phenotypes can vary widely across
  mutations

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Newborn Screening

• Infants can easily be diagnosed with a blood test
• Elevated levels of trypsinogen indicate CF
• Screening programs identify 10 of cases at birth

• Most hospitals do not screen for CF at birth.
• Should they?

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Genetic Carrier Testing

• Tests for common CF mutations are available.
• The type of defective CF gene can affect the type
  of CF symptoms.
• However, genetic testing cannot fully determine
  how severe a person's CF will be in advance.

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Carrier Screening

• Population-based screening
• Particular genetic carrier tests offered to
  everyone in the general population
• Targeted population-based screening
• Carrier screening limited to particular groups of
  people determined to be at higher risk for
  specific genetic disorders
• e.g. Ethnicity-based carrier screening

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Carrier Testing

• To determine an individuals carrier status for a
  specific genetic disease
• Not usually offered on a population basis

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Carrier Testing

• Available to clients with a family history of an
  autosomal recessive or X-linked genetic condition
  for which carrier testing available
• e.g. Fragile X syndrome, Duchenne muscular
  dystrophy, Hemophilia A or B
• e.g. PKU, Alpha-1-antitrypsin deficiency,
  Galactosemia

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Ethnicity-Based Genetic Carrier Screening

• Purpose To detect couples at risk for prenatally
  diagnosable genetic diseases
• Types of tests offered based on clients ethnic
  background
• Offered to all individuals of that ethnic
  background (targeted population screening)

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CARRIER FREQUENCIES BASED ON ETHNIC ORIGIN
Condition
Carrier Frequency
Population
African-American Sickle Cell Cystic Fibrosis Beta-Thalassemia 1 in 10 1 in 65 1 in 75
Ashkenazi Jewish Gaucher disease Cystic Fibrosis Tay-Sachs disease Dysautonomia Canavan disease 1 in 15 1 in 26 - 1 in 29 1 in 30 1 in 32 1 in 40
Asian Alpha-Thalassemia Beta-Thalassemia 1 in 20 1 in 50
European American Cystic Fibrosis 1 in 25 - 1 in 29
French Canadian, Cajun Tay Sachs disease 1 in 30
Hispanic Cystic Fibrosis Beta-Thalassemia 1 in 46 1 in 30 - 1 in 50
Mediterranean Beta-Thalassemia Cystic Fibrosis Sickle Cell 1 in 25 1 in 29 1 in 40
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CF Carrier Results

• Many tests detect a majority but not all carriers
• Detection rates differ by ethnicity
• Negative results do not eliminate risk
• Different mutations may confer different risks
• Example CFTR R117H mutation and 5T allele
• Genetic consultation is available to carriers and
  strongly advised for carrier/carrier couples

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Carrier Rates Cystic Fibrosis
Ethnic Group Carrier Frequency Detection Rate Carrier risk after negative test
Northern European Caucasian 1/25 1/29 85-90 1 in 250
Ashkenazi Jewish 1/26 1/29 97 1 in 930
Southern European Caucasian 1/29 70-80 1 in 97 to 1 in 140
Hispanic 1/46 57 1 in 105
African American 1/65 72 1 in 232
Asian 1/90 (?) 30 (?) Not available
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Drug Therapies

• Medication are often aerosolize and can be
  inhaled
• Bronchodilators
• Mucolytics
• Decongestants
• Antibiotics to fight lung infections
• Enzyme supplements

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• How can Gene Therapy be used to treat patients
  who suffer from Cystic Fibrosis (CF)?

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Gene Therapy

• SmaRT-discovered by Xiaoming Li
• Interferes with the processing of RNA
• Synthesis of DNA probe to locate the gene
• make copies of normal and abnormal gene
• Problems with gene therapy
• Ethical issues
• Fooling with Mother Nature

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Gene Therapy

• Gene therapy is currently the most ambitious
  approach to curing Cystic Fibrosis
• Nondefective copies of the CFTR gene are
  introduced into affected cells, where they are
  taken up and used to create the CFTR protein
• This will result in the mucus functioning
  properly and a stop to Cystic Fibrosis

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Gene Therapy

• It has been attempted to deliver the normal CFTR
  gene to the bronchial epithelium by aerosol
  spray, using a viral vector (usually an
  adenovirus)
• This viral vector would contain the genetic
  material for the CFTR gene
• The vector would be able to enter a cell inside
  the body and insert the genetic material for the
  CFTR gene into the cells DNA

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Gene Therapy

• This would result in the cell having a normal
  CFTR gene and an end to CF
• Thus far the attempts have not been completely
  successful, as most patients develop an immune
  response against the virus during the course of
  therapy

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Thank You