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Genetic Testing for Cystic Fibrosis

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1 Genetic Testing for Cystic Fibrosis
Dee Quinn, MS, CGC August, 2006

2 “Gene Testing Going Mainstream”
Cystic Fibrosis Gene Test Offered By Lauran Neergaard AP Medical Writer Monday, Oct. 1, 2001; 9:53 p.m. EDT WASHINGTON –– Gene testing is going mainstream: Starting this month, tens of thousands of white Americans will be offered testing to see if they carry a gene mutation that causes cystic fibrosis even if no one in their family has the disease.

3 CFTR Gene Identified in 1989
Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) 27 exons, 250kB of genomic DNA Chromosome 7q Lap-Chee Tsui and Francis Collins headed team that reported the discovery of the gene, linkage analysis (a lot of families to use) and positional cloning Name CFTR-they hypothesized the gene was involved with transport and conductance across the membrane Large gene Located on the long arm of chromosome 7



6 CFTR Gene cAMP-dependent chloride channel
Expressed in epithelial cells of: respiratory tract sweat and salivary glands pancreas intestine reproductive tract Reason CF has the clinical presentation that it does: CFTR forms a regulated cell membrane Cl channel with several domains, allowing chloride to be transported out of the cell to the epithelial surface, so mutations cause defective chloride transport Look at each individually 2. Respiratory: too much chloride kept in cell, leads to dehydrated secretions, thick mucus instead of thin watery secretions Sweat glands: high levels of NaCl (salt) not reabsorbed Pancreas-obstruct the digestive system and prevent pancreatic enzymes from reaching the small intestine Intestine-presence of thickened mucus and lack of digestive enzymes, lead to increased risk for bowel obstruction, can sometimes be seen by ultrasound echogenic bowel (1-13% risk) Reproductive

7 Clinical Phenotype Respiratory GI, pancreas Reproductive
CF is typically a multi-system disease. Most patients have both respiratory and digestive problems, while others only have respiratory problems. Pulmonary manifestations-range from very mild pulmonary symptoms to severe progressive chronic lung disease, vast majority of patients with CF die as a result of pulmonary complications Pancreatic insufficiency and intestinal malabsorption are present in 85% of affected individuals Men with CF (more than 95%) are infertile due to azoospermia, because of congenital bilateral absence of the vas deferens Also see men without any pulmonary or gastrointestinal manifestations of CF may have CBAVD resulting in azoospermia Salt loss syndromes like salt depletion or chronic metabolic alkalosis Intelligence and appearance are typically not affected in those with CF.

8 CFTR Mutations Over 1,000 mutations identified
Most labs test for mutations 13 of the identified CF mutations occur in more than 1% of CF chromosomes 72% of whites with CF are homozygous or heterozygous for 8 mutations 5 classes of CFTR mutations Genotype/phenotype correlations may not be helpful With that many mutations, how could screening possibly be accomplished? Most of the remaining mutations occur infrequently, usually only in a particular family Helpful to organize into 5 classes

9 Class I premature termination signals and splicing abnormalities, no protein production
Class II- defective folding and disruption of protein biosynthesis, can’t make to membrane Class III- generate protein that reaches the plasma membrane but channels show defective regulation Class IV- generate correcting localized CFTR with defective pore properties, reduced Cl current, less Cl in and out and the channel is open a shorter time Class V-reduced protein production In general Class 1,2 or 3 mutations are expected to have more serious phenotypic consequences than class 4 or 5 Call attention to deltaF508 and R117h

10 CFTR Mutations- ΔF508 Most common mutation in North America, 70-75% of the mutations Deletion of codon 508  Results in shortening of the protein product Protein product still functioning as Cl channel, but is retained in the endoplasmic reticulum ΔF508 (homozygosity): classical phenotype Varies from ethnic group Classical phenotype-typically have severe pancreatic insufficiency combined with variable pulmonary function

11 Epidemiology One of the most common autosomal recessive diseases in Caucasians Occurs in 1 in 3300 births 30,000 affected persons in the United States Proposed advantages cholera infection, higher fertility and resistance to a variety of infectious agents.


13 Recessive Pedigree

14 Epidemiology 97 1/29 1/3,300 Ashkenazi Jews 30 1/90 1/32,100 Asian
Americans 69 1/60-65 1/15,300 African Americans 80-90 1/52 1/3,970-1/1,500 Native Americans 57 1/46 1/8-9,000 Hispanics Caucasians (United States) Sensitivity Carrier Frequency Incidence Group 80-90 Sensitivity depends on ethnic background of Caucasians, N. Europeans have 90% detection rate, S. Europeans 80% Native Americans founder effect, geographic isolation

15 Congenital Bilateral Absence of the Vas Deferens (CBAVD)
Vas deferens – carries sperm from the epididymis to the ejaculatory ducts Absence of vas occurs in 95% of males with CF CBAVD: distinct genetic disorder which overlaps with CF and causes infertility Noncoding region of CFTR gene involved: intron 8 with thymidine tracts (5T/7T/9T) 60-70% of men with CBAVD carry one mutation in the CFTR gene. 5T reduces the number of functional Cl channels Intricately tied to CF CBAVD is a distinct genetic disorder. Condition overlaps with CF, but also represents a distinct entity so it occurs outside of the CF phenotype and is responsible for 1-2% of infertility due to azoospermia Mutations in the CFTR gene have also been identified in patients with CBAVD which suggests this condition is a genital form of CF % of men with CBAVD carry one mutation in the CFTR gene. 5T reduces the number of functional Cl channels

16 Congenital Bilateral Absence of the Vas Deferens (CBAVD)

17 Genotype - Phenotype Correlation
CFTR Genotype First Allele Second Allele Range of Phenotypes Classic (e.g., F508) Classic Classic >> nonclassic Mild (e.g., A455E) Classic or mild Nonclassic > classic R117H/5T Classic or mild Nonclassic > classic R117H/7T Classic or mild Asymptomatic female or CBAVD > nonclassic 5T/TG13 or TG12 Classic or mild CBAVD or nonclassic CF >> asymptomatic carrier 5T/TG11 Classic or mild Asymptomatic > CBAVD 7T or 9T Classic or mild Asymptomatic 7T or 9T 7T or 9T Asymptomatic (From 1. Patterns reflect dominant effect of "milder” alleles in compound heterozygotes. Classic alleles generally refer to Class I-III mutations; mild alleles refer to Class IV-V mutations exclusive of R117H and 5T alleles.

18 Newborn Screening Blood spots from infants taken within days of birth to identify infants at increased risk for a specific genetic disorders Justifications: Early treatment of respiratory illnesses Evidence for nutritional benefit Currently offered or in planning in many states – AZ to begin newborn screening by 9/07

19 Newborn Screening Initial screen tests levels of IRT (immunoreactive trypsinogen) Screening program should include: Specific provider and patient educational materials Protocol for addressing positive screening results Sweat chloride DNA testing Development of systems in collaboration with specialty care providers to track short-term and long-term child outcomes and identify resources to support this activity

20 Carrier Screening for CF
1997- NIH convened a Consensus Conference 1998- ACOG/ACMG formed Steering Committee 10/2001- “Preconception and Prenatal Carrier Screening for Cystic Fibrosis” 1997- several pilot studies before consensus, recommended that CFTR mutation testing be available to all pregnant couples and those contemplating pregnancy 1998-steering committee to coordinate the implementation of the consensus conference recommendations, develop guidelines for provider education, laboratory testing, interpretation and genetic counseling, and patient education and informed consent 10/2001- document includes guidelines for providers, patient education, and informed consent.

21 ACOG/ACMG Recommendations
Offer screening to: Individuals with a family history of CF Reproductive partners of individuals with CF Couples in whom one or both are Caucasian and are planning a pregnancy or seeking prenatal care Other individuals must be given written information Elicit family history, determine who in family has CF, refer to genetics professional because of interpretation of test results and estimation of risk may be more complex than in the general population (bring up referral to genetic professional, clearly states so that obs and PCPs will not have to provide these services and so that insurance will cover) Clarify couple’s risk of having a child with CF, majority are aware of their increased risk for having a child with CF, again referral to genetics professional Offered when both the frequency of carriers and the detection rate of the carrier test are relatively high in the ethnic groups of the partners Made available to other ethnic and racial groups, informed of their delectability through educational brochures. For example, Asian Americans w/o significant Caucasian admixture should be informed of the rarity of the disease and the very low yield of the test in their population. Problem may be difficult to classify patients ethnic background, especially in a busy clinical setting, advocate universal screening, most often happen in the PCP’s office or the OB’s office

22 ACOG/ACMG Recommendations
Provider’s Role: Purpose of screening Voluntary nature of screening Symptoms of CF, treatment and prognosis Genetics of CF and population frequencies Meaning of positive and negative test results Factors to consider in deciding to have or not to have screening Common patient belief that any test offered by a physician must be necessary and advantageous, see this with amnio Booklet address these issues, makes explicit the optional nature of the testing and presents information in written form so that the patient and partner can consider their concerns and formulate questions about testing before discussing it with the support staff or provider, suggested that booklet be sent to a woman before her initial clinic visit, reinforces idea that CF screening is a personal decision between the couple Factors for screening, if risk seems high and insurance covers, against doesn’t detect all carriers Booklets are intended to supplement not replace communication with providers Informed consent: counseling and offer of screening and the couple’s decision about screening should be discussed and documented in the medical record, written informed consent should be obtained only after the woman and her partner have opportunity to review the educational material and receive pretest counseling. Lower risk ethnic groups, CF screening should be made available, printed information provided, screening and counseling provided upon request

23 Additional Indications for Screening
Echogenic bowel detected on prenatal ultrasound Infertility in males

24 Diagnostic Prenatal Tests
Testing offered when: When both members of a couple are carriers, ie: 25% risk of having a baby with CF When one member of a couple is carrier and other member not available for testing Testing options: Chorionic villus sampling (CVS) 9-11 weeks Amniocentesis After 14 weeks

25 Other Approaches Procedure Caveats In vitro fertilization
One cell removed from early embryo to test for mutations which were found in parents Cell without a CF genotype transferred to mother’s uterus Caveats Technically demanding and complex procedure Available on a limited basis Expensive: $4,000 - $12,000 Ethical implications

26 ACOG/ACMG Recommendations
Laboratory’s Role: Reports should include results of screening and an interpretation: Negative  Residual risk given Positive  Test other partner So doctor can easily understand and have available residual risk See chart 1 QA: Time constraint, expect in 2 weeks Use accredited labs that will provide high quality testing and interpretation DNA based test, use oligonucetide primers

27 Limitations of CF Screening
Does not detect all carriers Estimate of residual risk applies only when family history is negative and to the current pregnancy Cannot make reliable predictions for outcome based on mutations Non-paternity Not all CF mutations can be detected in screening panels. Impt to go over residual risk based on the racial or ethnic group of the partner, the specific mutations the test covered, and whether one or both partners were tested like we just looked at At end: All of these issues should be discussed with couple before testing and reviewed after results given

28 Genetic Counseling Various outcomes of prenatal and newborn will generate need for genetic counseling: Newly diagnosed child with CF Healthy males who carry mutations associated with infertility Identification of positive/negative couples who request additional mutational analyses or counseling to clarify residual risk Positive/positive couples

29 Ethical, Legal, and Social Implications of CF Screening
Unnecessary anxiety created Inadequate pretest information Legal Informed Consent Insurance discrimination Social Expense swell health costs Societal pressure not to bear affected offspring NIH conducted pilot studies before issuing consensus statement. Ethical, legal and social issues were looked at in deciding whether CF screening should be implemented. Ethical: Anxiety levels: no increase in anxiety levels by CF education or DNA testing Less than half (44%) whose result was negative understood that they could still have a child with CF, providers in that study spent little time explaining CF carrier screening and generally relied in patient comprehending written material (article) Legal: Without proper education, can’t really be considered an informed consent to testing, suggestions have been made to use quizzes that would test patient understanding, provider cannot be held liable for couples that have children with CF 1/3 of those express concern about insurance discrimination, laws in place that prevent discrimination, most of the concern with insurance discrimination is with predictive gene tests and insurance companies reducing benefits on the basis of predictive gene test. Social: 57% offered screening accepted, factors against religious beliefs, 1/3 insurance fears Most decided on their own rather than on the basis of physician, reasons were logical, most women who would not terminate a pregnancy for CF chose not to be tested Overall, NIH pilot studies have not shown results that substantiate concern over any of these implications, except for adequate pretest education. Several studies have been done since then and these same issue has arisen, that providers aren’t familiar with the information about CF and in turn do not pass along to their patients

30 Resources Cystic Fibrosis Foundation
GeneTests and GeneReviews National Society of Genetic Counselors Mountain States Genetics Network

31 Conclusions “It will be very important to see how this goes. Certainly it requires the obstetricians to become more familiar with genetics than many of them have previously had occasion to do.” -Francis Collins Implications for other gene testing

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