Download Genetics Chapter 13 p258

Chapter 13 – Genetic Testing and Gene Therapy (pages 258-273 ONLY)
Genetic testing is defined as analysis of chromosomes, DNA, RNA, proteins, or other analytes to detect
abnormalities that can cause a genetic disease.
1. Population Screening for Genetic Disease
a. Designed to detect treatable disease early
i. Pap smears, hypercholesterolemia
b. Population screening is large scale testing for disease to see who probably has or
doesn’t have a test
i. First step to identify a subset of the population, follow up with more tests to
confirm diagnosis
c. Genetic screening – population screening for gene that causes disease
i. Newborn screening
ii. Heterozygote detection – e.g. for Tay-Sachs
GOAL OF SCREENING: Early recognition of disorder so intervention can prevent/reverse disease process
(aka: Newborn metabolism errors) or to make informed reproductive decisions (heterozygous
carriers/recessive mutations). General test first, followed by specific diagnostic test.
d. Principles of Screening
i. Should be serious and relatively common condition
1. PKU – inexpensive test
a. Importance of tests validity (sensitivity and specificity)
i. Sensitivity – detection of positive
ii. Specificity – detection of negative
1. These are reciprocally related (if sensitivity
increases, specificity decreases and vice versa)
b. Predictive values for a positive or negative test are just the
specific population (affected/those who tested positive)
i. Refer to Table 13-1 and a/a+b
c. CAH – Congenital Adrenal Hyperplasia
i. Deficiency of 21-hydroxylase
ii. Test is highly accurate
The main point: As prevalence of a disease increases, the value of the positive screening test also
increases with more true positive tests. (More validity of the test)
e. Newborn screening for inborn errors of metabolism (PKU, galactosemia)
i. Sickle cell testing, Duchenne Muscular Dystrophy
ii. Screening useful for public health in treatable disorders
iii. Tandem mass spectrometry has increased the number of detectable diseases
f. Heterozygote Screening
i. Scanning people who are at risk already
ii. Example : Tay-Sachs
1. Autosomal recessive lysosomal storage disorder
a. Beta-hexoamninidase A is deficient
b. Ganglioside builds up in neuronal lysosomes
c. Causes blindness, seizures, hypotonia
d. Ashkenazi Jews predisposed
iii. Beta Thalassemia
1. Mediterranean and South Asians at risk
Heterozygote screening constists of testing (at the phenotype or genotype level) a target population to
identify unaffected carriers of a disease gene. The carriers are then given info about risk and
reproduction options.
g. Presymptomatic diagnosis
i. Can help with reproductive decisions
ii. Can help with detecting issues early
1. E.g. Breast cancer mutation patients get early mammograms
2. RET mutations  thyroidectomy
3. APC and HNPCC
h. Psychosocial implication of genetic screening and diagnosis
i. Weighing anxiety, cost, and potential stigmatization
ii. The issue of accurate testing with possibly increasing healthcare costs.
iii. Some autosomal dominant issues can help with survival but many genetic issues
are too variable in other “disease causing genes”
2. Molecular Tools for screening and Diagnosis
a. Linkage Analysis
i. Marking loci in a family to follow specific mutations/alleles
ii. Need multiple samples in a family and recomibination and uninformative mating
make this difficult
b. Direct Mutation Analysis
i. Can be used to directly analyze a mutation if it affects a restriction site
c. Allele-Specific Oligonucleotides
i. Hybridizing a persons DNA with allele-specific oligonucleodtide.
ii. Useful if the DNA sequence is known and if the number of disease mutations is
limited
d. Other methods of direct diagnosis
i. Mass spectrometry
1. Detects differences in PCR-amplified DNA
ii. Tandem mass spectrometry
1. Used more for newborns, especially protein variations that characterize
newborn disorders
3. Prenatal Diagnosis of Genetic Disorders and Congenital Defects
a. 15 weeks, alpha-fetoprotein
i. Positive test indicates further testing needed for aneuploidy, NTDs
ii. Subsequent amniocentesis needed
b. Amniocentesis
i. Around 15-17 weeks
ii. Amniocytes withdrawn, cultured for 7 days, and most results available by day 12
post-test
1. FISH can be done quickly (1-2 days) and give quick aneuploidy result
iii. Can suggest NTD
1. NTD may be causing elevated AFP because of underestimated
gestational age, fetal death, twins, blood contamination, malformations
(omphalocele or gastroschisis)
iv. Rate of fetal loss because of this procedure is about 1/200 above background
risk level. Can be performed earlier but increases risk
c. Chorionic Villus Sampling
i. Can be done at 10-11 weeks, better to detect earlier
ii. Confined placental mosaicism (1-2% of time)
1. May show a problem in vili that’s not actually wrong with fetus
a. Can be confirmed if it is an actual fetal problem with follow up
amnio about a month later
iii. If performed before 10 weeks, some evidence that may affect fetal limbs
d. Other methods of fetal tissue sampling
i. Cordocentesis (Percutaneous umbilical blood sampling, PUBS)
1. After 16th week
2. Three application of PUBS
a. Cytogenic analysis of blood
b. Hematological diseases (immunological)
c. True vs. false mosaicism
e. Ultra sonography
i. Non-invasive, used in conjunction with invasive tests like amnio, and CVS
f. Maternal serum screening in 1st and 2nd trimesters
i. Link between elevated maternal serum AFP and NTDs
ii. Around 15-17 weeks
iii. Issue lies with sensitivity and specificity
1. Can increase sensitivity for detecting Downs with Quadruple Screen
g. Preimplantation Genetic diagnosis
i. During in-vitro fertilization, few cells removed from blastomere, and amplified
with PCR to detect early genetic issues
ii. PCR may cause inaccuracies
iii. Polar Body Diagnosis (the inactivated meiotic divided cell)
1. Can test its DNA to check for disease mutations
2. Can’t test paternal DNA with this
h. Fetal DNA in maternal circulation
i. Isolating fetal DNA from maternal blood, low risk
ii. But difficult to isolate, unknown accuracy
4. Fetal Treatment
a. Treatment of inborn errors and hormone deficiencies
i. Biotin responsive carboxylase deficiency
1. Treated with oral biotin – resulted in healthy newborn
ii. CAH
1. Treated at 10 weeks with oral dexamethasone
iii. Surgical approaches are moderately successful
1. Success in myelomeningocele (to restore CSF flow)