Genetic Screening

Question 1

What is targeted screening?

Answer 1

Targeted Screening
– Screening of populations known to be at risk:
• Affected individual in the family – Reduced penetrance or late onset
• Screening of high risk ethnic groups
– Persons at risk of having children with a genetic disease (carriers)

Question 2

What is population screening?

Answer 2

Population Screening
– Screening all members of a designated population regardless of family history
• PAP, breast, prostate
• Prenatal screening and Newborn screening

Question 3

What is the purpose of prenatal screening?

Answer 3

Typically used to detect 3 abnormalities
• Trisomy 21 (Down Syndrome) ~1/830 live births
• Trisomy 18 ~1/7,500 live births
Trisomy 13 has similar results to 18 but is ~1/22,700 live births
• Neural tube defects ~1/2,000 live births

Question 4

How does AFP from fetal to Maternal Serum?

Answer 4

Alpha Fetoprotein (AFP) is synthesized in: Yolk sac
Fetal GI tract Fetal liver
Detectable in fetal serum at 6 weeks Peak level in 12-14 weeks

To maternal circulation

Question 5

What are the types of prenatal screenings?

Answer 5

Non-invasive
Maternal Serum Screening- 16 weeks
First trimester screen- 11-14 weeks+1 day
Second trimester screen- 14 weeks+2 days-20 days

Ultrasound (fetal anomaly scan) -18 weeks
Nuchal translucency- 11-14 weeks

Invasive

Chronic villus sampling- 10-14 weeks
Amniocentesis- 15-18 weeks

Question 6

What are the maternal serum screening markers?

Answer 6

First Trimester Tests: 11-14 weeks + 1 day
Pregnancy associated plasma protein-A (PAPP-A) Human chorionic gonadotropin (-hCG)

Second trimester tests: 14 weeks + 2 days to 20 weeks

Triple Test: looks for 3 markers
AFP (alpha fetoprotein)
Estriol (unconjugated estriol, or uE3) Human chorionic gonadotropin (-hCG)

Quad test: Includes another marker
Inhibin A

Question 7

What are the maternal screening patterns?

Answer 7

-Low serum AFP, estriol and PAPP-A with high β-hCG and Inhibin A indicate a risk for Trisomy 21 (Down Syndrome)

• Low serum AFP, estriol, PAPP-A and β-hCG and indicate a risk for trisomy 18 (Edward syndrome)
• Trisomy 13 (Patau syndrome, most rare) first trimester test often shows reduced PAPP-A and β-hCG, however these reductions tend towards normal by the second trimester, thus second trimester test is uninformative for trisomy 13

Question 8

What are the challenges of Non-invasive Prenatal Screening (NIPS or NIPT)?

Answer 8

Challenges/complications in testing
– Young maternal age
– Twin or other multiples
– Previous pregnancies

Question 9

What is the purpose of the Non-invasive Prenatal Screening (NIPS or NIPT)?

Answer 9

• Detects Cell Free DNA in Maternal Serum, 10 weeks till delivery – cell-free DNA (cfDNA) from the fetus in the maternal circulation
• Fetal fraction (amount of fetal DNA in maternal circulation)
• Small segments of DNA 25-30 bp
– Used primarily for detection of aneuploidy
• Trisomy 13, 18, 21 or extra or missing X and Y
– May be used for detecting mutations in specific genes

• This is a screening test that predicts risk for an affected fetus
– Analysis of fetal DNA in maternal blood (non-invasive) has the advantages of earlier detection of
trisomy and eliminates the risk of spontaneous abortion by amniocentesis or CV sampling
– Requires follow-up prenatal testing to confirm any findings

Question 10

What is the main purpose of ultrasonography?

Answer 10

At 18 weeks can detect many different structural abnormalities
Neural tube defects are best detected by ↑maternal AFP combined with ultrasound

Nasal bone, hypoplastic or absent may suggest Trisomy 21

Question 11

What is Enlarged Nuchal Translucency (NT) 11- 14 weeks?

Answer 11

Increased NT thickness is associated with chromosome aneuploidy
Trisomies 21, 18, 13,
Triploidy
Turner syndrome (45,X)

Question 12

Describe maternal serum screening prenatal genetic counseling

Answer 12

• Prior to screening, a Genetic Counselor will inform the family that this is a SCREENING and not a diagnostic test
• Informed consent will force couples to consider their fetus may have a birth defect
• A positive screening result may require invasive testing
• Information comes either just before or in the second trimester when
family bonding has begun
• Inherent risk the invasive procedure may harm the pregnancy
• Issue of considering pregnancy termination if an abnormality is detected
• Conflict over additional bonding with fetus if an abnormality is detected

Question 13

What is Chorionic Villus Sampling?

Answer 13

• 10-14 weeks
• Involves removal of fetal cells by aspiration from the inner surface of placenta
• Direct chromosomal analysis by FISH allows rapid results

Question 14

What is amniocentesis ?

Answer 14

15-18 weeks

10-20 ml of amniotic fluid is aspirated trans-abdominally guided by ultrasound

• Fetal cells are pelleted by centrifugation and used for chromosome analysis
• supernatant can be used for AFP assay

Question 15

What is Percutaneous umbilical blood sampling PUBS?

Answer 15

• Usually done after 18 weeks of gestation
• Performed when there is a delayed suspicion of a chromosomal abnormality usually detected by ultrasound in 2nd trimester

Question 16

What situations call for prenatal genetic counseling for maternal screening?

Answer 16

Prenatal genetic counselor involvement during pregnancy
• Abnormal serum screening (neural tube defects or chromosome abnormalities)
• Abnormal ultrasound finding
• Positive invasive diagnostic testing

Prenatal genetic counselor involvement prior to conception
• History of infertility, miscarriages or stillbirths
• Couples older than 35 years
• Previous child with chromosome abnormality or other genetic disorder
• Previous child with birth defect (neural tube defect) or multiple congenital anomalies
• Specific ethnicity that may have a higher incidence of certain disorders

Question 17

What are preimplantation genetics?

Answer 17

• Preimplantationgeneticsinvolvesusingartificialreproductive technology (ART).
• This involves fertilizing the egg with sperm by in vitro fertilization and testing the embryo for a specific condition before it is implanted into the mother

Question 18

What is genetic counseling?

Answer 18

Non-directiveCounseling
• Geneticcounselingistheprocessofhelpingpeopleunderstandand adapt to the medical, psychological and familial implications of genetic contributions to disease.

Question 19

What is integrated into genetic counseling?

Answer 19

Thisprocessintegratesthefollowing:

Discuss the cause of the family’s disease
Interpretation of family and medical histories to assess the chance of disease
occurrence or recurrence (in offspring)
Education about inheritance, testing, management, prevention, resources and
research.
Counseling to promote informed choices and adaptations to the risk or condition
Support resources
Help treat psychosocial issues:

Guilt, fear, shame, grief, search for meaning

Question 20

What criteria should be considered for undertaking neonatal screening?

Answer 20

Criteria for Undertaking Neonatal Screening
– The disorder produces irreversible damage if untreated early in life
– there is a treatment available for the disorder
– early intervention is effective
– a reliable lab test for detection
(A positive result should be confirmed immediately by retesting)

Question 21

How is newborn screening done?

Answer 21

• Traditional Testing: based on signs, symptoms, or family history •NBS: done on all to identify those children to treat
• Preclinical: before symptoms, disease or death

Question 22

Describe phenylketonuria unique in screening

Answer 22

Newborn screening originally by a bacterial inhibition test that detects elevated levels of phenylalanine in circulation

• Today, positive newborn screening test has to be immediately confirmed by elevated Phe levels in circulation by a more specific assay (Quantitative mass spectrometry)
• Management involves the lifelong restriction of dietary Phe, that results in a marked improvement and normal mental development in the child

Question 23

How can PKU newborn screening be done by Tendem MS (MS/MS) ?

Answer 23

• Molecules (amino acids) are ionized & injected into mass analyzer of MS/MS
• Selected ions are fragmented
• Fragments separated by second mass analyzer & amino acids are detected based on their signature fragment pattern (see control from previous slide)

Question 24

Describe the detection and treatment of the sickle cell disease

Answer 24

Point mutation in β-globin resulting in anemia
Detection
• Hemoglobin electrophoresis • DNA test:
-Southern blot, PCR-RFLP, ASO test Treatment
•Prevention of sickling crisis, blood transfusions, use of the drug hydroxyurea

Question 25

What is thalassemia (a and B)?

Answer 25

• Both show autosomal recessive inheritance

• Both can be detected by mean corpuscular hemoglobin and
hemoglobin electrophoresis

• Screening of young adults for the β-thalassemia carrier status has markedly reduced the birth incidence of β-thalassemia in Cyprus, Greece

Question 26

Explain the detection and management of cystic fibrosis

Answer 26

Mutation in the CF transmembrane conductance regulator (CFTR)
FYI- Incidence: 1 in 3,000 Caucasians or Ashkenazi Jewish, 1 in 8,000 Hispanics, 1 in 15,000 African Americans, 1 in 32,000 Asians.

Detection
• Immuno-reactive trypsinogen (IRT) in blood followed by a DNA test • DNA looking for ~32 of the most common mutations
• Sweat chloride test (confirmatory test)

Management
• Antibiotics to combat respiratory infections, gene therapy, management of associated malabsorption

Question 27

Describe the SCID(Severe Combined Immunodeficiency)

Answer 27

SCID is the result of genetic defects which impair the normal development of T-cells
(x-linked recessive and autosomal recessive)
SCID infants are phenotypically normal but acquire life-threatening infections within a few months of life

Detection
•Healthy T lymphocyte development will show presence of the by-product T cell receptor excision circles (TRECs)

•TRECs are normally made in large numbers, while barely detectable in infants with
SCID

•TRECs are quantitated by real-time quantitative polymerase chain reaction (RT-qPCR) on DNA extracted
from routine NBS cards

Management
• bone marrow transplant

Question 28

Explain the story of NBS & the genetic counselor

Answer 28

• “My first patient is a 4-week-old girl coming in due to a positive newborn screen for cystic fibrosis (CF). Before coming to see me this afternoon, she will have a sweat test.”
• She has no medical history for respiratory or gastrointestinal problems. Upon her arrival I learn that she has had a negative sweat test and may be a carrier of cystic fibrosis.
• This is the first thing I explain to the parents, adding that being a carrier does not have any medical implications for their daughter. They look very relieved.
• During the remainder of the session, we talk about how CF is inherited in an autosomal recessive fashion, and most likely that one of the parents is a CF carrier.
• In addition, when their daughter is planning her family, she may have a similar consultation with genetics.
• Finally, as it is possible that both parents could be carriers but only one passed on the mutant CF gene, I suggest that both parents be tested for carrier status. The parents opt to come back to have their carrier status tested and an appointment is made for next week.

Question 29

What is the impact of carrier screening for parents with affected child?

Answer 29

• Most couples learn that they are carriers only after the birth of an affected child
• Carrier screening offers the opportunity to identify couples at risk of having a child with a genetic disease

• Screening for heterozygotes biochemical abnormalities
– Specific molecular diagnostic tests (ASO array tests) or mutation
analysis
– Biochemical testing on tissue detects 50%, no need to know specific mutation

Question 30

What are the challenges of carrier screening program?

Answer 30

• Allelic heterogeneity

• For cystic fibrosis, the DNA mutation panel can identify 25 of the most
common mutations in the US population

• A person who has been identified as a “non-carrier” – has the residual risk of being a carrier of an infrequent mutant allele, that is not included in the panel

Question 31

What are the options for carrier screening?

Answer 31

• Options for a couple in which both partners are carriers

– Choosing not to have biological children

– Artificial insemination or egg donation

– Prenatal diagnosis and termination of an affected pregnancy

– Prenatal diagnosis and planning for the care of an affected child – No prenatal testing

Question 32

What are the ethical and legal issues in Medical Genetics “GENETHICS”?

Answer 32

• The most common issues involve:
– Autonomy: client has the right to make own decisions
• Complications include patient age or economic status
• What to do when asked “what would you do?” Avoid answering!
– Informed consent: knowledge of all possible consequences
– The right to know or not know: Late onset diseases?
– Coercion: is the client being persuaded by family or friends?
– Confidentiality: no release of data to third parties including relatives • Insurance?
– Reproductive decision making: DMD is tested but not Wilson’s?? • Sex selection???
– Testing of minors or mentally challenged persons: controversial, • Will the patient benefit from the test?
• Is there a treatment or intervention?

Question 33

What is the significance of the SHH gene?

Answer 33

The SHH gene encodes a protein that functions extracellularly
to control a signaling pathway. Since SHH is expressed from
specifically located cells, a concentration gradient of SHH
protein is formed, with the highest concentration near the cells
that secretes the protein. This concentration gradient is then
interpreted via the pathway to direct tissue differentiation.
• At the end of the SHH pathway, gene expression is indeed regulated via the two transcription factors (DNA binding proteins) CREBP and GLI

Question 34

What is the significance of SHH gene in achondroplasia?

Answer 34

A receptor tyrosine kinase is not found in the SHH pathway; achondroplasia (a different disorder), is caused by a dominant gain of function variant in the FGFR3 gene.

• Cholesterol is a lipid, SHH is a gene, SHH is a protein. To become activated, a portion of the SHH protein is first cleaved off, then what is left binds to cholesterol. For this reason, an enzyme deficiency disorder that leads to reduced cholesterol synthesis has a similar presentation as reduced SHH activity
• SHH is extracellular, and it does not bind to actin.

Question 35

What are HOX genes?

Answer 35

The HOX genes and many other genes involved in regulation of gene expression contain the homeodomain which is a highly conserved set of amino acids that can bind to DNA

Question 36

What is the significance of HOX genes?

Answer 36

• The homeodomain can bind to the DNA
• Then the bound protein can exert its regulatory effects on the DNA via
transcriptional regulation
• Some genes experience increased expression
• Some genes might be repressed
• The end result is appropriate cell determination and differentiation
• Appropriate tissue structures are formed, and the fetus develops