Transcript Genetic screening of diseases (It`s worth & potential)

Genetic Screening of
Diseases
(It’s worth & potential)
Dr Derakhshandeh
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First trimester screening for
Down syndrome and trisomy 18
• The availability and acceptability of early
invasive diagnostic methods (eg, chorionic
villus sampling)
• The continued need for second trimester
screening for open fetal neural tube
defects
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• Women with singleton pregnancies
underwent: first-trimester combined
screening:
–measurement of nuchal translucency
–pregnancy-associated plasma protein
A [PAPP-A]
– The free beta subunit of human
chorionic gonadotropin (HCG) at 10
weeks 3 days through 13 weeks 6
days of gestation
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Nuchal translucency screening involves the
measurement by ultrasound of the skin
thickness at the back of the neck of a first
trimester fetus
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Nuchal translucency
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Identification about 85-90% of
affected fetuses in the first-trimester
• maternal age was combined with fetal
NT
• and maternal serum biochemistry
(free β-hCG and pregnancyassociated plasma protein (PAPP-A))
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Second-trimester
quadruple screening
–measurement of:
• alpha-fetoprotein
• total human chorionic
gonadotropin
• unconjugated estriol
• inhibin A at 15 through 18 weeks
of gestation
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Maternal blood sampling for fetal
blood cells
• This is a new technique
• use of the phenomenon of fetal blood cells
gaining access to maternal circulation
through the placental villi
• only a very small number of fetal cells
enter the maternal circulation in this
fashion
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Maternal serum alphafetoprotein (MSAFP)
• fetus has two major blood proteins:
• albumin and alpha-fetoprotein (AFP)
• Since adults typically have only albumin in
their blood
• the MSAFP test can be utilized to
determine the levels of AFP from the fetus
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• the gestational age must be known with
certainty
• the amount of MSAFP increasses with
gestational age
• the MSAFP can be elevated for a
variety of reasons
• which are not related to fetal neural
tube or abdominal wall defects, so this
test is not 100% specific
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Neural tube defect
• a neural tube defect in the fetus:
• from failure of part of the
embryologic neural tube to close
• there is a means for escape of
more AFP into the amniotic fluid
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Neural tube defect
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Prenatal screening and
diagnosis of neural tube defects
• Neural tube defects (NTD): second most
prevalent congenital anomaly in the United
States
• Two factors have played a significant role in
the prevention of this disorder in developed
countries:
– Sonographic imaging combined with
amniocentesis for diagnosis of affected
fetuses
– folic acid supplements for prevention of the14
Anencephaly (failure of closure at
the cranial end of the neural tube)
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Spina bifida (failure of closure at the
caudal end of the neural tube)
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Environmental factors
• The frequency of NTDs is increased with
exposure to certain environmental factors:
– drugs (valproic acid, carbamazepine, Folic acid
deficiency)
– diabetes mellitus
– Obesity
• Adequate folate is critical for cell division due to
its essential role in the synthesis of:
– nucleic
– certain amino acids
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Genetic factors
the observations that NTDs have a high rate:
– in monozygotic twins
– more frequent among first degree relatives
– more common in females than males
• The risk of recurrence for NTDs: approximately 2
to 4 percent when there is one affected sibling
• With two affected siblings, the risk is
approximately: 10 percent
• The risk of NTD according to family history:
– to be higher in countries such as Ireland where
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the prevalence if NTDs is high
NOTE:
• The genetic polymorphisms :
• mutations in the methylene tetrahydrofolate
reductase gene
• may increase the risk for NTDs
• Folate is a cofactor for this enzyme
• which is part of the pathway of homocysteine
metabolism in cells
• The C677T and the A1298C mutations are
associated with elevated maternal
homocysteine concentrations and an increased
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risk for NTDs in fetuses
Prevention of neural tube defects
• can be accomplished by supplementation
of the maternal diet with only 4 mg of folic
acid per day
• but this vitamin supplement must be taken
a month before conception and through
the first trimester
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Maternal serum beta-HCG
• This test is most commonly used as a test for
pregnancy
• Later in pregnancy: in the middle to late second
trimester
• the beta-HCG can be used in conjunction with
the MSAFP to screen for chromosomal
abnormalities, and Down syndrome in particular
• An elevated beta-HCG coupled with a
decreased MSAFP suggests Down syndrome
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Maternal serum estriol
• made by the fetal adrenal glands
• Estriol tends to be lower when Down
syndrome is present
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Inhibin-A
• An increased level of inhibin-A is
associated with an increased risk for
trisomy 21
• A high inhibin-A may be associated
with a risk for preterm delivery
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The facial features of Down
syndrome
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overlapping are typical for
trisomy 18
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BRCA1 AND BRCA2 GENES
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BRCA1 AND BRCA2 GENES
• Breast cancer develops in about12 percent of
women who live to age 90
• a positive family history is reported by 15 to 20
percent of women with breast cancer
• Just only 5 to 6 percent of all breast cancers
are associated with an inherited gene mutation
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• Two major susceptibility genes for
breast cancer, BRCA1 and BRCA2
• Testing for mutations in these
genes, is available
• Clinicians and patients must decide
when it is appropriate to screen for
their presence
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The BRCA1 and BRCA2 genes
function
• As an essential part of the normal mechanisms
that repair double-strand DNA breaks (ionizing
radiation and DNA cross linking agents such as
cisplatin)
• through recombination with undamaged,
homologous DNA strands
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Cisplatin
• Cisplatin is a platinum-based
chemotherapy drug
• used to treat various types of cancers,
such as sarcomas, some carcinomas,
lymphomas and germ cell tumors
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Cisplatin
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Cisplatin
• works by crosslinking across DNA interstrands
• making it impossible for rapidly dividing
cells to duplicate their DNA for cell division
(mitosis)
• The damaged DNA sets off DNA repair
mechanisms which fails to work
• so in turn activate cell death processes
(apoptosis)
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BRCA mutations
• The reason why BRCA mutations
predispose mainly to breast and ovarian
cancers is unclear
• intact BRCA1 represents a barrier to
transcriptional activation of the estrogen
receptor
• that functional inactivation could lead to
altered hormonal regulation of mammary
and ovarian epithelial proliferation
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BRCA1 and BRCA2 gene
abnormalities
• Cancer risk with a high penetrance
• women who have inherited mutations
• the lifetime risk of breast cancer is
between 65 and 85 percent by age 70
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Ovarian cancer
• Ovarian cancer is also linked to the
presence of BRCA mutations
• the lifetime risk of ovarian cancer:
– between 45 and 50 percent in women who
have a deleterious BRCA1 mutation
– and 15 to 25 percent for those with a BRCA2
mutation
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BRCA1 and BRCA2-associated
cancers
•
•
•
•
prostate cancer
male breast cancer
pancreatic cancer
Although the risk of male breast cancer
and pancreatic cancer may be under 10
percent
• the risk of prostate cancer in BRCA2
carriers may be as high as 35 to 40
percent
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BRCA 1
• The gene Locus for BRCA1:
17q21
• a large gene
• 24 exons
• encoding a 220 kD
• 1863 amino acids
• Two recognizable motifs
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BRCA2
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•
•
•
•
BRCA2 (13q12.3)
was identified by Wooster et al. in 1995
It encodes for 384 kD nuclear protein
3418 amino acids
BRCA2 bears no homology to any
known tumour supressor genes
• contains 27 exons
• spread over 70 kb of genomic DNA 39
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Breast Cancer Families
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Significance of family history
• Degree of relatedness to affected relatives
• Number of affected relatives
• The age of the relative(s) when breast
cancer occurred
• Laterality of the disease in affected
relatives
• Whether there is a family history of ovarian
cancer
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Pedigree of Breast cancer Family
With BRCA1 mutation
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SSCP Analysis
Exon 11pi BRCA1 MS R1347G
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Mutations in BRCA1/2 gene
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Screening for colorectal
cancer
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Screening for colorectal cancer
• Colorectal cancer (CRC) is
– a common
– Lethal
– preventable disease
• It is infrequent before age 40
• the incidence rises progressively to
3.7/1000 per year by age 80
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Clinical detection of increased risk
• Before deciding how to screen:
• clinicians should decide whether the individual patient is at
average or increased risk
• based on his or her medical and family history
• A few simple questions are all that is necessary:
• Have you ever had colorectal cancer
• or an adenomatous polyp
• Have you had inflammatory bowel disease (Crohn disease)
• Has a family member had colorectal cancer or an
adenomatous polyp
• If so, how many
• was it a first-degree relative (parent, sibling, or child)
• and at what age was the cancer or polyp first diagnosed
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Crohn’s disease
• an inflammatory bowel disease
• causes inflammation of the
gastrointestinal tract in both men
and women
• persistent diarrhea, abdominal
pain, fever, and at times rectal
bleeding
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Crohn’s disease
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screening for Colorectal cancer
(CRC)
• Patients at highest risk with familial
syndromes (HNPCC, FAP)
• should be screened for CRC with
colonoscopy at frequent specified
intervals
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People at high risk
• a first-degree relative with colon cancer
• or adenomatous polyp diagnosed at age <60
years
• or two first-degree relatives diagnosed at any
age
• should be advised to have screening
colonoscopy starting at age 40 years
• or 10 years younger than the earliest diagnosis
in their family
• whichever comes first, and repeated every five
years
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Adenomatous polyp
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Microsatellite Instability in Adenomas as
a Marker for Hereditary Nonpolyposis
Colorectal Cancer
•Hereditary nonpolyposis colorectal cancer
(HNPCC)
•the most common of the well-defined
colorectal cancer syndromes
•accounting for at least 2% of the total
colorectal cancer
•carrying a greater than 80% lifetime risk of
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cancer
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Microsatellite instability
(MSI)
• can be detected in approximately 90%
of tumors from individuals with
Hereditary Non-Polyposis Colorectal
Cancer (HNPCC)
• MSI is also reported in approximately
15% of sporadic colorectal
carcinomas
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Mutations in the human mismatch
repair genes (MLH1, MSH2, PMS1, PMS2, MSH6)
• responsible for the MSI of the HNPCC
tumors
• In contrast to the classical tumor
suppressor pathway
• the mismatch-repair gene tumor pathway
accumulates mutations in genes involved
in tumorigenesis
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Reduction in cancer morbidity
and mortality of HNPCC patients
• can be accomplished by appropriate
clinical cancer screening of HNPCC
patients with mutations in mismatch
repair (MMR) genes
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Germline mutation analysis
• In individuals with cancer
• mutation detection can be accomplished
relatively efficiently by germline mutation
analysis of individuals (blood) whose
cancers show microsatellite instability
(MSI)
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• Among 378 adenoma patients
• six (1.6%) had at least one MSI adenoma
• Five out of the six patients (83%) had a
germline MMR gene mutation
• MSI analysis is a useful method of
prescreening colorectal adenoma patients for
HNPCC
Microsatellite Instability in Adenomas as a Marker for Hereditary
Nonpolyposis Colorectal Cancer
Anu Loukola et al. American Journal of Pathology. 1999;155:1849-1853
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Somatic mutations
• Cells deficient for both alleles of a mismatch
repair gene, leading to somatic mutations
• which can be demonstrated by analyzing
microsatellite sequences in the tumor DNA
• These sequences display frequent somatic
deletions and insertions, often referred to as
microsatellite instability (MSI).
• HNPCC patients form adenomas at a slightly
but not strikingly increased rate as compared
with the general population
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The frequency of the MSI
• The frequency of the MSI is 80 to 95% in
HNPCC cancers
• 10 to 15% in sporadic colorectal cancers
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Factor V Leiden Assay on
LightCycler
• The presence of the factor V mutation can
cause an increased risk of venous
thrombosis
• Individuals heterozygous for the Factor V
Leiden (FacV) mutation :
– carry an eight-fold greater risk for thrombosis
– while homozygosity confers an estimated
ninety-fold increased risk
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SMA Carrier Testing
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Screening for cardiovascular
risk with C-reactive protein
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Screening for cardiovascular
risk with C-reactive protein
• The most extensively studied biomarker of
inflammation in cardiovascular diseases
• serum C-reactive protein (CRP)
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• Elevated serum CRP was a strong
independent risk factor for cardiovascular
disease
• added to the predictive value of other
factors, such as serum total cholesterol
• Elevated serum CRP was a stronger
predictor of cardiovascular events than
LDL cholesterol (LDL-C) At eight years
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Screening for coronary heart
disease
• Although mortality from coronary heart
disease (CHD) has fallen substantially
over the past three decades
• it remains the leading cause of death in
adults
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High-risk patients
• risk factors for CHD, including:
– Hypertension
– Hypercholesterolemia
– Smoking
– a family history of premature CHD
– and diabetes mellitus
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The Association Between
Apolipoprotein E
Polymorphism and
Cardiovascular Risk Factors
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Association Between Apolipoprotein E
Polymorphism and Cardiovascular Risk
Factors
• the allelic and genotypic frequencies
related to apolipoprotein E (ApoE)
polymorphism
• association of the genotypes with risk
factors
• and cardiovascular morbidity in
population
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• the gene amplification technique through
the polymerase chain reaction-restriction
fragment length polymorphism
• (PCR-RFLP) and cleavage with the
restriction
• enzyme Hha I to identify the ApoE
genotypes The most frequent
• Individuals with the E3E4 had a mean age
greater than those with the E3E3
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Cystic fibrosis: Prenatal genetic
screening
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Cystic fibrosis: Prenatal genetic
screening
• Cystic fibrosis (CF) is a chronic pulmonary
• and exocrine pancreatic disease
• the most common monogenic disorder in
Caucasians of Northern European
• classic form it is marked by abnormal sweat
chloride levels
• chronic pulmonary disease
• pancreatic insufficiency
• liver disease
• and obstructive azoospermia in males
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Cystic fibrosis
• an autosomal recessive disease
• with a carrier rate of 1 in 22 to 25 in
Caucasian Americans of Northern
European background
• the most common mutation in this group is
called F508 and accounts for 75 percent of
all CF cases
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The effect of ethnicity on CF
screening
• The key to all CF screening is knowing
which mutations to test for
• knowledge of the most common CF
mutations in individuals of different
heritages
• Native Americans are at increased risk of
CF (carrier frequency 1 in 31
• Jewish heritage (carrier rates ranging from
1 in 24 to 1 in 29 if the patient's ancestors
originated in Greece, Bulgaria, or Libya)
• to 1 in 90 if the ancestors originated in Iran
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or Iraq)
PHENOTYPE of CF
• The diagnosis and treatment is complicated:
• not everyone who is homozygous for the CF
mutation has the classic form of the disease
• Some individuals have an atypical presentation:
– pulmonary disease associated with
pancreatic sufficiency
• have only isolated features of the disease:
– pancreatitis
– liver disease
– nasal polyps
– congenital bilateral absence of the vas
deferens (CBAVD)
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Vas deferens (CBAVD)
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Prenatal testing for the
hemoglobinopathies and thalassemias
• hemoglobinopathies as two general types:
– the thalassemias
– decreased globin chain production
• hemoglobin variants (eg, sickle cell anemia and
its variants, hemoglobin C disease)
• chronic, debilitating, and often fatal
• new therapies:
– including hydroxyurea (XMNI, g
mRNA>active)
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– hematopoietic cell transplantation
Identification of candidates for
prenatal testing
Family history of a relative with a
hemoglobinopathy or thalassemia
• couples from extended families living in
endemic areas
• consanguineous marriages are common,
may be at highest risk
• Ethnic background:
– at low risk for hemoglobinopathies are
northern Europeans
– Japanese
– Native Americans
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Red blood cell indices
• mean corpuscular volume (MCV) <80
femtoliters (fL), MCH< 28
• in the absence of iron deficiency (alpha or beta
thalassemia)
• Hemoglobin electrophoresis:
• this test will identify carriers:
– hemoglobin variants
– beta thalassemia
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