Medical Genetics
18 疾病的 产前 诊断
Prenatal Diagnosis of Disease
Medical Genetics
Genetic diseases and congenital
malformations occur in
approximately 2 to 5 % of all live
births,account for up to 30% of
paediatric admissions to hospital,
and are an important cause of death
under the age of 15 years,
Medical Genetics
Furthermore,the psychological
stress on families with children with
serious genetic disorders is
incalculable,Until gene therapy
becomes a practical reality,the only
option available for the control of
genetic disease is prenatal diagnosis,
Medical Genetics
1,Indications for prenatal diagnosis
The use of prenatal diagnosis is
determined by balancing the risk of
the birth of an abnormal child
against the risk of an investigative
procedure,
Medical Genetics
The general indications of
prenatal diagnosis include maternal
age and the results of noninvasive
serum biochemical screening,
Medical Genetics
Specific indications include a
positive family history and the birth
of a previous child affected by a
particular genetic disease,
Medical Genetics
2,Methods for obtaining fetal tissues for prenatal diagnosis
To perform prenatal diagnosis,
fetal-derived tissues must first be
obtained,All of the commonly used
methods that yield fetal tissues are
invasive,
Medical Genetics
A,Amniocentesis
Amniocentesis is the
withdrawal of amniotic fluid from
the amniotic sac surrounding the
fetus,For over two decades this has
been the primary technique utilised
for the diagnosis of fetal genetic
disorders,
Medical Genetics
Traditionally amniocentesis has
been performed around 15 to 16
weeks gestation,At this time the
uterus is easily accessible to a
transabdominal approach,and a
sufficient volume of amniotic fluid
(approximately 200 ml) exists to
permit 20 to 30 ml to be withdrawn
safely,
Medical Genetics
Amniocentesis is normally
performed as an outpatient facility,
An ultrasound examination is
normally done immediately before
the procedure to evaluate fetal
number and viability,perform fetal
biometric measurements,establish
placental location,and estimate
amniotic fluid volume,
Medical Genetics
Amniocentesis performed concurrently with
ultrasound scanning,
Medical Genetics
Safety of amniocentesis
Any procedure that involves
passing a device into an organ,
especially the pregnant uterus,
carries with it risks; amniocentesis is
no exception,
Medical Genetics
Amniocentesis involves potential
danger to both mother and fetus,
Serious maternal risks are quite low
but include amnionitis which can lead
to fetal loss,haemorrhage or injury
to an intra-abdominal viscus and
leakage of amniotic fluid,
Medical Genetics
Fetal risks include spontaneous
abortion,needle puncture injuries,
placental abruption,chorioamnionitis,
preterm labour,and amniotic band
formation,
Medical Genetics
Several controlled studies have
been done to evaluate the risks of
amniocentesis,The data indicate that
the risk of pregnancy loss
attributable to amniocentesis may be
as high as 0.5%,
Medical Genetics
In general,risks will depend on
(1) the experience of the obstetrician
performing the procedure;
(2) clinical characteristics of a
particular case (e.g.,presence or
absence of biochemical markers of
fetal abnormality);
(3) the quality of the high-resolution
ultrasound utilised,
Medical Genetics
Early amniocentesis
With development of higher
resolution ultrasound equipment,
some centres have begun offering
amniocentesis before 15 weeks
gestation,usually between 10 and
14 weeks,The majority of
procedures have been performed
during the 13th and 14th weeks of
gestation,
Medical Genetics
There is evidence that early
amniocentesis is associated with a
higher fetal loss rate and a more
frequent occurrence of certain
congenital abnormalities,
Medical Genetics
B,Chorionic villus sampling
Chorionic villus sampling (CVS)
is the only tested method for first-
trimester fetal genetic diagnosis that
is currently in clinical use and is
usually performed between 9 and 11
weeks,
Medical Genetics
The procedure involves the
passing of a sampling instrument
into the chorion (developing
placenta),A good procedure yields
from 10 to 25 mg of tissue which is
adequate for cytogenetic,enzymatic
or DNA analysis,
Medical Genetics
The main advantage of CVS over
amniocentesis is the applicability of
CVS earlier in gestation,This results
in considerably reduced social,
emotional and psychological stress
for the couple,
Medical Genetics
Safety of CVS
Maternal complications include
bleeding and infection,Fetal loss
following CVS has been reported to
be around 2%,There are also
reports of limb reduction defects in
infants born to mothers who have
had CVS between 56 and 66 days of
gestation,
Medical Genetics
C,Fetal blood sampling (FBS)
Fetal blood can be safely and
directly sampled from approximately
18 weeks gestation onwards,FBS
can be used for both diagnostic and
therapeutic purposes,
Medical Genetics
Indications for fetal blood sampling
Diagnostic
Rapid karyotyping Fetal anomaly on ultrasound
Late attending patients who require fetal karyotyping
Alloimmunisation Rhesus
Platelet antigens
Fetal infection Toxoplasmosis
Cytomegalovirus infection
Genetic Haemoglobinopathies
Metabolic disorders and enzyme deficiencies
Fetal well being Severe intrauterine growth retardation
Therapeutic
Transfusion Red cell alloimmunisation
Transplantation Stem cells
Medical Genetics
FBS is contraindicated if the
mother is suffering from infections
that can be transmitted to the fetus
by the procedure,Examples include
human immunodeficiency virus and
hepatitis B virus infection,
Medical Genetics
Safety of FBS
Maternal complications from FBS
are uncommon but include
amnionitis,infection,rhesus
sensitisation and transplacental
haemorrhage,
Medical Genetics
Fetal loss rates following FBS
have been reported to be
approximately 1% in several large
series,The presence of structural
abnormalities or severe growth
retardation of the fetus is associated
with a much increased fetal loss rate,
Medical Genetics
Other fetal complications include
infection,premature rupture of
membranes,haemorrhage,severe
bradycardia and umbilical cord
thrombosis,
Medical Genetics
D,Fetal biopsy
Although advances in molecular and
biochemical genetics have made the
diagnosis of many Mendelian disorders
possible by analysis of amniotic fluid cells
or chorionic villi,some conditions still
require direct analysis of tissues in which
the disorder is manifested,Tissues which
have been successfully biopsied include
fetal skin,liver and muscle,
Medical Genetics
Safety of fetal biopsy
Due to the relatively small
numbers performed in different
centres,no precise figures for the
safety of fetal biopsy is available,
Medical Genetics
3,Analytical methods
Following the acquisition of fetal
tissues,these materials are then
subjected to analysis using a variety
of techniques,
Medical Genetics
A,Cell culture and conventional
cytogenetics
These are the most commonly
used methods for the diagnosis of
chromosomal aneuploidies such as
Down syndrome,
Medical Genetics
B,Molecular cytogenetics using
FISH
FISH involves the hybridization of
DNA probes representing a specific
chromosome or chromosomal region
to target DNA such as metaphase
chromosomes or interphase nuclei,
where the probe binds to
homologous sequences in the cell,
Medical Genetics
Using FISH,several groups have
demonstrated that trisomies such as
trisomy 21 and trisomy 18 can be
detected in uncultured interphase
nuclei as three positive hybridisation
signals rather than the normal two,
Medical Genetics
The main advantage is speed,
thus results are available in 24 to 48
hours compared with the 10 to 14
days more typical of standard
culture-based cytogenetic analysis,
This type of technology can be
applied to fetal materials obtained
following amniocentesis,CVS or fetal
blood sampling,
Medical Genetics
C,DNA-based techniques
The main advantage of DNA-
based techniques is that any
nucleated fetal cell can be used,
Techniques which are used include
the polymerase chain reaction (PCR)
and Southern blotting,
Medical Genetics
PCR-based techniques allow a
rapid diagnosis to be made in several
hours,These methods can be used
for direct mutation detection or
linkage analysis,The latter type of
analysis is needed when the exact
mutation or gene causing the disease
is not known,
Medical Genetics
Medical Genetics
Genetic diagnosis is then carried
out by analysing DNA sequences
within the gene itself or DNA loci
closely linked to it,An analysable
difference or polymorphism must
exist between the disease-carrying
allele and the normal allele to
distinguish them,
Medical Genetics
Medical Genetics
Medical Genetics
D,Metabolic analysis of fetal
tissues
Fetal tissues or fluids can be
subjected to analysis to detect the
characteristic metabolic or cellular
defects of an inherited metabolic
disease,
Medical Genetics
For this type of analysis to be
carried out,the specific enzyme or
metabolite of interest must be
expressed in the fetal tissues
sampled,and the range of normal
values as well as the assay
sensitivity and reproducibility must
be established within the tissue of
interest,
Medical Genetics
Although an increasing number of
inherited metabolic diseases are
amenable to direct DNA-based
diagnosis,enzyme-based techniques
are still useful in situations where the
disease-causing gene has not been
identified or where the precise
mutation is not known,
Medical Genetics
E,Microarray Analysis
Much of the excitement today
centers on gene expression profiling
that uses a technology called
microarrays,
Medical Genetics
A DNA microarray is a thin-sized
chip that has been spotted at fixed
locations with thousands of single-
stranded DNA fragments
corresponding to various genes of
interest,
Medical Genetics
A single microarray may contain
10,000 or more spots,each containing
pieces of DNA from a different gene,
Fluorescent-labeled probe DNA fragments
are added to ask if there are any places
on the microarray where the probe
strands can match and bind,Complete
patterns of gene activity can be captured
with this technology,
Medical Genetics
Medical Genetics
4,New methods for prenatal diagnosis
A,Preimplantation diagnosis
Preimplantation diagnosis is the
performance of prenatal genetic
analysis on embryos or oocytes prior
to implantation,
Medical Genetics
This technology has the
advantage that it allows prenatal
diagnosis to be carried out much
earlier than existing methods such as
amniocentesis and CVS,
Medical Genetics
Furthermore,couples who are at
exceptionally high genetic risk and
those who have had previous
terminations for genetic indications
may find preimplantation diagnosis a
more acceptable form of prenatal
testing,
Medical Genetics
Medical Genetics
In the future,preimplantation diagnosis
may also be used in conjunction with gene
therapy,At present,given the expense of
the procedure and the small number of
centres equipped to perform this form of
diagnosis,preimplantation diagnosis is
unlikely to become a standard procedure
in the foreseeable future,
Medical Genetics
Access to oocyte and embryonic
cells
Preimplantation genetic analysis
can be carried on either embryonic
cells or oocytes,In the later situation,
diagnosis is carried out even prior to
fertilisation,Individual oocytes are
aspirated and their polar body
biopsied,
Medical Genetics
For preimplantation diagnosis
carried out on embryonic cells,the
embryo may be fertilised in vitro and
then individual blastomeres biopsied,
Alternatively,the embryo may be
fertilised in vivo and then the
embryos are obtained by uterine
lavage followed by biopsy and
genetic analysis,
Medical Genetics
For heterozygous women
carrying one mutant and one normal
allele of a disease-causing gene,in
the absence of chromosomal
crossing-over,the aspirated polar
body containing a mutant allele
would indicate that the primary
oocyte pronucleus is carrying the
normal allele,
Medical Genetics
In both situations,only the
embryos confirmed not to possess
the full disease-causing genotype are
then implanted back into the uterus,
Medical Genetics
Medical Genetics
Diagnostic methods
Preimplantation diagnosis may be
achieved using PCR,FISH or by
measurement of embryonic secretory
products such as certain enzymes,
Medical Genetics
This type of analysis has been carried
out successfully for the determination of
fetal sex for the avoidance of sex-linked
disorders such as Duchenne muscular
dystrophy and haemophilia A,and for the
diagnosis of single gene disorders such as
cystic fibrosis,alpha-1-antitrypsin
deficiency,Tay-Sach's disease,fragile X
and sickle cell anaemia,
Medical Genetics
Worldwide,preimplantation
diagnosis of embryos has been
attempted on over 1,200 in vitro
fertilisation cycles in 1997,with
clinical pregnancy resulted in 20%,
No increase in the occurrence of
abnormalities has been observed in
the liveborns,
Medical Genetics
B,Noninvasive prenatal diagnosis
using fetal cells isolated from
maternal blood
Circumstantial evidence that fetal
nucleated cells exist in maternal
peripheral blood can be traced back
to 1969,
Medical Genetics
However,convincing proof of the
existence of these cells have to await
the development of molecular
biological techniques,especially the
PCR,
Medical Genetics
Using the PCR,investigators are
able to demonstrate the presence of
cells possessing fetal genetic
markers circulating in the peripheral
blood of pregnant women,The
isolation of these cells offer the
possibility of a noninvasive and safe
method for prenatal diagnosis,
Medical Genetics
Fetal nucleated cell types in
maternal blood
Three populations of fetal
nucleated cells are currently known
to be present in maternal peripheral
blood,fetal lymphocytes,
trophoblasts and fetal nucleated red
cells,
Medical Genetics
At present,the isolation of fetal
nucleated red cells is receiving the
most attention from investigators in
the field due to the availability of
relatively specific monoclonal
antibodies against these cells,
Medical Genetics
Isolation of fetal cells and genetic
analysis
A combination of physical and
immunological methods are used to isolate
fetal nucleated cells from maternal blood,
Physical methods include density gradient
centrifugation and micromanipulation
techniques while immunological methods
include the use of monoclonal antibodies,
Medical Genetics
Genetic analysis of these isolated fetal
cells can be performed using PCR or FISH,
Fetal cells in maternal blood have been
successfully used on a research level to
diagnose trisomy 21,trisomy 18,beta-
thalassaemia and sickle cell anaemia,
Actual clinical use will have to await
further technological development to
improve its reliability and clinical trials to
assess the sensitivity and specificity of
this approach,
18 疾病的 产前 诊断
Prenatal Diagnosis of Disease
Medical Genetics
Genetic diseases and congenital
malformations occur in
approximately 2 to 5 % of all live
births,account for up to 30% of
paediatric admissions to hospital,
and are an important cause of death
under the age of 15 years,
Medical Genetics
Furthermore,the psychological
stress on families with children with
serious genetic disorders is
incalculable,Until gene therapy
becomes a practical reality,the only
option available for the control of
genetic disease is prenatal diagnosis,
Medical Genetics
1,Indications for prenatal diagnosis
The use of prenatal diagnosis is
determined by balancing the risk of
the birth of an abnormal child
against the risk of an investigative
procedure,
Medical Genetics
The general indications of
prenatal diagnosis include maternal
age and the results of noninvasive
serum biochemical screening,
Medical Genetics
Specific indications include a
positive family history and the birth
of a previous child affected by a
particular genetic disease,
Medical Genetics
2,Methods for obtaining fetal tissues for prenatal diagnosis
To perform prenatal diagnosis,
fetal-derived tissues must first be
obtained,All of the commonly used
methods that yield fetal tissues are
invasive,
Medical Genetics
A,Amniocentesis
Amniocentesis is the
withdrawal of amniotic fluid from
the amniotic sac surrounding the
fetus,For over two decades this has
been the primary technique utilised
for the diagnosis of fetal genetic
disorders,
Medical Genetics
Traditionally amniocentesis has
been performed around 15 to 16
weeks gestation,At this time the
uterus is easily accessible to a
transabdominal approach,and a
sufficient volume of amniotic fluid
(approximately 200 ml) exists to
permit 20 to 30 ml to be withdrawn
safely,
Medical Genetics
Amniocentesis is normally
performed as an outpatient facility,
An ultrasound examination is
normally done immediately before
the procedure to evaluate fetal
number and viability,perform fetal
biometric measurements,establish
placental location,and estimate
amniotic fluid volume,
Medical Genetics
Amniocentesis performed concurrently with
ultrasound scanning,
Medical Genetics
Safety of amniocentesis
Any procedure that involves
passing a device into an organ,
especially the pregnant uterus,
carries with it risks; amniocentesis is
no exception,
Medical Genetics
Amniocentesis involves potential
danger to both mother and fetus,
Serious maternal risks are quite low
but include amnionitis which can lead
to fetal loss,haemorrhage or injury
to an intra-abdominal viscus and
leakage of amniotic fluid,
Medical Genetics
Fetal risks include spontaneous
abortion,needle puncture injuries,
placental abruption,chorioamnionitis,
preterm labour,and amniotic band
formation,
Medical Genetics
Several controlled studies have
been done to evaluate the risks of
amniocentesis,The data indicate that
the risk of pregnancy loss
attributable to amniocentesis may be
as high as 0.5%,
Medical Genetics
In general,risks will depend on
(1) the experience of the obstetrician
performing the procedure;
(2) clinical characteristics of a
particular case (e.g.,presence or
absence of biochemical markers of
fetal abnormality);
(3) the quality of the high-resolution
ultrasound utilised,
Medical Genetics
Early amniocentesis
With development of higher
resolution ultrasound equipment,
some centres have begun offering
amniocentesis before 15 weeks
gestation,usually between 10 and
14 weeks,The majority of
procedures have been performed
during the 13th and 14th weeks of
gestation,
Medical Genetics
There is evidence that early
amniocentesis is associated with a
higher fetal loss rate and a more
frequent occurrence of certain
congenital abnormalities,
Medical Genetics
B,Chorionic villus sampling
Chorionic villus sampling (CVS)
is the only tested method for first-
trimester fetal genetic diagnosis that
is currently in clinical use and is
usually performed between 9 and 11
weeks,
Medical Genetics
The procedure involves the
passing of a sampling instrument
into the chorion (developing
placenta),A good procedure yields
from 10 to 25 mg of tissue which is
adequate for cytogenetic,enzymatic
or DNA analysis,
Medical Genetics
The main advantage of CVS over
amniocentesis is the applicability of
CVS earlier in gestation,This results
in considerably reduced social,
emotional and psychological stress
for the couple,
Medical Genetics
Safety of CVS
Maternal complications include
bleeding and infection,Fetal loss
following CVS has been reported to
be around 2%,There are also
reports of limb reduction defects in
infants born to mothers who have
had CVS between 56 and 66 days of
gestation,
Medical Genetics
C,Fetal blood sampling (FBS)
Fetal blood can be safely and
directly sampled from approximately
18 weeks gestation onwards,FBS
can be used for both diagnostic and
therapeutic purposes,
Medical Genetics
Indications for fetal blood sampling
Diagnostic
Rapid karyotyping Fetal anomaly on ultrasound
Late attending patients who require fetal karyotyping
Alloimmunisation Rhesus
Platelet antigens
Fetal infection Toxoplasmosis
Cytomegalovirus infection
Genetic Haemoglobinopathies
Metabolic disorders and enzyme deficiencies
Fetal well being Severe intrauterine growth retardation
Therapeutic
Transfusion Red cell alloimmunisation
Transplantation Stem cells
Medical Genetics
FBS is contraindicated if the
mother is suffering from infections
that can be transmitted to the fetus
by the procedure,Examples include
human immunodeficiency virus and
hepatitis B virus infection,
Medical Genetics
Safety of FBS
Maternal complications from FBS
are uncommon but include
amnionitis,infection,rhesus
sensitisation and transplacental
haemorrhage,
Medical Genetics
Fetal loss rates following FBS
have been reported to be
approximately 1% in several large
series,The presence of structural
abnormalities or severe growth
retardation of the fetus is associated
with a much increased fetal loss rate,
Medical Genetics
Other fetal complications include
infection,premature rupture of
membranes,haemorrhage,severe
bradycardia and umbilical cord
thrombosis,
Medical Genetics
D,Fetal biopsy
Although advances in molecular and
biochemical genetics have made the
diagnosis of many Mendelian disorders
possible by analysis of amniotic fluid cells
or chorionic villi,some conditions still
require direct analysis of tissues in which
the disorder is manifested,Tissues which
have been successfully biopsied include
fetal skin,liver and muscle,
Medical Genetics
Safety of fetal biopsy
Due to the relatively small
numbers performed in different
centres,no precise figures for the
safety of fetal biopsy is available,
Medical Genetics
3,Analytical methods
Following the acquisition of fetal
tissues,these materials are then
subjected to analysis using a variety
of techniques,
Medical Genetics
A,Cell culture and conventional
cytogenetics
These are the most commonly
used methods for the diagnosis of
chromosomal aneuploidies such as
Down syndrome,
Medical Genetics
B,Molecular cytogenetics using
FISH
FISH involves the hybridization of
DNA probes representing a specific
chromosome or chromosomal region
to target DNA such as metaphase
chromosomes or interphase nuclei,
where the probe binds to
homologous sequences in the cell,
Medical Genetics
Using FISH,several groups have
demonstrated that trisomies such as
trisomy 21 and trisomy 18 can be
detected in uncultured interphase
nuclei as three positive hybridisation
signals rather than the normal two,
Medical Genetics
The main advantage is speed,
thus results are available in 24 to 48
hours compared with the 10 to 14
days more typical of standard
culture-based cytogenetic analysis,
This type of technology can be
applied to fetal materials obtained
following amniocentesis,CVS or fetal
blood sampling,
Medical Genetics
C,DNA-based techniques
The main advantage of DNA-
based techniques is that any
nucleated fetal cell can be used,
Techniques which are used include
the polymerase chain reaction (PCR)
and Southern blotting,
Medical Genetics
PCR-based techniques allow a
rapid diagnosis to be made in several
hours,These methods can be used
for direct mutation detection or
linkage analysis,The latter type of
analysis is needed when the exact
mutation or gene causing the disease
is not known,
Medical Genetics
Medical Genetics
Genetic diagnosis is then carried
out by analysing DNA sequences
within the gene itself or DNA loci
closely linked to it,An analysable
difference or polymorphism must
exist between the disease-carrying
allele and the normal allele to
distinguish them,
Medical Genetics
Medical Genetics
Medical Genetics
D,Metabolic analysis of fetal
tissues
Fetal tissues or fluids can be
subjected to analysis to detect the
characteristic metabolic or cellular
defects of an inherited metabolic
disease,
Medical Genetics
For this type of analysis to be
carried out,the specific enzyme or
metabolite of interest must be
expressed in the fetal tissues
sampled,and the range of normal
values as well as the assay
sensitivity and reproducibility must
be established within the tissue of
interest,
Medical Genetics
Although an increasing number of
inherited metabolic diseases are
amenable to direct DNA-based
diagnosis,enzyme-based techniques
are still useful in situations where the
disease-causing gene has not been
identified or where the precise
mutation is not known,
Medical Genetics
E,Microarray Analysis
Much of the excitement today
centers on gene expression profiling
that uses a technology called
microarrays,
Medical Genetics
A DNA microarray is a thin-sized
chip that has been spotted at fixed
locations with thousands of single-
stranded DNA fragments
corresponding to various genes of
interest,
Medical Genetics
A single microarray may contain
10,000 or more spots,each containing
pieces of DNA from a different gene,
Fluorescent-labeled probe DNA fragments
are added to ask if there are any places
on the microarray where the probe
strands can match and bind,Complete
patterns of gene activity can be captured
with this technology,
Medical Genetics
Medical Genetics
4,New methods for prenatal diagnosis
A,Preimplantation diagnosis
Preimplantation diagnosis is the
performance of prenatal genetic
analysis on embryos or oocytes prior
to implantation,
Medical Genetics
This technology has the
advantage that it allows prenatal
diagnosis to be carried out much
earlier than existing methods such as
amniocentesis and CVS,
Medical Genetics
Furthermore,couples who are at
exceptionally high genetic risk and
those who have had previous
terminations for genetic indications
may find preimplantation diagnosis a
more acceptable form of prenatal
testing,
Medical Genetics
Medical Genetics
In the future,preimplantation diagnosis
may also be used in conjunction with gene
therapy,At present,given the expense of
the procedure and the small number of
centres equipped to perform this form of
diagnosis,preimplantation diagnosis is
unlikely to become a standard procedure
in the foreseeable future,
Medical Genetics
Access to oocyte and embryonic
cells
Preimplantation genetic analysis
can be carried on either embryonic
cells or oocytes,In the later situation,
diagnosis is carried out even prior to
fertilisation,Individual oocytes are
aspirated and their polar body
biopsied,
Medical Genetics
For preimplantation diagnosis
carried out on embryonic cells,the
embryo may be fertilised in vitro and
then individual blastomeres biopsied,
Alternatively,the embryo may be
fertilised in vivo and then the
embryos are obtained by uterine
lavage followed by biopsy and
genetic analysis,
Medical Genetics
For heterozygous women
carrying one mutant and one normal
allele of a disease-causing gene,in
the absence of chromosomal
crossing-over,the aspirated polar
body containing a mutant allele
would indicate that the primary
oocyte pronucleus is carrying the
normal allele,
Medical Genetics
In both situations,only the
embryos confirmed not to possess
the full disease-causing genotype are
then implanted back into the uterus,
Medical Genetics
Medical Genetics
Diagnostic methods
Preimplantation diagnosis may be
achieved using PCR,FISH or by
measurement of embryonic secretory
products such as certain enzymes,
Medical Genetics
This type of analysis has been carried
out successfully for the determination of
fetal sex for the avoidance of sex-linked
disorders such as Duchenne muscular
dystrophy and haemophilia A,and for the
diagnosis of single gene disorders such as
cystic fibrosis,alpha-1-antitrypsin
deficiency,Tay-Sach's disease,fragile X
and sickle cell anaemia,
Medical Genetics
Worldwide,preimplantation
diagnosis of embryos has been
attempted on over 1,200 in vitro
fertilisation cycles in 1997,with
clinical pregnancy resulted in 20%,
No increase in the occurrence of
abnormalities has been observed in
the liveborns,
Medical Genetics
B,Noninvasive prenatal diagnosis
using fetal cells isolated from
maternal blood
Circumstantial evidence that fetal
nucleated cells exist in maternal
peripheral blood can be traced back
to 1969,
Medical Genetics
However,convincing proof of the
existence of these cells have to await
the development of molecular
biological techniques,especially the
PCR,
Medical Genetics
Using the PCR,investigators are
able to demonstrate the presence of
cells possessing fetal genetic
markers circulating in the peripheral
blood of pregnant women,The
isolation of these cells offer the
possibility of a noninvasive and safe
method for prenatal diagnosis,
Medical Genetics
Fetal nucleated cell types in
maternal blood
Three populations of fetal
nucleated cells are currently known
to be present in maternal peripheral
blood,fetal lymphocytes,
trophoblasts and fetal nucleated red
cells,
Medical Genetics
At present,the isolation of fetal
nucleated red cells is receiving the
most attention from investigators in
the field due to the availability of
relatively specific monoclonal
antibodies against these cells,
Medical Genetics
Isolation of fetal cells and genetic
analysis
A combination of physical and
immunological methods are used to isolate
fetal nucleated cells from maternal blood,
Physical methods include density gradient
centrifugation and micromanipulation
techniques while immunological methods
include the use of monoclonal antibodies,
Medical Genetics
Genetic analysis of these isolated fetal
cells can be performed using PCR or FISH,
Fetal cells in maternal blood have been
successfully used on a research level to
diagnose trisomy 21,trisomy 18,beta-
thalassaemia and sickle cell anaemia,
Actual clinical use will have to await
further technological development to
improve its reliability and clinical trials to
assess the sensitivity and specificity of
this approach,
