Zagreb University School of Medicine , Croatia
The standard measures for population health outcomes is based on
maternal, infant and under five mortality rates. Health care of
mother and unborn child is the most important part of population
health. Therefore the care for mother and child health during
pregnancy and delivery, assessment of all risks during pregnancy is
of utmost importance of any health care system.
It is known that perinatal mortality is caused in 20-25 percent
of cases by inhaerited anomalies of fetuses and many of theese
might be explained by genetic disorders. In general genetic
disorder is a condition caused by abnormalities in genes or
chromosomes. Chromosomes are complex bodies in cell nucleus as
carriers of genes. While some diseases are due to genetic
abnormalities acquired in a few cells during life, the term
"genetic disease" most commonly refers to diseases present in all
cells of the body and present since conception. Some genetic
disorders are caused by chromosomal abnormalities due to errors in
meiosis, the process which produces reproductive cells such as
sperm and eggs. Examples include Down syndrome (extra chromosome
21), Turner Syndrome (45X0) and Klinefelter's syndrome (a male with
2 X chromosomes). Other genetic changes may occur during the
production of germ cells by the parent. One example is the triplet
expansion repeat mutations which can cause fragile X syndrome or
Huntington's disease. Defective genes may also be inherited intact
from the parents. In this case, the genetic disorder is known as a
hereditary disease. This can often happen unexpectedly when two
healthy carriers of a defective recessive gene reproduce.
Chromosomal abnormalities are disruptions in the normal
chromosomal content of cell and are a major cause of genetic
diseases in humans; some chromosomal abnormalities do not cause
disease in carriers such as translocations or chromosomal
inversions although they may lead to higher proportions of
chromosomal disorder in child. Abnormal number of chromosomes or
chromosome sets caled aneuploidy may cause letal condition or give
rise in genetic disorders. Furthermore the gain or loss of
chromosome material may lead to genetic disorder (deletion, extra
copy as trisomy). Chromosomal mutations produce changes in whole
chromosomes (more than one gene) or in the number of chromosomes
1.1 The major chromosomal abnormalities
The risk for chromosomal abnormalities increases with increasing
maternal age, mainly because non-dysfunctional events in meiosis
are more likely, and result in trisomies. To make it more complex
the "mosaicism" must be added. A "mosaic" is a person with a
combination of two cell lines with different karyotypes (normal and
abnormal). When karyotyping is performed, multiple cells are
analyzed to rule out this possibility. The mosaic condition is not
as severe as the completely abnormal karyotype, and the features
may not be as marked, and live births may be possible. Sometimes
the mosaic's is confined to the placenta ("confined placental
A placenta with an abnormal karyotype (confined placental
mosaicism) may lead to stillbirth, even though the fetus has a
normal karyotype; conversely, a placenta with a normal karyotype
may allow longer survival for a fetus with a chromosomal
abnormality. Rarely, a translocation of part of one chromosome to
another in the parent will be passed on to the child as a partial
trisomy (such as 6p+ or 16p+) which may not be as severe as a
complete trisomy .
� Trisomy 21 ( extra chromosome 21) : Down syndrome;
incidence based upon maternal age, though translocation type is
familial; features can include: epicanthal folds, simian crease,
brachycephaly, cardiac defects.
� Trisomy 18 (47, XY,+18): Features
include micrognathia, overlapping fingers, horseshoe kidney, rocker
bottom feet, cardiac defects, diapragmatic hernia, omphalocele.
� Trisomy 13 ( Patau Syndrome also called D-Syndrome):
Features include microcephaly, cleft lip and/or palate,
polydactyly, cardiac defects, holoprosencephaly.
� Trisomy 16: Seen in abortuses from first trimester.
� Monosomy X: Turner's syndrome (45,X 0); can survive to
adulthood; features include short stature, cystic hygroma of neck
(leading to webbing), infertility, coarctation.
� Klinefelter's syndrome (XXY, a male with 2 X
chromosomes); features include elongated lower body, gynecomastia,
testicular atrophy (incidence: 1/500 males)
� Triploidy: There is often a partial hydatidiform mole of
placenta. Fetal features include 3-4 syndactyly, indented nasal
bridge, small size.
� Idic 15 or isodicentric 15 :inverted duplication of
chromosome 15 or tetrasomy 15
� Jacobsen syndrome also called the terminal 11q deletion
disorder. This is a very rare
disorder. Those affected have normal intelligence or mild mental
retardation, with poor expressive language skills. Most have a
� XYY syndorm. XYY boys are usually taller than their
siblings. Like XXY boys and XXX girls, they are somewhat more
likely to have learning difficulties.
� Triple XXX syndrome. XXX girls tend to be tall and thin.
They have a higher incidence of dyslexia.
A host of other chromosomal abnormalities are possible. In
general, fetal loss earlier in gestation, and multiple fetal
losses, more strongly suggests a possible chromosomal
1.2 Prenatal diagnosis
Prenatal diagnosis employs a variety of techniques to determine
the health and condition of an unborn fetus. Without knowledge
gained by prenatal diagnosis, there could be an untoward outcome
for the fetus or the mother or both.
Specifically, prenatal diagnosis is helpful for:
� Managing the remaining weeks of the pregnancy
� Determining the outcome of the pregnancy
� Planning for possible complications with the birth
� Planning for problems that may occur in the newborn
� Deciding whether to continue the pregnancy
� Finding conditions that may affect future
There are a variety of non-invasive and invasive techniques
available for prenatal diagnosis. Each of them can be applied only
during specific time periods during the pregnancy for greatest
Indications for prenatal diagnostic testing include:age of
mother, Down syndrome in previous pregnancy or family, structural
aberrations in previous pregnancies or in family members, autosomal
genopaties, X-linked genetic disorders, neuronal tube defects in
previous pregnancies, mental retardation in family (linked to
fragile X) present ultrasound suspicion, consanguinity,
pathological finding in prenatal serum screening, other reasons
(viral infection, radiation).
1.3 Source of samples for prenatal testing
Prenatal diagnosis of chromosomopathies as well as genetic
disorders is based on invasive and non-invasive techniques.
Chorionic villi sampling (CVS)In this procedure, a catheter is
passed via the vagina through the cervix and into the uterus to the
developing placenta under ultrasound guidance. Alternative
approaches are transvaginal and transabdominal. The introduction of
the catheter allows sampling of cells from the placental chorionic
villi. These cells can then be analyzed by a variety of techniques.
The most common test employed on cells obtained by CVS is
chromosome analysis to determine the karyotype of the fetus. The
cells can also be grown in culture for biochemical or molecular
biologic analysis. CVS can be safely performed between 9.5 and 12.5
CVS has the disadvantage of being an invasive procedure, and it
has a small but significant rate of morbidity for the fetus; this
loss rate is about 0.5 to 1% higher than for women undergoing
amniocentesis. Rarely, CVS can be associated with limb defects in
the fetus. The possibility of maternal Rh sensitization is present.
There is also the possibility that maternal blood cells in the
developing placenta will be sampled instead of fetal cells and
confound chromosome analysis. The obtained material is used for
fluorescent in situ hybridization (FISC), short tandem repeats
(STR) , DNA and some biochemical analyses.
Amniocenthesis (transvaginal aspiration of amnionic fluid 15-20
weeks of pregnancy) is the most used method ( risk below 0,5 %) for
sample for all kind of analyses.
Preconception - preimplantation diagnosis is possibility applied
in connection with in vitro fertilization (IVF) to make diagnosis
at the gamete stage or performing the biopsy of one or two
blastomeres by aspiration with micropipette. Preimplantation
diagnosis is now offered as an alternative to conventional prenatal
diagnosis in following cases: recessive or dominant hereditary
disorders linked to chromosome X, monogenic disorders of authosomal
inheritance (recessive or dominant) and the detection of
translocations (couples who are carriers of chromosome abnormality
of number or structure).
Maternal blood sampling for fetal blood cells is a new
non-invasive technique that makes use of the phenomenon of fetal
blood cells gaining access to maternal circulation through the
placental villi. Ordinarily, only a very small number of fetal
cells enter the maternal circulation in this fashion (not enough to
produce a positive Kleihauer-Betke test for fetal-maternal
hemorrhage). The fetal cells can be sorted out and analyzed by a
variety of techniques to look for particular DNA sequences, but
without the risks that these latter two invasive procedures
inherently have. Fluorescence in-situ hybridization (FISH) is one
technique that can be applied to identify particular chromosomes of
the fetal cells recovered from maternal blood and diagnose
aneuploid conditions such as the trisomies and monosomy X.
The problem with this technique is that it is difficult to get
many fetal blood cells. There may not be enough to reliably
determine anomalies of the fetal karyotype or assay for other
1.4 Molecular analysis
The technologies developed for the Human Genome Project, the
recent surge of available DNA sequences resulting from it and the
increasing pace of gene discoveries and characterization have all
contributed to new technical platforms that have enhanced the
spectrum of disorders that can be diagnosed prenatal. The
importance of determining the disease-causing mutation or the
informative ness of linked genetic markers before embarking upon a
DNA-based prenatal diagnosis is, however, still emphasized.
Different fluorescence in situ hybridization (FISH) technologies
provide increased resolution for the elucidation of structural
chromosome abnormalities that cannot be resolved by more
conventional cytogenetic analyses, including micro deletion
syndromes, cryptic or subtle duplications and translocations,
complex rearrangements involving many chromosomes, and marker
chromosomes. Interphase FISH and the quantitative fluorescence
polymerase chain reaction are efficient tools for the rapid
prenatal diagnosis of selected aneuploidies, the latter being
considered to be most cost-effective if analyses are performed on a
large scale. There is some debate surrounding whether this approach
should be employed as an adjunct to karyotyping or whether it
should be used as a stand-alone test in selected groups of
Interphase and metaphase FISH, either as a single probe
analysis, or using multiple chromosome probes, can give reliable
results in different clinical situations.
It should be noted that there may be variation in probe signals
both between slides (depending on age, quality, etc. of metaphase
spreads) and within a slide. Where a deletion or a rearrangement is
suspected, the signal on the normal chromosome is the best control
of hybridisation efficiency and control probe also provides an
internal control for the efficiency of the FISH procedure.
Depending on the sensitivity and specificity of the probe and on
the number of cells scored, the possibility of mosaicism should be
considered, and comments made where appropriate. By using
locus-specific probes at least 5 cells should be scored to confirm
or exclude an abnormality. Multiprobe analysis: three cells per
probe should be scored to confirm a normal signal pattern. Where an
abnormal pattern is detected, confirmation is advisable. In
prenatal interphase screening for aneuploidy signals should be
countered in at least 30 cells for each probe set. A minimum 100
cells should be scored.
When hybridisation is not optimal, the test should be repeated.
When a deletion or another rearrangement is suspected, the results
must be confirmed with at least one other probe.
Results should preferably be followed up by karyotype analysis.
This is essential when there are discrepancies between the expected
laboratory findings, and the clinical referral.
Before introducing interphase FISH as a diagnostic technique,
staff need appropriate training on the type of samples to be
analysed. Laboratories should set standards for classification of
observations and interpretation of results.
More recently new method for fast identification of chromosomal
abnormalities has been developed as high resolution array
comparative genomic hybridization (aCGH) which provide genome-wide
analysis of chromosome copy number and structural change .The chip
technology provide investigation of genetic causes associated with
dysmorphic features, mental retardation, developmental delay,
multiple congenital abnormalities .The commercial chip include more
tha 40 abnormalities including duplications and microdeletion
regions. It is expected that evaluation of this technique will
prove scientifically based evidence for named advantages.
Recommended literature :
1� Baric I, Stavljenic-Rukavina A. Racionalna dijagnostika
nasljednih i prirodenih bolesti. Medicinska naklada Zagreb,
2� Borovecki F, Lovrecic J, Zhou J, Jeong H, Rosas H.D ,
Hersch S.M, et al . Genome-wide expression profiling of
human blood reveals biomarkers for Huntington's disease. Available
at URL address: www.pnas.org/cgi/doi/10.1073/pnas.0504921102
3� Brown TW, Jenkins C. The fragile X syndrome. In: Friedman T.: Molecular genetic Medicine .
Academic Press inc. San Diego, New York, Boston, London, Sydney,
Tokyo, Toronto 1992;2:39-65.
4� Bui TH, Blenow E, Nordenskjold M. Prenatal diagnosis:
molecular genetics and cytogenetics. Best Pract Res Clin Obstet
5� Elles R. Molecular diagnosis of genetic diseases.
Humana Press. 2000;Totowa, New Jersey.
6� Miny P, Tercanli S, Holzgreve W. Developments in
laboratory techniques for prenatal diagnosis. Curr Opin Obstet
7� Morris J.K, Wald NJ, Watt HC. Fetal loss in Down
syndrom pregnancies. Prenat.Diagn 1999;19:142-5.
8� Khoury JM, Burke W, Thomson E. Genetics and public
health in the 21 century. Oxford University Press 2000.