Prenatal
Chromosome Analyses
Introduction
Prenatal diagnosis can include chromosome analysis, molecular testing, and biochemical investigations. While molecular analysis is applied in special instances, chromosome analysis is often part of routine prenatal testing, in particular in
pregnant women of advanced age. This leaflet provides information about chromosome analysis.
Chromosomes
Chromosomes are located in the nucleus of cells. They are
composed of nucleic acids and proteins. One type of nucleic
acid, desoxyribonucleic acid (DNA), encodes the information
required for development and function of all cells and thus
of an entire organism. The genetic information is encoded by
genes (segments of the DNA), which are defined by the order
of four different nucleotides. Each nucleotide is composed of a
sugar, phosphate, and one of the four bases adenine, thymine,
Chromosomes
Gene
DNA
(Desoxyribonucleic acid)
Bases / Nucleotides
Cytosine– blue
Guanine – yellow
Thymine – red
Adenine – green
Fig. 1
The heredity molecule DNA
3
guanine, and cytosine (figure 1). Humans have about 25 000
genes.
The human chromosome complement is made up of 22 pairs
of autosomes, and 2 sex chromosomes. Females have two
X-chromosomes, males one X and one Y chromosome. A female karyotype (46,XX) is given in figure 2, a male karyotype
(46,XY) is depicted in figure 3.
Fig. 2
Female set of
46 chromosomes (46,XX)
Fig. 3
Male set of
46 chromosomes (46,XY)
4
During germ cell formation (formation of oocytes and sperms)
the number of chromosomes is divided in half. Thus each germ
cell contains 23 chromosomes. While all female germ cells are
identical (22 autosomes and 1 X chromosome), male germ
cells can include either an X or a Y chromosome in addition to
the 22 autosomes. Depending on whether an X chromosome
or a Y chromosome bearing sperm fertilizes an oocyte, a girl
or a boy develops.
Chromosome analysis
Special techniques facilitate the preparation of chromosomes
from cells of the body. The chromosomes are stained and evaluated under a light microscope. This evaluation (“chromosome
analysis”) allows the determination of number and structure of
the chromosomes. The findings are recorded according to an
international classification system (ISCN-classification). A chromosome analysis results in an individual´s karyotype, which is
documented as 46,XX for a normal female, and as 46,XY for a
normal male (figures 2, 3).
Why are chromosome analyses performed
A structurally or numerically abnormal karyotype is found in
about 0.5 % of all life-born babies. Clinical consequences of
such chromosomal aberrations depend on their nature and
can range from death to mental retardation to more subtle
anomalies. Chromosome analysis can be performed prenatally
on fetal cells. The incidence of an abnormal number of chromosomes increases with the age of the mother.
Trisomy 21 with three instead of two copies of this chromosome is the most common autosomal numerical chromosome
anomaly. The ISCN-formula is 47,XX,+21 for a female and
47,XY,+21 for a male patient with trisomy 21. Trisomy 21 is
also known as “Down syndrome”, a designation used in honor of John Langdon Haydon Down who first described this
syndrome. Trisomies of the sex chromosomes such as the Klinefelter syndrome (47XXY) occur similarly often. Additional clinically relevant autosomal trisomies include Ewards syndrome
5
(trisomy 18; 47,XX,+18 or 47,XY,+18) and Patau syndrome
(trisomy 13; 47,XX,+13 or 47,XY,+18).
Apart from numerical chromosomal anomalies structural aberrations occur. These include various chromosomal rearrangements such as translocations, deletions, duplications, and
inversions.
Indications for chromosome analyses
Maternal age
Children with a chromosome anomaly can be born to women
of all ages. However, the incidence of numerical chromosome
aberrations increases with maternal age. It is 1 / 1300 in children born to mothers of 25 years of age, 1 / 900 in children
born to 30 year-old mothers, and 1 / 380 in children of 35
year-old mothers.
Abnormal ultrasound findings
Several abnormal ultrasound findings in fetuses such as increased nuchal transparency or hygroma colli (increased accumulation of fluid in the lateral regions of the neck) are suggestive
of a chromosomal anomaly.
Prenatal risk analyses (blood screening during pregnancy)
Risk estimates are possible based on investigations of certain proteins within a woman´s blood during the first (first
trimester screening) or second (second trimester screening)
trimester of pregnancy. Estimated risks higher than a certain
threshold make a fetal chromosomal anomaly more likely than
a statistical risk based on the woman´s age alone.
Chromosomal structural anomalies in a parent
Chromosomal rearrangements can also occur in healthy persons. These rearrangements are “balanced” translocations
that involve parts or entire chromosomes. However, the overall amount of chromosomal material is not affected and these persons are healthy. Parents with a “balanced” translocation have an increased risk for children with an “unbalanced”
translocation, i.e. loss or gain of chromosomal material. This
is caused by unequal distribution of chromosomal material
6
during germ cell formation. An unbalanced translocation commonly results in malformations, mental retardation, and other
serious health problems.
Previous child with malformations / chromosomal
anomalies.
Malformations are commonly caused by chromosomal anomalies. About 30 % of life-born children with malformations have
an aberrant karyotype. Furthermore, a chromosomal anomaly
is detected in about 5 % of stillbirths without malformation.
Chromosomal anomalies in a child increase the recurrence risk
for subsequent children.
Habitual abortions
Recurring abortions can be indicative of chromosomal anomalies. At least 50 % of spontaneously aborted fetuses have an
abnormal karyotype. The recurrence risk for a chromosomal
anomaly in future pregnancies is increased. Recurring abortions can also be indicative of a balanced chromosomal rearrangement in a parent.
Summary
In case one of the above findings applies, genetic counselling
is recommended. Here individual results of previous investigations are discussed, possibilities of future prenatal diagnoses are provided and their indications are given. Based on this
information, the patient can make informed decisions for or
against additional diagnostic measures. These diagnoses can
include chromosome analyses on fetal cells that can be obtained by chorionic villus sampling or amniocentesis.
7
Sampling of fetal cells
Prenatal chromosome analysis is performed on cells of the
developing fetus.
Fetal cells are obtained by invasive procedures such as chorionic villus sampling (CVS) and amniocentesis. Both procedures
are associated with an increased risk of inducing a miscarriage. This risk is 0.5 –1 % in the case of amniocentesis and somewhat higher in CVS. Therefore a decision whether or not to
have either procedure performed needs to be carefully considered. Apart from amniocentesis and CVS fetal cells are occasionally obtained by cordocentesis (collection of blood from
the umbilical cord).
Amniocentesis can be performed from the 13 th week of pregnancy onwards. Here amniotic fluid (7 –15 ml) is collected that
contains fetal cells. These cells are cultured and chromosomes
can be prepared after several days to weeks of culture. For
detection of the common trisomies (see above) a rapid test
can be performed that allows detection of autosomal trisomies 21, 18, 13, and of sex chromosomal trisomies within 24
hours. This rapid test only detects said numerical aberrations
and is therefore followed by conventional chromosome analysis to rule out other numerical as well as structural anomalies.
CVS can be performed as early as the 10th week of pregnancy.
Here 15 –20 mg of chorionic tissue that contains fetal cells is
collected. Preliminary results on the karyotype are available after 24 – 48 hours and definite ones after 8 –10 days. The early
time of performance is the main advantage of CVS.
Sometimes fetal cells are obtained by puncture of the umbilical cord. This procedure allows collection of 1 – 2ml of fetal
blood and is performed from week 20 onwards. Cordocentesis is applied if a fetal anomaly is detected relatively late during pregnancy and rapid results on the fetus´ karyotype are
required.
8
amniotic fluid
Wall of the uterus
Cells originating
from the fetus
Fig. 4
Amniocentesis
Perspective – Investigation of fetal DNA from
maternal blood
During the last few years non-invasive methods have been
developed that allow more precise risk estimates of fetal numeric chromosome anomalies. These tests have resulted in a
decrease in invasive tests for direct chromosome anomalies.
Although this is a welcome trend, these non-invasive tests
only give risk estimates and no definite results. Therefore, human geneticists have tried during the last 25 years to obtain
fetal cells from the maternal blood for analysis. It was shown
that fetal cells do indeed get to the maternal blood via the
placenta. Although some promising results have been recently
published, comprehensive, large-scale studies are required before such methods can be applied in a routine setting.
9
Requisition and sample material
Requisition form
bio.logis provides requisition form ”Prenatal Diagnosis“ and
required shipping supplies such as flasks and packaging material.
Please contact the bio.logis client service directly by phone:
+49 (0) 69 - 530 84 37- 0 or visit our website
www.bio.logis.com
Samples
Prenatal chromosome analyses are performed on amniotic
fluid cells, chorionic villi, tissue from abortuses, and from fetal
umbilical blood.
Since these samples are used for tissue culture they must not
be frozen. For further details please contact the bio.logis
client service directly by phone: +49 69 - 530 84 37- 0 or visit
our website www.biologis.com
Shipment
By courier or by post.
For questions related to the coordination of sample shipment, please contact the client service of bio.logis by phone:
+49 69 - 530 84 37- 0 or email: [email protected].
Turn-around time (TAT)
Average turn-around time for prenatal chromosome analyses
is 8 days after receipt of the sample at bio.logis. Results of
rapid tests are available within 1– 2 days.
10
Notes
11
bio.logis Center for Human Genetics is located at the
Frankfurt Biotechnology Innovation Center (FIZ)
ch
lba
Ka
A5
Bad
Kas Homb
urge
s
Dor el
r Kr
tmu
euz
nd
Flughafen /
airport
Darmstadt
Basel
No
Cit rdwe
y
stz
en
tru
m
e
lle
ga
er
b
ed
Ri
r.
-St
ue
-La
n
o
x-v
Ma
Zu
rK
alb
ac
he
r
Autobahnanschluss / Exit
Heddernheim
Riedberg
Mertonviertel
Hö
he
A
661
Mertonviertel
uz
re
rK
he
ac
nb
rg t
ffe
O
bu ad
A3 ürz st
W arm
D
Ros
a-Lu
xem
burg
-Str
.
d
Ba
A5
Ko
nra
d-Z
M
use
ar
-St
ie
r.
-C
ur
ie
-S
tr.
Am
ee
rall
öfe
enh
Alt
Oberursel
erg
er B
hen
c
ir
ißk
We
uz
Kre
ger
r
u
mb
Ho
© bio.logis Center for Human Genetics 06.2015 Design: msgd-studio.de, Frankfurt
bio.logis
Center for Human Genetics
FIZ
By Car:
Public Transport:
Coming from Frankfurt Airport
A 5 towards Bad Homburg Exit Bad Homburger Kreuz onto A 661 towards Offenbach
Exit Heddernheim / Riedberg Turn right at
the second stoplight into the Altenhöferallee
towards FIZ Follow the street until a roundabout traffic Take the second exit and you
have reached the FIZ
Coming from Frankfurt Airport
S8 or S9 or a regional train towards Hauptbahnhof (Central Station) or Hauptwache
Coming from the North
A 661 towards Offenbach Exit Heddernheim /
Riedberg Proceed as explained above
From the South
A 661 towards Bad Homburg Exit Heddernheim / Riedberg Proceed as explained above
Parking spaces
In front of the main entrance
Garage
Entrance: take the third exit in the roundabout
into the Max von Laue Street After 50 meters
on your right
From Central Station towards Hauptwache
Trains S1- S6, S8, S9; underground trains U4, U5
From Hauptwache
U1 towards Ginnheim Exit Nordwestzentrum
U2 towards Bad Homburg, Gonzenheim Exit
Sandelmühle Bus 29 towards Frankfurt / Main
Kalbach Exit Uni Campus Riedberg
U3 towards Oberursel-Hohemark Exit Niederursel U9 towards Nieder-Eschbach Exit Uni
Campus Riedberg
U8 towards Riedberg Exit Uni Campus
Riedberg
From Nordwestzentrum
Bus 29 towards Frankfurt / Main Hohe Brück
Exit Uni Campus Riedberg
Bus 251 towards Kronberg im Taunus Berliner
Platz Exit Max-Planck-Institut / FIZ
bio.logis Center for Human Genetics
Prof. Dr. med. D. Steinberger
Human Geneticist
Altenhöferallee 3
60438 Frankfurt am Main
T + 49 69 - 530 84 37- 0
F + 49 69 - 530 84 37- 11
[email protected]
www.biologis.com
Authors
Prof. Dr. med. Daniela Steinberger
Dr. biol. hum. Jochen Bruch
Prof. Dr. med. Ulrich Müller
Dr. rer. nat. Sabine Naumann
Dr. phil. Maike Post
accredited by:
College of American Pathologists (CAP)