Prenatal Chromosome Analyses

IntroductionPrenatal diagnosis can include chromosome analysis, mole-cular testing, and biochemical investigations. While molecu-lar analysis is applied in special instances, chromosome ana-lysis is often part of routine prenatal testing, in particular in pregnant women of advanced age. This leaflet provides infor-mation about chromosome analysis.

ChromosomesChromosomes 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

Bases / NucleotidesCytosine – blueGuanine – yellowThymine – redAdenine – green

Gene

Fig. 1The heredity molecule DNA

DNA(Desoxyribonucleic acid)

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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 fe-male karyotype (46,XX) is given in figure 2, a male karyotype (46,XY) is depicted in figure 3.

Fig. 2Female set of 46 chromosomes (46,XX)

Fig. 3Male set of 46 chromosomes (46,XY)

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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 analysisSpecial techniques facilitate the preparation of chromosomes from cells of the body. The chromosomes are stained and eva-luated 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 chro-mosome 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 performedA 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 chromo-somes increases with the age of the mother.

Trisomy 21 with three instead of two copies of this chromo-some 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 ho-nor of John Langdon Haydon Down who first described this syndrome. Trisomies of the sex chromosomes such as the Kli-nefelter syndrome (47XXY) occur similarly often. Additional cli-nically relevant autosomal trisomies include Ewards syndrome

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(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 ab-errations occur. These include various chromosomal rearran-gements such as translocations, deletions, duplications, and inversions.

Indications for chromosome analysesMaternal ageChildren 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 child-ren 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 incre-ased nuchal transparency or hygroma colli (increased accumu-lation 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 cer-tain 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 parentChromosomal rearrangements can also occur in healthy per-sons. These rearrangements are “balanced” translocations that involve parts or entire chromosomes. However, the ove-rall amount of chromosomal material is not affected and the-se persons are healthy. Parents with a “balanced” transloca-tion 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

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during germ cell formation. An unbalanced translocation com-monly results in malformations, mental retardation, and other serious health problems.

Previous child with malformations / chromosomal anomalies.Malformations are commonly caused by chromosomal anoma-lies. 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 abortionsRecurring abortions can be indicative of chromosomal anoma-lies. At least 50 % of spontaneously aborted fetuses have an abnormal karyotype. The recurrence risk for a chromosomal anomaly in future pregnancies is increased. Recurring abor-tions can also be indicative of a balanced chromosomal rear-rangement in a parent.

SummaryIn case one of the above findings applies, genetic counselling is recommended. Here individual results of previous investi-gations are discussed, possibilities of future prenatal diagno-ses 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 ob-tained by chorionic villus sampling or amniocentesis.

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Sampling of fetal cellsPrenatal chromosome analysis is performed on cells of the developing fetus.

Fetal cells are obtained by invasive procedures such as chorio-nic villus sampling (CVS) and amniocentesis. Both procedures are associated with an increased risk of inducing a miscarria-ge. This risk is 0.5 –1 % in the case of amniocentesis and so-mewhat higher in CVS. Therefore a decision whether or not to have either procedure performed needs to be carefully consi-dered. Apart from amniocentesis and CVS fetal cells are occa-sionally obtained by cordocentesis (collection of blood from the umbilical cord).

Amniocentesis can be performed from the 13 th week of preg-nancy 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 triso-mies 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 analy-sis 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 af-ter 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 umbi-lical cord. This procedure allows collection of 1 – 2ml of fetal blood and is performed from week 20 onwards. Cordocente-sis is applied if a fetal anomaly is detected relatively late du-ring pregnancy and rapid results on the fetus´ karyotype are required.

Perspective – Investigation of fetal DNA from maternal bloodDuring the last few years non-invasive methods have been developed that allow more precise risk estimates of fetal nu-meric 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, hu-man 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 be-fore such methods can be applied in a routine setting.

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amniotic fluid

Fig. 4Amniocentesis

Cells originating from the fetusWall of the uterus

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Requisition and sample materialRequisition formbio.logis provides requisition form ”Prenatal Diagnosis“ and required shipping supplies such as flasks and packaging ma-terial.

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

SamplesPrenatal 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

ShipmentBy courier or by post.

For questions related to the coordination of sample ship-ment, please contact the client service of bio.logis by phone: +49 69 - 530 84 37- 0 or email: info@biologis.com.

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.

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Notes

Am Weißkirchener Berg

A5

Altenhöferallee

Rosa-Luxemburg-Str.

Marie-

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Riedbe

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Kalbach

Bad Homburger Kreuz

KasselDortmund

Bad Homburger Kreuz

A5

Offenbacher Kreuz

A3Würzburg

Darmstadt

Nordwestzentrum

CityMax-vo

n-Laue-Str.

Konr

ad-Z

use-

Str.

Zur Kalbacher Höhe

Merton-viertel

A661

Flughafen / airportDarmstadtBasel

Autobahnanschluss / ExitHeddernheimRiedbergMertonviertel

bio.logis Center for Human Genetics is located at the Frankfurt Biotechnology Innovation Center (FIZ)

By Car:

Coming from Frankfurt AirportA 5 towards Bad Homburg Exit Bad Hom- burger 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 round- about traffic Take the second exit and you have reached the FIZ

Coming from the NorthA 661 towards Offenbach Exit Heddernheim / Riedberg Proceed as explained above

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Parking spacesIn front of the main entrance

GarageEntrance: take the third exit in the roundabout into the Max von Laue Street After 50 meters on your right

Public Transport:

Coming from Frankfurt AirportS8 or S9 or a regional train towards Hauptbahn-hof (Central Station) or Hauptwache

From Central Station towards HauptwacheTrains S1- S6, S8, S9; underground trains U4, U5

From HauptwacheU1 towards Ginnheim Exit NordwestzentrumU2 towards Bad Homburg, Gonzenheim Exit Sandelmühle Bus 29 towards Frankfurt / Main Kalbach Exit Uni Campus RiedbergU3 towards Oberursel-Hohemark Exit Nieder-ursel U9 towards Nieder-Eschbach Exit Uni Campus RiedbergU8 towards Riedberg Exit Uni Campus Riedberg

From NordwestzentrumBus 29 towards Frankfurt / Main Hohe Brück Exit Uni Campus Riedberg Bus 251 towards Kronberg im Taunus Berliner Platz Exit Max-Planck-Institut / FIZ

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bio.logisCenter for Human GeneticsFIZ

bio.logis Center for Human GeneticsProf. Dr. med. D. Steinberger Human Geneticist

Altenhöferallee 360438 Frankfurt am MainT + 49 69 - 530 84 37- 0F + 49 69 - 530 84 37- 11info@biologis.comwww.biologis.com

AuthorsProf. Dr. med. Daniela SteinbergerDr. biol. hum. Jochen BruchProf. Dr. med. Ulrich MüllerDr. rer. nat. Sabine NaumannDr. phil. Maike Post

accredited by:College of American Pathologists (CAP)