Karyotyping
In the Name of GODClinical Cytogenetics
M.Dianatpour MLD, PhD
DefinitionCytogenetics is the study of chromosomes, their
structure and their inheritance.
Clinical cytogenetics is the study of chromosomes, their
structure and their inheritance, as applied to the practice of
medical genetics.
Chromosome abnormalities Microscopically visible changes in the
number or structure of chromosomes, could account for a number of
clinical conditions that are referred to as chromosome
disorders.
Chromosome disordersChromosome disorders form a major category
of genetic disease.
They account for a large proportion of all reproductive
abnormalities, congenital malformations, and mental retardation and
play an important role in the pathogenesis of malignant
disease.Chromosome disordersChromosome disorders are collectively
more common than all the mendelian single-gene disorders together.
Cytogenetic disorders are present in nearly 1% of live births, in
about 2% of pregnancies in women older than 35 years who undergo
prenatal diagnosis, and in fully half of all spontaneous
first-trimester abortions.KaryotypingCells for karyotyping must be
capable of growth and rapid division in cell culture medium.
The most accessible cells are WBCkaryotypingCell
CultureHarvestingBandingAnalysisChromosome classification Human
chromosomes are often classified by the position of the centromere
into three types:
metacentric central centromeresubmetacentric off-center
centromereAcrocentric the centromere near oneend.
A potential fourth type of chromosome, telocentric, with the
centromere at one end and only a singl arm, does not occur in the
normal human karyotype but it is occasionally observed in
chromosome rear rangements and is a common type in some other
species.Chromosome stainingG bandingQ banding (quinacrine mustard)R
banding High Resolution banding(prometaphase banding)Fragile
site
Indications for Chromosome Analysis 1- Problems of early growth
and development.Failure to thrive, Developmental delay, Dysmorphic
faces, Multiple malformations, Short stature, Ambiguous
genitalia,Mental retardation Indications for Chromosome Analysis 2.
Stillbirth and neonatal death.The incidence of chromosome
abnormalities is much higher among still- births (up to
approximately 10%) than among live births (about 0.7%). It is also
elevated among infants who die in the neonatal period (about
10%).
Chromosome analysis should be performed for all still- births
and neonatal deaths.Indications for Chromosome Analysis 3.
Fertility problems. Chromosome studies are indicated for women
presenting with amenorrhea and for couples with a history of
infertility or recurrent miscarriage. A chromosome abnormality is
seen in one or the other parent in a significant proportion (3%to
6%) of cases in which there is infertility or two or more
miscarriages. 4. Family history. A known or suspected chromosome
abnormality in a first-degree relative is an indication for
chromosome analysis under some circumstances. 5. NeoplasiaVirtually
all cancers are associated with one or more chromosome
abnormalities . 6. Pregnancy in a woman of advanced age. There is
an increased risk of chromosome abnormality in fetuses conceived by
women older than about 35 years. Fetal chromosome analysis should
be offered as a routine part of prenatal care in such
pregnancies.
Other cells for karyotypingFibroblast (cultured from skin
biopsy)Bone marrowFetal cells Amniotic fluid CVS (chorionic villus
sampling)Molecular karyotypingFISH (Fluorescent insitu
Hybridization) CGH (Comparative Genome Hybridization)Array
CGHQF-PCR (Quantitative Fluorescent PCR)MLPA (Multiplex Ligation
Probe Amplification)FISH DNA probes specific for individual
chromosomes, chromosomal regions, or genes can be used to identify
particular chromosomal rearrangements or to rapidly diagnose the
existence of an abnormal chromosome number in clinical material
Gene-specific or locus-specific probes can be used to detect the
presence, absence, or location of a particular gene, both in
metaphase chromosomes and in interphase cells. specific chromosomal
loci including centromeres, telomeres, and regions of
heterochromatin. Whole chromosome painting
CGH
Array CGH
CHROMOSOME ABNORMALITIESChromosome abnormalities
Abnormalities of chromosomes may be either numerical or
structural and May involve one or more autosomes, sex chromosomes,
or both simultaneously.Neumerical (Heteroploidy) Aneuploidy: <
or > 2n, For example 47,45, 48,
Polyploidy: Triploidt or tetraploidyTriploid (3n) and tetraploid
(4n), are occasionally observed in clinical material. Both
triploidy and tetraploidy have been seen in fetuses, and although
triploid infants can be liveborn, they do not survive long.
Triploidy is observed in 1% to 3% of recognized conceptions, and
among those that survive to the end of the first trimester, most
result from : Fertilization by two sperm (dispermy) Failure of one
of the meiotic divisions, resulting in a diploid egg or sperm,
Tetraploids are always 92,XXXX or 92,XXYY;
Tetraploidy results from failure of completion of an early
cleavage division of the zygote. AneuploidyAneuploidy is the most
common and clinically significant type of human chromosome
disorder, occurring in at least 5% of all clinically recognized
pregnancies.Most aneuploid patients have either trisomy or, less
often, monosomy Either trisomy or monosomy can have severe
phenotypic consequences. The most common type of trisomy in
liveborn infants is trisomy 21 (karyotype 47,XX or XY,+21), Other
trisomies observed in liveborns include trisomy 18 and trisomy 13.
It is notable that these autosomes(13, 18, and 21) are the three
with the lowest number of genes located on them
Monosomy Monosomy for an entire chromosome is almost always
lethal; an important exception is monosomy for the X chromosome
(Turner syndrome)
Although the causes of aneuploidy are not well understood, it is
known that the most common chromosomal mechanism is meiotic
nondisjunction. The failure of a pair of chromosomes to disjoin
properly during one of the two meiotic divisions, usually during
meiosis I
Structural Chromosome Abnormalities
Structural rearrangements result from chromosom breakage,
followed by reconstitution in an abnormal combination.
Rearrangements are less common than aneuploidy; overall, structural
abnormalities are present in about 1 in 375 newborns.Structural
rearrangements:Balanced, if the chromosome set has the normal
complement of chromosomal material unbalanced, if there is
additional or missing material Unbalanced Rearrangements
In unbalanced rearrangements, the phenotype is likely to be
abnormal because of deletion, duplication, or (in some cases) both.
Duplication of part of a chromosome leads to partial trisomy;
deletion leads to partial monosomy. Deletion
Deletions Deletions involve loss of a chromosome segment,
resulting in chromosome imbalance.A deletion may occur at the end
of a chromosom (terminal) or along a chromosome arm
(interstitial)Cytogenetically visible autosomal deletions have an
incidence of approximately 1 in 7000 live births. Smaller,
submicroscopic deletions detected by microarray or
FISHDuplications
Duplications, like deletions, can originate by unequal crossing
over or by abnormal segregation from meiosis in a carrier of a
translocation or inversion. Duplication appears to be less harmful
than deletion. isochromosome Isochromosomes is a chromosome in
which one arm is missing and the other duplicated in a mirror-image
fashion. A person with 46 chromosomes carrying an isochromosome,
therefore, has a single copy of the genetic material of one arm
(partial monosomy) and three copies of the genetic material of the
other arm (partial trisomy). Balanced Rearrangements
Chromosomal rearrangements do not usually have a phenotypic
effect if they are balanced because all the chromosomal material is
present Carriers of balanced translocations are likely to produce a
high frequency of unbalanced gametes and therefore have an
increased risk of having abnormal offspring range from 1% to as
high as 20%.There is also a possibility that one of the chromosome
breaks will disrupt a gene, leading to mutation. This is a
well-documented cause of X-linked diseases in femaleInversions An
inversion occurs when a single chromosome undergoes two breaks and
is reconstituted with the segment between the breaks inverted.
Inversions are of two types: paracentric (not including the
centromere), in which both breaks occur in one arm pericentric
(including the centromere), in which there is a break in each
arm.An inversion does not usually cause an abnormal phenotype in
carriers because it is a balanced rear- rangement. Its medical
significance is for the progeny;
TranslocationsTranslocation involves the exchangeof chromosome
segments between two, usually nonhomologous, chromosomes. There are
two main types:Reciprocal Robertsonian.
Mosaicism
When a person has a chromosome abnormality, the abnormality is
usually present in all of his or her cells. Sometimes, two or more
different chromosome complements are present in an individual; this
situation is called mosaicism. Mosaicism may be either
Genomic Imprinting
Differences in gene expression between the allele inherited from
the mother and the allele inherited from the father are the result
of genomic imprinting. For some disorders, the expression of the
disease phenotype depends on whether the mutant allele or abnormal
chromosome has been inherited from the father or from the
mother.Imprinting is a normal process caused by alterations in
chromatin that occur in the germline of one parent, but not the
other, at characteristic locations in the genome. These alterations
include the covalent modification of DNA, such as methylation of
cytosine to form 5-methyl-cytosine, or the modification or
substitution in chromatin of specific histone types, which can
influence gene expression within a chromosomal region.
Prader-Willi and Angelman Syndromes
Perhaps the best-studied examples of the role of genomic
imprinting in human disease are Prader-Willi syndrome and Angelman
syndrome. Prader- Willi syndrom a relatively common dysmorphic
syndrome characterized by obesity, excessive and indiscriminate
eating habits, small hands and feet, short stature, hypogonadism,
and mental retardation In approximately 70% of cases of the
syndrome, there is a cytogenetic deletion involving the proximal
long arm of chromosome 15 (15qll-q13), occurring only on the
chromosome 15 inherited from the patient's father.Angelman
syndromeCharacterized by unusual facial appearance, short stature,
severe mental retardation, spasticity, and seizures, there is a
deletion of approximately the same chromosomal region but now on
the chromosome 15 inherited from the mother. Patients with Angelman
syndrome, have genetic information in 15qll-q13 derived only from
their fathers. This unusual circumstance demonstrates strikingly
that the parental origin of genetic material (in this case, on
chromosome 15) can have a profound effect on the clinical
expression of a defect.
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