CONVENTIONAL AND MOLECULAR CYTOGENETICS

To detect and prevent

Conventional Cytogenetics
Cytogenetics is the study of an individual's chromosomes (fetus, infant, child or adult) in order to detect numerical or structural abnormalities. In a normal cell, there are 46 chromosomes, 23 chromosomes of maternal origin and 23 chromosomes of paternal origin (23 pairs). Numerical abnormalities can be a trisomy (an extra copy of a chromosome, see photo 2) or a monosomy (a missing chromosome). Structural abnormalities can be a translocation (a portion of one chromosome that is transferred to another), a deletion, or an inversion in one chromosome.


Photo 2 Three 21 chromosomes (individual with Down syndrome)


Photo 2 Two 21 chromosomes ( normal individual)

Analysis
Cytogenetic analysis can be performed on various types of cells, such as:

Lymphocytes (white blood cells) found in venous blood,

Amniocytes (cells from the fetus to be born) found in the amniotic fluid,

Tumor cells found in the bone marrow.

Once the cells have been cultured and the chromosomes have become easily observed under the microscope, a labeling technique is carried out. This technique gives chromosomes their characteristic dark and clear bands. This marking will facilitate chromosome analysis and will serve in matching chromosomes according to their size, their centromere position and for comparing the dark and clear band sequences to detect an abnormality. A band karyotype will be set up. (see photo 1).


Photo 1 - G Band Karyotype of a normal male

Indications for Chromosome Analysis
in the Fetus:

Positive prenatal screening test,

Advanced maternal age (AMA),

Abnormal ultrasound results.

In the Newborn:

Dysmorphic syndrome, congenital malformation, psychomotor delay,

Sexual ambiguity,

Stillborn baby.

During Childhood or Adolescence:

Mental retardation or delayed development,

Congenital malformation,

Dysmorphism, visceral malformation gone undetected at the neonatal stage,

Hypogonadism (Turner, Klinefelter's syndrome).

In Adulthood:

Infertility workup, premature menopause, primary amenorrhea or spermatogenesis problem,

Repeated miscarriages,

Family history of chromosome abnormalities.

Cancer:

To specify the type of leukemia at the time of diagnosis,

Evaluation of the prognosis and to direct therapeutic decisions,

Evaluation of the therapeutic response.

Molecular Cytogenetics
Molecular cytogenetics uses probes, specifically small DNA sequences where the usual location on a chromosome is known. These probes are labeled with a fluorescent molecule easily detected by means of a fluorescence microscope. (See photo 3). They are placed in contact with chromosomes or nucleus preparations and they will hybridize (attach themselves) specifically to the complementary DNA of the preparation. Probes can be used alone or in combination to obtain several colors. This technology makes it possible to highlight a numerical chromosome abnormality (an extra or missing chromosome), or to underscore chromosome reorganization. It is also possible to find out the origin of a fragment or to highlight a very small loss of material (micro-deletion). (See photo 4)


Photo 3 - Molecular cytogenetics, AneuVysion(tm) probe (Vysis),
trisomy 21 example ( three red signals), patient with Down Syndrome


Photo 4 - Micro deletion, DiGeorge Probe (Vysis),
only one red signal, patient with DiGeorge Syndrome