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Chromosome analysis (metaphase cytogenetics) is now considered an obligatory test method for the diagnosis of hematological neoplasia. The findings contribute to substantiating the diagnosis, although the prognostic significance that can be confirmed from the karyotype of malignant cells is far more important. Neoplasia-associated chromosome aberrations occur only in malignant cells. These are acquired genetic mutations. Hence, the other cells in the body of a patient suffering from hematological neoplasia are cytogenetically normal.
Between 5 and 10 ml of bone marrow with heparin anticoagulant are needed for chromosome analysis, as the proportion of malignant cells is usually larger than in peripheral blood, and the cells obtained from the bone marrow exhibit the highest level of proliferation activity. Cytogenetic testing should only be conducted on peripheral blood (heparin vial) if it is not possible to obtain bone marrow. Chronic lymphocytic leukemia is an exception, and peripheral blood anticoagulated with heparin is the ideal test material for chromosome analysis in this case. Given that vital cells are needed for chromosome analysis, the obtained test material should arrive at a cytogenetic laboratory within 24 hours after extraction. The cells must not be frozen and should be stored at room temperature.
Chromosome analysis requires a sufficient number of metaphases in good quality. Therefore, the bone marrow or blood cells are cultured for 24-72h depending on the cell type to be analysed and then arrested at the metaphase stage by adding colcemid. Cytokines can be added to the culture medium to stimulate the malignant cell population and increase the metaphase yield during cultivation. Swelling of the cells is induced by adding a hypotonic potassium chloride solution; they are then fixed in this state with a methanol/glacial acetic acid. The cell suspension is then dripped onto slides. It is mandatory to conduct chromosome banding in order to ensure an unequivocal identification of the individual chromosomes. The most frequently used techniques are G- (giemsa), Q- (quinacrine) and R- (reverse) banding. The various banding techniques produce light and dark bands on the chromosomes that are specific to each chromosome and that hence permit unequivocal identification of the individual chromosomes. According to international consensus, 20–25 should be fully analyzed in order to produce a reliable diagnosis (ISCN).
International System of Cytogenetic Nomenclature (ISCN)
Chromosomes are classified according to their size, the centromere location (which separates the two chromosome arms) and their characteristic banding pattern. Each chromosome has a short arm (p) and a long arm (q). Based on this banding pattern, chromosomes are divided into regions and bands that are numbered outward from the centromere to the telomere. An international cytogenetic nomenclature (ISCN: International System of Cytogenetic Nomenclature) provides an exact description of all numerical and structural aberrations in a karyotype formula. The karyotype formula first states the number of chromosomes, followed by the statement of gender chromosomes. Hence, the normal female karyotype is 46,XX, while the normal male karyotype is 46,XY.
Two types of chromosome aberration
Among the numerical chromosome aberrations are monosomy (loss of one chromosome) and trisomy (gain of one chromosome). Duplications of the entire chromosome set may also occur. Normally, cells in the body will contain a double (diploid) set of chromosomes. Triple or quadruple chromosome sets are called triploids and tetraploids.
The most frequent structural chromosome aberrations are deletions (loss of chromosome parts), translocations (exchange of chromosome parts between different chromosomes), inversions (rotation of a chromosome section by 180°) and isochromosomes (chromosomes consisting of two short or long arms, with absence of the other arm).
The karyotype formula
The karyotype formula denotes the gain of a chromosome with a “+” and the loss of a chromosome with “-”, e.g. 47,XX,+8: trisomy of chromosome 8, i.e. 45,XY,-7, describes monosomy of chromosome 7. Abbreviations are defined for chromosome aberrations at international level, e.g. “t” for translocation and “inv” for inversion: t(8;21)(q22;q22) means that a breakpoint has occurred in band q22 of chromosome 8, another one in band q22 of chromosome 21, and that translocation of the fractions has taken place between the chromosomes. The karyotype formula uses a semicolon (;) to separate chromosomes and breakpoints in different chromosomes, while breaks within a single chromosome are stated in a direct line without punctuation, e.g. inv(16)(p13q22): i.e. breaks took place in the chromosome bands p13 and q22 of the same chromosome 16, and the segment between both breakpoints was inverted by 180°. Another example: del(5)(q13q31), i.e. the breaks took place in the bands q13 and q31 of the same chromosome 5, and the region between q13 and q31 has been lost.
Clonality of chromosome aberration
Chromosome aberrations are clonal if an identical structural aberration or the gain of one chromosome in at least two metaphases or the loss of the same chromosome in at least three metaphases is observed. According to ISCN only clonal chromosome abnormalities should be reported in the karyotype formula.