It is important for the hematopathologist to have a basic understanding of cytogenetic nomenclature, as he/she is often asked to incorporate this data into an integrated or comprehensive report including all clinical laboratory analytic data on individuals with hematologic malignancies (i.e., flow cytometry, molecular and cytogenetic analytic data). The International System of Cytogenetic Nomenclature (ISCN)
18 is the accepted method of describing the karyotype of an individual or tumor. There are very specific rules for how this information is presented. This is the internationally accepted cytogenetic language that, using alpha/numeric/symbolic string text allows one laboratory to describe what was observed in the karyotype and another laboratory to understand what that means. Every few years this system of nomenclature is updated. The most recent update was in 2009. ISCN first came into existence in 1978; however, there were several conferences held from 1960 until then to codify the human karyotype, with banded ideograms first introduced in 1971. An ideogram is a scientific representation of the light and dark bands, sub-bands
and sub-sub-bands observed by metaphase chromosome analysis. Each chromosome has its own particular set of recognized bands, which allows it to be identified as such (
Fig. 3.5). For instance, all human chromosome #1’s look very similar to one another, having the same pattern of light and dark bands, with the exception of a known variant region near the centromere. Chromosomes are divided into short arm and long arm by the centromere or primary constriction, which mediates attachment to the spindle fiber apparatus in mitosis. Bands in the short arm are labeled “p,” while bands in the long arm are labeled “q.” Each chromosome arm has landmark bands which demarcate the regions of the chromosome arm (this is the first number indicated after the p or q designation). These regions are then divided into bands, and possibly sub-bands or sub-sub-bands. Bands are numbered in increasing order starting at the centromere and proceeding toward the end of the chromosome arm (or telomere). The total number of chromosomes observed is stated first, with the sex chromosome designation given following a comma. There are normally no spaces between the numbers, letters, and punctuations that make up the karyotype designation. As an example, a female patient with the Philadelphia chromosome would have a karyotype written as “46,XX,t(9;22)(q34;q11.2)[18]/46,XX[2],” meaning that she has the Ph or t(9;22) in 18 of her metaphases (18 in []), a slash designating a second normal cell line with 46,XX (or normal chromosomes) in two metaphases (2 in []). The breakpoint in chromosome #9 is at band 9q34 (long arm or q arm, region 3, band 4 or band three four, not thirty-four) and the breakpoint in chromosome #22 is at sub-band 22q11.2 (long arm or q arm, region 1, band 1, sub-band .2 or band one one point two, not eleven point two). There are rules as well for describing both interphase and metaphase FISH (
Table 3.2).