New Insights in the Cytogenetic Practice: Karyotypic Chaos, Non-Clonal Chromosomal Alterations and Chromosomal Instability in Human Cancer and Therapy Response

Abstract

Recently, non-clonal chromosomal alterations previously unappreciated are being proposed to be included in cytogenetic practice. The aim of this inclusion is to obtain a greater understanding of chromosomal instability (CIN) and tumor heterogeneity and their role in cancer evolution and therapy response. Although several genetic assays have allowed the evaluation of the variation in a population of cancer cells, these assays do not provide information at the level of individual cells, therefore limiting the information of the genomic diversity within tumors (heterogeneity). The karyotype is one of the few available cytogenetic techniques that allow us not only to identify the chromosomal alterations present within a single cell, but also allows us to profile both clonal (CCA) and non-clonal chromosomal alterations (NCCAs). A greater understanding of CIN and tumor heterogeneity in cancer could not only improve existing therapeutic regimens but could also be used as targets for the design of new therapeutic approaches. In this review we indicate the importance and significance of karyotypic chaos, NCCAs and CIN in the prognosis of human cancers.

Keywords: cancer; chromosomal instability; clonal chromosomal alterations; non-clonal chromosomal alterations; therapy resistance; therapy response.

Figure 1

Examples of chromosomal chaos observed in cancer. (A) DAPI image of a metaphase with numerical alterations. In these cells, the absence of cell division leads to endoreduplication, a form of nuclear polyploidization that results in multiple uniform changes connected at the centromere. (B) DAPI image of a metaphase with polyploidy. In these cells, the chromosome number is greater than 46. (C) DAPI image of a metaphase with structural chaos, where chromatid breakage and single sister chromatids are visible (indicated by arrows). (D) DAPI image of a metaphase showing a defective mitotic figure. In this metaphase it is possible to observe the co-existence of condensed chromosomes and undercondensed chromatin fibers (indicated by the arrow) within one mitotic figure. (E) DAPI image of a metaphase showing early stage C-Frag where most chromosomes are intact. The chromosome being degraded is denoted by the arrow. (F) DAPI image of a metaphase with chromosome breakage and several large-scale chromosome fusions. Broken and fused chromosomes can be clearly seen (indicated by arrows).

Figure 2

Role of chromosomal instability (CIN) in cancer. CIN is characterized by the presence of both clonal (CCAs) and non-clonal chromosomal alterations (NCCAs). CCAs and NCCAs can conduce to clonal selection and expansion of chromosomal alterations and thus to an overall increase in heterogeneity. Both clonal selection and heterogeneity reflect system instability and drive cancer evolution by increasing population diversity. In addition, CIN and tumor heterogeneity have been linked to tumor cell survival and therapy response.

Figure 3

Role of CIN in cancer evolution and therapy response. (A) CIN, including both clonal (black circles) and NCCAs (red circles), could impact cell responses to drug treatments. NCCAs can lead to variable drug sensitivity, promoting the survival of a fraction of the tumor cell population. (B) The administration of high-dose chemotherapeutics can result in the generation of new NCCAs (red circles), ultimately giving the disease a chance for recovery and resistance. (C) CIN can conduce to clonal expansion of NCCAs and thus to an overall increase in heterogeneity, favoring cell survival and therapy resistance. Chromosomes with two colors indicate the presence of structural alterations (translocations).

Figure 4

Numerical and structural CIN induced by radiotherapy. Radiation exposure can cause both (A) DNA double-strand breaks (DSBs) and (B) mitotic chromosome segregation errors (MCSE). While DSBs misrepair can lead to chromosomal rearrangements such as (C) deletions and translocations (structural CIN), MCSE can conduce to whole chromosome gains and losses (numerical CIN). Both DSBs and MCSE predispose chromosomes to subsequent (D) structural (sub-chromosomal gains, losses and translocations) and (E) numerical alterations (monosomies and trisomies), thereby increasing CIN.