4 Koss_Diagnostic_Cytology Principles of Cytogenetics

Embed Size (px)

Text of 4 Koss_Diagnostic_Cytology Principles of Cytogenetics

Ovid: Koss' Diagnostic Cytology and Its Histopathologic Bases

Page 1 of 63

Editors: Koss, Leopold G.; Melamed, Myron R. Title: Koss' Diagnostic Cytology and Its Histopathologic Bases, 5th Edition Copyright 2006 Lippincott Williams & Wilkins> Table of Contents > I - General Cytology > 4 - Principles of Cytogenetics

4 Principles of Cytogenetics*Linda A. Cannizzaro The events governing the developmental evolution of cells as they progress from the fertilized ovum to mature tissues are not fully understood as yet. It is known, however, that this process involves extensive proliferation and differentiation of embryonal stem cells and their selective destruction by programmed cell death or apoptosis (see Chap. 6). These processes are governed by messages inscribed in the nuclear deoxyribose nucleic acid (DNA) (see Chap. 3). The key feature in cell proliferation is cell division. There are two forms of cell division, one occurring during the formation of gametes (e.g., the spermatozoa and ova), known as meiosis, and the other affecting all other cells (somatic cells) known as mitosis. The purpose of meiosis is to reduce the number of chromosomes by one half (in humans from 46 to 23) in the gametes, so that the union of a spermatozoon and an ovum (fertilization of the ovum) will result in an organism that carries the full complement of chromosomes (in humans, 46) in its somatic cells. The purpose of mitosis is the reproduction of somatic cells, each carrying the full complement of chromosomes. Both forms of cell division are discussed in this chapter. P.80 The events encompassing the life of a cell from its birth until the end of the mitotic division are known as the cell cycle, during which the genomic identity of the cell, vested in the DNA, must be preserved. Molecular genetic technology has considerably advanced our knowledge of the processes involved in the progression of the cell cycle. The normal cell cycle has developed complex mechanisms for the detection and repair of damaged DNA. Upsetting the intricate balance of these cellular processes has dramatic and usually tragic consequences. Dysregulation of meiosis oftentimes is manifested as a genetic disorder, while dysregulation of mitosis may result in a malignant disorder. Since the demonstration of the specificity of chromosomal changes in many disease states and their utilization in diagnosis, the cytogenetic aspects of human diseases have become of direct concern to the practicing physician. This chapter summarizes the salient features of cell division, as well as some of the inherited and malignant conditions that directly result from faulty or anomalous events during meiosis and mitosis. Recent introduction of several powerful molecular cytogenetic methods has facilitated the identification of chromosomal alterations previously irresolvable by high-resolution cytogenetic analysis. These technologies, including the recent mapping of the human genome (Caron et al, 2001; International Human Genome Sequencing Consortium, 2001; Venter et al, 2001; Peltonen and McKusick, 2001) have enormously impacted our knowledge of human genetic disease and the contributions made by these innovations will be made evident in the forthcoming narrative.

THE CELL CYCLEThe cell cycle is composed of several phases, which have, for their purpose, the preservation of the genomic heritage of the cell to be transmitted to the two daughter cells. The phases of the cell cycle are as follows: G0 (resting phase) G1 (gap1) S (synthesis) G2 (gap2) M (mitosis)

mk:@MSITStore:C:\Users\YOUSSEF\Desktop\Koss_Diagnostic_Cytology_Its_Histo... 01/02/2013

Ovid: Koss' Diagnostic Cytology and Its Histopathologic Bases

Page 2 of 63

The events in the phases of cell cycle are described below.

Events Preparatory to Cell DivisionGenetic information in the form of DNA is stored within the interphase nucleus in thread-like, tangled structures called chromatin. During the process of cell division, the DNA condenses and divides into several distinct pairs of linear segments or chromosomes. Each time the cell divides, the hereditary information carried in the chromosomes is passed on to the two newly formed cells. The DNA in the nucleus contains the instructions for regulating the amount and types of proteins made by the cell. These instructions are copied, or transcribed, into messenger RNA (mRNA), which is transported from the nucleus to the ribosomes located in the cytoplasm, where proteins are assembled (see Chap. 3). Most somatic cells spend the greater part of their lives in G0, or the resting phase of the cell cycle, because such cell populations are not actively dividing. Before a cell can divide, it must double its mass and duplicate all of its contents. This ensures the ability of the daughter cells to begin their own cycle of growth followed by division. Most of the work involved in preparing for division goes on invisibly during the growth phase of the cell cycle, known as the interphase, which comprises the G1, S, and G2 phases of the cell cycle (Fig. 4-1). The interphase nucleus is the seat of crucial biochemical activities including the synthesis of proteins and the duplication of its chromosomal DNA in preparation for subsequent cell division.

Cell DivisionThe process of cell division (see Fig. 4-1) can be readily visualized in the microscope and consists of two sequential P.81 events: nuclear division (mitosis) followed by cytoplasmic division (cytokinesis). The cell-division phase is designated as the M phase (M = mitosis). The period between the end of the M phase and the start of DNA synthesis is the G1 phase (G = gap). In G1, RNAs and proteins, including the essential components needed for DNA replication, are synthesized without replication of DNA. Once all the ingredients are synthesized in G1, DNA replication takes place in the ensuing synthesis phase (S-phase) of the cell cycle.

mk:@MSITStore:C:\Users\YOUSSEF\Desktop\Koss_Diagnostic_Cytology_Its_Histo... 01/02/2013

Ovid: Koss' Diagnostic Cytology and Its Histopathologic Bases

Page 3 of 63

Figure 4-1 Schematic presentation of the phases of the mitotic cycle. After the M phase, which consists of nuclear division (mitosis) and cytoplasmic division (cytokinesis), the daughter cells enter the interphase of a new cycle. Interphase begins with the G1 phase in which the cells resume a high rate of biosynthesis after a relatively dormant state during mitosis. The S phase starts when DNA synthesis begins and ends when the DNA content of the nucleus has been replicated (doubled); each chromosome now consists of two sister chromatids. The cell then enters the G2 phase, which ends with the start of mitosis (M). The latter begins with mitosis and ends with cytokinesis. During the early part of the M phase, the replicated chromosomes condense from their elongated interphase state and can be seen in the microscope. The nuclear membrane breaks down, and each chromosome undergoes organized movements that result in the separation of its pair of sister chromatids as the nuclear contents are divided. Two nuclear membranes then form, and the cytoplasm divides to generate two daughter cells, each with a single nucleus. This process of cytokinesis ends the M phase and marks the beginning of the interphase of the next cell cycle. Although a 24-hour cycle is shown in this figure, cell cycle times vary considerably in cells, with most of the variability being in the duration of the G1 phase. (Courtesy of Dr. Avery Sandberg, Scottsdale, AZ.)

The period between the completion of DNA synthesis and the M phase is known as the G2 phase, in which additional cellular components are synthesized in preparation for the cell's entry into mitosis. The interphase thus consists of successive G1, S, and G2 phases that normally constitute 90% or more of the total cell cycle time (see Fig. 4-1). However, following the completion of mitotic division, most normal somatic cells leave the division cycle and enter a postmitotic resting phase (G0), rather than the new G1 phase. The unknown trigger mechanism for cell division

mk:@MSITStore:C:\Users\YOUSSEF\Desktop\Koss_Diagnostic_Cytology_Its_Histo... 01/02/2013

Ovid: Koss' Diagnostic Cytology and Its Histopathologic Bases

Page 4 of 63

is activated during the G0 phase; as a result, the cell enters G1 phase and is committed to divide (Brachet, 1985; Levitan, 1987; Therman, 1993; Nicklas, 1997; Hixon and Gualberto, 2000). In fact, experiments have shown that the point of no return, known as the restriction point (R point), occurs late in G1. After cells have passed this point, they will complete the rest of the cycle at their normal rate, regardless of external conditions. The time spent by cells in G2 and S phases is relatively constant (Brachet, 1985; Gardner, 2000). One interesting exception is the epidermis of the skin, in which some cells remain in the G2 phase and thus are able to undergo rapid division in wound healing. Studies of the cell cycle in yeast have shown that the cell proceeds from one phase of the cell cycle to the next by passing through a series of molecular checkpoints (Li and Murray, 1983). These checkpoints determine whether the cell is ready to enter into the next phase of the cell cycle. These biochemical checkpoints involve the synthesis of new proteins and degradation of already existing proteins. Both the S phase and the M phase are activated by related protein kinases, which function at specific stages of the cell cycle. Each kinase consists of at least two subunits, one of which is cyclin, so named because of its role in the cell cycle. There are several cyclins involved in regulating entry into different parts of the cell cycle, and they are degraded after serving their purpose or as the cell progresses in the cycle and through mitosis (Rudn