Riview Of Lit 2

Review of literature Toxic chemicals can cause genetic damage. The genetic material of a cell consists of genes, which exist in chromosomes. Genes and chromosomes contain the information that tells the cell how to function and how to reproduce (form new cells). Some chemicals may change or damage the genes or chromosomes. This kind of change, or damage in a cell is called a mutation. Anything that causes a mutation is called a mutagen. Mutations may affect the way the cell functions or reproduces. The mutations can also be passed on to new cells that are formed from the damaged cell. This can lead to groups of cells that do not function or reproduce the same way the original cell did before the mutation occurred. Some kinds of mutation result in cancer. Most chemicals that cause cancer also cause mutations. However, not all chemicals that cause mutations cause cancer. Tests for the ability of a chemical to cause a mutation take little time and are relatively easy to perform. If testing shows a chemical to be a mutagen, additional testing must be done

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Exposure to chemical substances may affect your children or your ability to have children. Toxic reproductive effects include the inability to conceive children (infertility or sterility), lowered sex drive, menstrual disturbances, spontaneous abortions (miscarriages), stillbirths, and defects in children that are apparent at birth or later in the child’s development. Teratogens are chemicals which cause malformations or birth defects by directly damaging tissues in the fetus developing in the mother’s womb. Other chemicals that harm the fetus are called fetotoxins. If a chemical causes health problems in the pregnant woman herself, the fetus may also be affected. Certain chemicals can damage the male reproductive system, resulting in sterility, infertility, or abnormal sperm. There is not enough information on the reproductive toxicity of most chemicals. Most chemicals have not been tested for reproductive effects in animals. It is difficult to predict

risk in humans using animal data. There may be “safe” levels of exposure to chemicals that affect the reproductive system. However, trying to determine a “safe” level is very difficult, if not impossible. It is even more difficult to study reproductive effects in humans than it is to study cancer. At this time, only a few industrial chemicals are known to cause birth defects or other reproductive effects in humans.1

CHROMOSOMAL ABBERATION INTRODUCTION Chromosomes are the organized form of DNA found in cells. Chromosomes contain one very long, continuous piece of DNA, which contains many genes, regulatory elements and other intervening nucleotide sequences. A broader definition of "chromosome" also includes the DNA-bound proteins which serve to package and manage the DNA. The word chromosome comes from the Greek (chroma, color) and (soma, body) due to their property of being stained very strongly with vital and supravital dyes. 2,3 chromosome abnormality reflects an abnormality of chromosome number or structure. Chromosome abnormalities usually occur when there is an error in cell division following meiosis or mitosis. There are many types of chromosome abnormalities. Visible changes to chromosome structure and morphology have played a very important part as indicators of genetic damage in both clinical and cancer studies.Most of the changes encountered in clinical studies are “secondary” or “derived” aberrations. This is true also in cancer studies, except that here, there is an ongoing production of aberrations, so that in some cells, a mixture of primary and secondary changes is present, and a continuously changing karyotype (true chromosomal instability).To appreciate these observed secondary changes we need to understand the primary changes from which they are derive.4 CLASSIFICATION OF PRIMARY CHANGES A chromosome abnormality reflects an abnormality of chromosome number or structure. There are many types of chromosome abnormalities. However, they can be organized into two basic groups:

Numerical Abnormalities: When an individual is missing either a chromosome from a pair (monosomy) or has more than two chromosomes of a pair (trisomy). An example of a condition caused by numerical abnormalities is Down Syndrome, also known as Trisomy 21 (an individual with Down Syndrome has three copies of chromosome 21, rather than two). Turner Syndrome is an example of monosomy, where the individual - in this case a female - is born with only one sex chromosome, an X. Structural Abnormalities: When the chromosome's structure is altered. This can take several forms: •

Deletions: A portion of the chromosome is missing or deleted.

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Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material.

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Translocations: When a portion of one chromosome is transferred to another chromosome. There are two main types of translocations. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation, an entire chromosome has attached to another at the centromere.

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Inversions: A portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted.

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Rings: A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.

Most chromosome abnormalities occur as an accident in the egg or sperm. Therefore, the abnormality is present in every cell of the body. Some abnormalities, however, can happen after conception, resulting in mosaicism, where some cells have the abnormality and some do not. Chromosome abnormalities can be inherited from a parent (such as a translocation) or be "de novo" (new to the individual). This is why chromosome studies are often performed on parents when a child is found to have an abnormality.5 For purely pragmatic and diagrammatic purposes, we can regard the chromosomal changes we see down the microscope as being the result of “breaks” followed by “rejoins” of the chromosome thread. However, we must always remember that, in reality, their origin is much more complicated

Since the chromosome we see and score at metaphase has two (sister-) chromatids, it is convenient (and conventional) to divide all aberrations into two broad types: Chromosome-type where the breaks and re-joins always affect both sister-chromatids at any one locus. Examples in Figure 1.

Chromatid-type where the breaks and re-joins affect only one of the sister-chromatids at any one locus (Fig 2

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If the breaks are situated in the arms of different (non-homologous or homologous) chromosomes we have the category of INTERCHANGES.

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If the breaks are in the opposite arms of the same chromosome, we have the category of INTER-ARM INTRACHANGES.

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If the two breaks are both in the same arm of a chromosome, we have the category of INTRA-ARM INTRACHANGES. These three categories are often referred to collectively as EXCHANGES.

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Finally, some aberrations appear to arise from a single, open break in just one arm. This category we term “BREAKS” or “DISCONTINUITIES”. Many (perhaps all) of them are, in reality, intra-arm intrachanges where one end has failed to join up properly, though the limitations of microscopical resolution do not permit us to be certain that the re-joining is really incomplete.5,6

Interaction between the four ends of two breaks can obviously take place in three ways :  Join back to re-form the original chromosomes (“RESTITUTION”) so that no aberration is produced  Re-join in such a way that an acentric fragment is always formed (ASYMMETRICAL RE-JOINING)  Re-join in a way that never leads to an acentric fragment unless one of the re-joins is incomplete (SYMMETRICAL RE-JOINING)6

RELEATIONSHIP WITH CELL CYCLE Conventionally, the period between successive mitoses (“INTERPHASE”) is sub-divided into three phases G1, S and G2 . For critical work, further sub-division of S is possible. G1 is the pre-duplication period, when the cell begins to prepare for DNA synthesis and the next mitosis. If the cell is not going to divide again, it passes out of cycle during this phase into another phase termed G0. From this phase it may, or may not, be possible to

call it back into a division cycle. Usually, however, cells pass on to irreversible differentiation with their chromosomes unduplicated. S-phase is a discrete period of interphase of a few hours duration during which the chromosomal DNA and protein is duplicated, and the new chromatin segregated into the sister-chromatids. Each chromosome has a precise programme of replication, closely associated with its G-band pattern. Pale G-bands always replicate early in S-phase, dark G-bands later, and constitutive heterochromatin tends to be among the very last regions to replicate. During G2 , the newly replicated chromosomes undergo a rapid programme of condensation, packing and coiling to produce the familiar metaphase chromosomes where we normally identify and score aberrations. These condensed chromosomes facilitate transport of the genetic material to the daughter cells at mitosis. This condensation and packing readily obscures, modifies and disguises aberrations which are produced during interphase - a point that should always be borne in mind when interpreting what we see down the microscope.7,8,9 Most aberration-inducing agents can introduce lesions into the chromatin at all stages of the cell cycle, but relatively few of them can produce actual structural changes in G1,( and therefore give rise to primary chromosome-type changes) or in S and G2 (producing primary chromatid-types,cobalt60 causes chromosome types of changes 10 Ionising radiation, restriction endonucleases, and a few chemicals like bleomycin and some antibiotics are amongst those that can. Almost all remaining aberration producing agents are “S-dependent”; surviving unrepaired lesions from G1 or G2 have to pass through a scheduled S-phase to convert them into exclusively chromatid-type aberrations. Any interference with or abnormality in the processes of chromatin replication also leads to chromatid-type aberrations visible at next mitosis. It is almost certain that the vast majority of “spontaneous” and de novo aberrations arise in this way. Chromosome instability syndromes also probably produce aberrations via defective S-phase pathways.

However they are produced, the resulting chromatid-type aberrations are qualitatively (but not quantitatively) identical RECIPROCAL TRANSLOCATION Reciprocal translocations (rcp) are among the most common constitutional chromosomal aberrations in man. 11In a reciprocal translocation there is a mutual exchange of chromosomal segments between two different chromosomes. This exchange can take place between any two chromosomes and at various sites along the length of the chromosome.12 PERICENTRIC INVERSION An inversion in which the breakpoints occur on both arms of a chromosome. The inverted segment spans the centromere.13 Pericentric inversion in chromosome 7 may play a role in the etiology of the family's miscarriages 14 PARACENTRIC INVERSION A chromosomal inversion that does not include the centromere.

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at the chromosome level unless they are very large (many megabases of DNA). Again the re-joining points can disrupt important genetic sequences, and reverse segments of the reading frame. Large inversions will give problems at meiosis.

INTERSTITIAL DELETION Deletion that does not involve the terminal parts of a chromosome called interstitial deletion.16 A case of autism with an interstitial deletion on 4q leading to hemizygosity for genes encoding for glutamine and glycine neurotransmitter receptor sub-units and neuropeptide receptors 17

TERMINAL DELETION Deletion is the loss of genetic material. . Terminal Deletion - a deletion that occurs towards the end of a chromosome. Cryptic terminal deletion of chromosome 9q34 a novel cause of syndromic obesity in childhood 18

Structural chromosome instability An elevated frequency of structural chromosome aberrations could be directly caused by an abnormally high incidence of DNA double-strand breaks. Chromosomal breakage can result in a number of different structural rearrangements, some of which give rise to abnormalities of chromosomal segregation at mitosis. For example, terminal deletions due to a break of a single chromatid will result in a centric derivate chromosome plus an acentric fragment. Because of its failure to bind the mitotic spindle, the fragment may be permanently lost in the subsequent cell division, and may be seen as a lagging chromatin body at metaphase or anaphase. Such lagging is a common finding in cell populations exposed to ionising radiation .It has also been described in a number of solid tumours, such as head and neck, and breast carcinomas . Chromosomal instability is a common feature of cancer cells. Several cellular mechanisms lead to numerical and structural chromosomal instability in cancer cells, including defects in chromosomal segregation, cellular checkpoints that guard against reproduction of abnormal cells, telomere stability, and the DNA damage response. Human papillomavirus interferes with these processes, causing chromosomal instability and tumor formation in some of the epithelial cells which it infects. The rate of discoveries about the mechanisms leading to chromosomal instability in cancer cells is increasing rapidly. Although these mechanisms were thought to be unrelated, they are intimately intertwined, connecting the complex network of cellular pathways. Since chromosomal instability is undoubtedly a major cause of tumor evasion of therapy, understanding the mechanisms leading to chromosomal instability has major translational significance. 19 THE BFB CYCLE — A CHAIN REACTION OF CHROMOSOMAL BREAKAGE Concurrent breaks in two different chromosomes may either give rise to translocations or dicentrics. Whereas translocation derivatives are stably transmitted through cell division, the dicentric chromosomes may be stretched out between the spindle poles to form bridges at anaphase (Figure 1A). These bridges may subsequently break, and the chromosomes are transmitted to the daughter cells with broken ends that may recombine

further during the subsequent interphase. Similarly, ring chromosomes having undergone sister chromatid exchange, may be stretched out at anaphase, break, and then be transmitted to the daughter cells as broken chromosomes (Figure 1B; ) The broken chromosome ends may fuse into novel dicentrics and rings, which may again break at the next cell division. Thus, chromosomal damage may not only result in static aberrations, such as translocations, inversions, deletions, and duplications; it may also result in mitotically unstable chromosomes, which may trigger a series of breakage-fusion-bridge (BFB) events. Such BFB cycles have been shown to occur in many malignant solid tumours with complex chromosome abnormalities, including head and neck, pancreatic, and ovarian carcinomas, as well as leiomyosarcoma, osteosarcoma, malignant fibrous histiocytoma, and atypical lipomatous tumours.Data from in vitro and animal studies indicate that the BFB cycles may be initiated by shortening of telomeric repeat sequences, leading to impaired integrity of chromosome termini and telomeric associations between chromosomes . It is probable that similar mechanisms are responsible for the high frequency of BFB instability in human.

Figure 1 Chromosome breakage-fusion-bridge (BFB) cycles: Dicentric (A) and ring (B) chromosomes may form bridges at anaphase and the broken ends of the two chromatids (red and white) may fuse into novel dicentric and ring-shaped structures in the daughter cells.

Different mechanisms or steps in a single process In most tumours exhibiting chromosomal instability, BFB events and centrosomal abnormalities occur together. In analogy, most malignant tumours exhibit both structural and numerical chromosome abnormalities. However, in many low-grade mesenchymal and neuroglial tumours, BFB events involving telomeric associations and ring chromosomes are seen at mitosis, in the absence of major numerical. However, at progression of these tumours towards higher malignancy, numerical aberrations as well as highly complex structural aberrations become more frequent. This implies a sequence of parallel cytogenetic and molecular steps, where telomeric dysfunction and BFB events occur at an early stage, and numerical instability develops later. First, a continued proliferation of cells with reduced telomere length requires an inhibition of cell cycle control mechanisms. In vitro, this may be induced by partial inhibition of normal TP53 and RB1 function, for instance by SV40 transfection. The corresponding in vivo changes remain to be elucidated, although recent findings indicate that some cell types, such as human mammary epithelial cells, may spontaneously proliferate beyond the normal telomere length and acquire chromosomal changes such as dicentrics and rings. Further survival of cell populations with disrupted telomeric integrity, resulting in frequent BFB events, would require additional impairment or total abrogation of the systems normally causing arrest or apoptosis in cells with double-strand DNA breaks. This step appears to be associated with inactivating mutations in TP53 or high-level amplification of MDM2. Also, the numerical chromosome instability associated with abnormal centrosome function indicates that highly malignant cells have acquired some tolerance to massive genomic imbalances

Figure2 Hypothetical scenario of progressive mitotic instability: Telomere shortening, seen as absence of detectable TTAGGG repeats (top image) may compromise the integrity of chromosome ends, leading to the formation of rings and dicentrics. These may form bridges at anaphase, which either breaks and initiate a series of BFB-events, or induce cytokinetic failure leading to the formation of binucleate cells with supernumerary centrosomes. Cells with an abnormal centrosome number may form multipolar mitoses at the next cell division. Thus, telomeric dysfunction may result both in structural and numerical chromosome instability. The common concurrence of BFB instability and centrosome abnormalities suggests that these phenomena are mechanistically linked. Although it is true that both these instabilities may be associated with similar molecular genetic lesions, such as TP53 mutation, their causal relationship, if any, remains unclear. There may be one rather straightforward relationship, however. It is well established that anaphase bridging may cause collapse of the cytokinetic process, leading to formation of cells with a duplicated genome . Tumours with BFB events show a high frequency of binucleated cells . These cells would not only carry the double amount of genetic material, but also twice the normal number of centrosomes. After the next round of replication, such cells may thus enter mitosis with abnormal centrosome configurations, leading to either tri- or tetrapolar cell divisions (Figure 2). Incomplete cytokinesis could then easily explain the connection between telomere shortening and BFB events, on one hand, and mitotic multipolarity, on the other hand. 20

GENETIC DISORDER A genetic disorder is a disease that is caused by an abnormality in an individual's DNA. Abnormalities can range from a small mutation in a single gene to the addition or subtraction of an entire chromosome or set of chromosomes.21

SINGLE GENE DISORDER Over 10 000 human diseases are caused by defects in single genes. These single gene disorders, which are also described as unifactorial or monogenic diseases, are individually very rare but they affect about 1 per cent of the population as a whole. Since only a single gene is involved in each case, these diseases generally have simple inheritance patterns in family pedigrees. This means they can be traced through families and their occurrence in later generations can be predicted (see Genetic counselling ). The defective version of the gene responsible for the disease is known as a mutant allele or a disease allele. Single gene disorders can be divided into a number of different categories according to how they are transmitted from generation to generation. Some are described as dominant diseases because only one mutant allele is required, and such diseases tend to crop up in every generation. Other diseases are described as recessive because both copies of the gene must be defective in order for the disease to occur. These recessive diseases often skip generations because mutant alleles can be carried without any effect if a normal allele is also present. Many single gene disorders affect both sexes equally. However, where the relevant gene is present on the X-chromosome, the associated disease tends to be more common in males 22 AUTOSOMAL DOMINANT Only one mutated copy of the gene is needed for a person to be affected by an autosomal dominant disorder. Each affected person usually has one affected parent. There is a 50% chance that a child will inherit the mutated gene. Common example of autosomal dominant disorders includes Huntingtons disease, Neurofibromatosis 1, Marfan Syndrome. Hereditary non-polyposis colorectal cancer (HNPCC) is an autosomal dominant disease, characterized by the occurrence of predominantly colon and endometrial cancer and, less frequently, cancer of the small bowel, stomach, hepatobiliary tract, ureter, renal pelvis, ovaries and brain23

AUTOSOMAL RECESSIVE Two copies of the gene must be mutated for a person to be affected by an autosomal recessive disorder. An affected person usually has unaffected parents who each carry a single copy of the mutated gene (and are referred to as carriers) common autosomal recessive disorder includes Cystic fibrosis, Sickle cell anemia(Also Partial Sickle Cell Anemia), Tay-Sachs disease, Spinal muscular atrophy. A recent study proved that Parkinson's disease (PD) is a movement disorder of high prevalence in the elderly. It is characte-rized by a loss of dopaminergic neurons and the presence of intracytoplasmic inclusions named Lewy bodies. To date six familial PD-associated proteins have been identified so far. Some of them are implicated in the development of either autosomal dominant (alpha-synuclein and LRRK2 (leucine-rich repeat kinase 2/dardarin) or earlyonset recessive (parkin, DJ-1, PINK1 (PTEN-induced kinase-1) and ATP13A2) PD forms. A number of genetic studies have shown that 50% of the recessive forms are linked to mutations on parkin gene,24 X-LINKED DOMINANT TRAIT X-linked dominant disorders are caused by mutations in genes on the X chromosome. Only a few disorders have this inheritance pattern. Males are more frequently affected than females. Eg Hypophosphatemia, Aicardi Syndrome, Chokenflok Syndrome Y-LINKED Y-linked disorders are caused by mutations on the Y chromosome. Only males can get them, and all of the sons of an affected father are affected. Since the Y chromosome is very small, Y-linked disorders only cause infertility, and may be circumvented with the help of some fertility treatments. E.g. male infertilty Multifactorial and polygenic disorders Genetic disorders may also be complex, multifactorial or polygenic, this means that they are likely associated with the effects of multiple genes in combination with lifestyle and environmental factors. Multifactoral disorders include heart disease and diabetes. Although complex disorders often cluster in families, they do not have a clear-cut pattern of inheritance. This makes it difficult to determine a person’s risk of inheriting or passing

on these disorders. Complex disorders are also difficult to study and treat because the specific factors that cause most of these disorders have not yet been identified. On a pedigree, polygenic diseases do tend to “run in families”, but the inheritance does not fit simple patterns as with Mendelian diseases. But this does not mean that the genes cannot eventually be located and studied. There is also a strong environmental component to many of them (e.g., blood pressure). •

autism

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cancers

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Mental retardation 25

MUTAGEN mutagen is a substance or agent that causes an increase in the rate of change in genes (subsections of the DNA of the body's cells). These mutations (changes) can be passed along as the cell reproduces, sometimes leading to defective cells or cancer. Examples of mutagens include certain biological and chemical agents as well exposure to ultraviolet light or ionizing radiation.Mutagenesis is the formation of mutations. There are many types of mutations, some of which are harmful and others which have little or no effect on the body's function.. The problem of medication safety came to public attention largely through a chemotherapy error and the high toxicity and low therapeutic index of anticancer medications make safety in their prescription and administration critical. The anti cancer drugs are known to be mutagenic, teratogenic and carcinogenic therefore all efforts must be done to reduce accidents from cancer chemotherapy and to implement safe chemotherapy practices.26

ANTIMUTAGENIC HERBS Terminalia arjuna is an important medicinal plants widely used in the preparation of Ayurvedic formulations proved to have antimutagenic potential 27 Curcumin (diferuloymethane), a yellow colouring agent present in the rhizome of Curcuma longa Linn (Zingiberaceae), has been reported to possess anti-inflammatory, antioxidant, antimutagenic and anticarcinogenic activities. Curcumin exerts its chemoprotective and chemopreventive effects via multiple mechanisms 28 Mutagenic and antimutagenic properties of essential oil (EO) of basil and its major constituent Linalool, reported to possess antioxidative properties 29 Capsaicin (trans-8-methyl-N-vanillyl-6-nonenamide), a major pungent ingredient of red pepper, is reported to have antimutagenic and anticarcinogenic properties.30 Mucuna collettii Lace is a Thai herb with a long record of consumption among mature Thai males for the promotion of sexual potency proved to have antimutagenic potential.31 Anacardium occidentale L. species, popularly known as the cashew, which has several therapeutic indications, such as cicatrizing, antihypertensive, hypoglycemic and antitumoral properties proved to have antimutagenic activity.32 The major food items of Indian cuisine include rice, wheat, diary products, and abundant fruits and vegetables. Beside these, there are several kinds of herbs and spices as important ingredients, containing many phytochemicals with medicinal properties, adding taste to Indian cuisine. An impressive body of data exists in support of the concept that Indian food ingredients can be used in preventive strategies aimed at reducing the incidence and mortality of different types of cancers because of their antioxidative, antimutagenic and anticarcinogenic properties 33

Antioxidant An Antioxidant is a molecule capable of slowing or preventing the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons from a substance to

an oxidizing agent. Oxidation reactions can produce free radicals, which start chain reactions that damage cells. Antioxidants terminate these chain reactions by removing free radical intermediates, and inhibit other oxidation reactions by being oxidized themselves. As a result, antioxidants are often reducing agents such as thiols or polyphenols.34 Although oxidation reactions are crucial for life, they can also be damaging; hence, plants and animals maintain complex systems of multiple types of antioxidants, such as glutathione, vitamin C, and vitamin E as well as enzymes such as catalase, superoxide dismutase and various peroxidases. Low levels of antioxidants, or inhibition of the antioxidant enzymes, causes oxidative stress and may damage or kill cells. As oxidative stress has been associated with the pathogenesis of many human diseases, the use of antioxidants in pharmacology is intensively studied, particularly as treatments for stroke and neurodegenerative diseases. However, it is unknown whether oxidative stress is the cause or the consequence of such diseases. Antioxidants are also widely used as ingredients in dietary supplements in the hope of maintaining health and preventing diseases such as cancer and coronary heart disease. Although some studies have suggested antioxidant supplements have health benefits, other large clinical trials did not detect any benefit for the formulations tested, and excess supplementation may occasionally be harmful. In addition to these uses in medicine, antioxidants have many industrial uses, such as preservatives in food and cosmetics and preventing the degradation of rubber and gasoline Reduced levels of antioxidants such as carotenoids and vitamins A and E can increase DNA damage caused by free radicals. Exposure to radiation has been proposed to reduce levels of antioxidants that are used for DNA repair and this reduction may be responsible for increased levels of mutation in radioactively contaminated areas.