Chromosomal Mutations

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Abstract:

Cytogeneticists have catalogued various types of structural changes in the chromosomes of organisms termed as chromosome rearrangements. These changes in addition to the changes in the number of chromosomes in their cells, facilitate a change in the genotype and the resultant phenotype of the individual. Such changes are called chromosomal mutations and their aetiological dimensions along with their implications have been discussed in this study.

Introduction:

The genetic information is carried through generations by discrete entities called chromosomes", which are characteristic of any particular species. Diploid human cells contain 46 chromosomes comprising of 44 autosomes and two sex chromosomes, which are XX in females and XY in males (Snustad, Peter D. and Simmons, Michael J 2012). Synthesis of proteins from ribonucleic acid (RNA) which in turn gets coded by the deoxyribonucleic acid (DNA) happens to be the basic criteria in molecular biology. Conversion of said genetic information coding a stipulated function in the DNA into a copy of RNA facilitates synthesis of protein that is attributable for development of a particular phenotype. 

According to Weatheral & Connor et al, “variations in phenotype might result from a number of causes ranging from both normal and abnormal differences, (1) Cytogenetic disorders, which result from changes in the chromosome structure (deviation in chromosomal morphology) or number (deviation from the normal number). The former arises either by centromeric misdivision or chromosome breakage and the latter arises by chromosome malsegregation. (2) Mendelian disorders, which result from a mutation at a single genetic locus, causing an abnormal allele. (3) Polygenic (multifactorial) disorders which could result from (a) the cumulative effect of several different genes, (b) multiples allele at a single locus, or (c) a combination of genetic and environmental factors and (4) teratogenic agents (Weatheral, 1991; Connor et al, 1991). Our subject of interest is confined to the first cause mentioned above, namely chromosomal mutations that have been classified under cytogenetic disorders.

Chromosome mutations due to variations in chromosome structure

The fundamental structure of a chromosome is subject to mutations, which will most likely occur during crossing over at meiosis. Chromosome mutations due to variations in chromosome structure are brought about by deletions, duplications, translocations, inversions and transpositions that are imposed on the same by radiation, chemicals or certain infections leading to breakage of the DNA.

 Such a breakage on being followed by a loss of the broken segment is called deletion; on its insertion into its respective homolog is called duplication; on its attachment to a different non-homologous chromosome is called translocation; and on its reattachment to the same chromosome that lost it in reversed orientation is called as inversion. These are some of the ways in which the chromosome structure can change, leading to a detrimental change in the phenotype and the genotype of the organism.

Chromosome mutations due to variations in chromosome number

Apart from these mutations that are caused due to changes in the chromosome structure, the second major category of mutations is aneuploidy, where the chromosome number is abnormal. While individuals with a normal set of chromosomes are called euploids, aneuploid refers to a certain organism whose chromosome number differs from the standard by part of a chromosome set. The nomenclature of aneuploids is based on the number of chromosomes and can thus be termed as monosomic (2n-1), disomic (n+1), trisomic (2n+1) or nullisomic.

While aneuploidy refers to a numerical change in part of the genome, usually just a single chromosome, polyploidy refers to a numerical change in a whole set of chromosomes. While aneuploidy implies a genetic imbalance polyploidy does not. Meiosis of high order, although appears to be a tightly regulated process, is prone for errors during the creation of the gametes. These errors result in the failure of chromosomes ranging from the homologous chromosomes during meiosis I or sister chromatids during meiosis II to segregate properly, a process called nondisjunction (NDJ). Non disjunction therefore occurs when the spindle fibers fail to separate during meiosis, resulting in some gametes with one extra chromosome while other gametes lacking in a chromosome. Non disjunction in mitosis or meiosis is the main reason for occurrence of aneuploidy.

Discussion:

Sex chromosome abnormalities can mean many things but commonly it refers to all abnormalities of sex chromosome number and structure. Chromosomal errors may be divided into: (A) Structural changes. (B) Numerical changes.

Structural changes : Depending on the changes that have been brought about in the chromosomal structure, the effects produced happen to vary. For example, in deletion where the broken segment is lost, the effects produced are serious if the missing segment contains genes that code for essential development. This is what is seen in cri-du-chat syndrome in humans where part of chromosome 5 is deleted leading to severe mental retardation and malfunctions.  In duplication, the effects may be mostly harmful while in some rare cases they have been reported to be beneficial in that they bring about superior evolutionary changes. In translocation, which is mostly reciprocal involving exchange of segments between two non-homologous chromosomes, effects may range from the chronic myelogenous leukemia (CML) and infertility to down’s syndrome. Inversions of the balanced kind do not produce any physical or mental abnormalities, as there is no net gain or loss of total genetic information.  On the other hand if they are unbalanced they produce physical or mental abnormalities on account of the genetic information lost.

Numerical chromosomal aberrations: If a haploid gamete or a diploid cell lacks the expected number of chromosomes ( n or 2n respectively), aneuploidy exists. When the complement contains one addition of a whole chromosome (2 n+l), trisomy exists. The term can be applied to sex chromosomal abnormalities (e. g. 47, XXX) but if an excess number of sex chromosomes represent it is customary to use the term polysomy. If one entire chromosome is missing (2n-1) monosomy exists (i. e. 45, X). Polyploidy happens when more than two haploid complements represent within a single cell. Triploidy (3n-69 chromosomes) and tetraploidy (4n--92 chromosomes) are the most common types of polyploidy. Both conditions are seen in spontaneous abortions and stillbirths and are essentially incompatible with life (Emery and Muller, 1992)

According to Therman and Susman, “Aneuploidy results from non- disjunction in either the meiotic divisions of the parents ( during gametogenesis) or in the early cleavage division of the affected individual (during embryogenesis). Aneuploidy for more than one chromosome is the result of non-disjunction in both meiotic divisions, non-disjunction in the meiosis of both parents (which must be a rare coincidence), multiple non- disjunction in the same mitosis or meiosis, or abnormalities in more than one mitosis (Therman and Susman1, 1993)”. The resulting NDJ produces gametes with the incorrect number of chromosomes, known as aneuploid cells. This aneuploidy can either have minimal effects on the cell in which the organism can survive or it can cause irreversible defects and/or death (Hawley, R.S et al 1993, Ashburner, M.et al 2005 ) Nondisjunction occurs in most organisms, but is usually a rare occurrence. It is, however, more common in humans than other organisms.

Aneuploidy is the main cause of birth defects as well as many other numerous human diseases, including Down syndrome, Turner syndrome, Klinefelter syndrome, and others. The meiotic NDJ rate in human males is only 1-2%, but the rate in females is approximately 20% (Hassold, T.et al 2001). Nine out of 10 birth defects caused by NDJ occur in the female germline during meiosis I (. Oliver, T.R., et al 2008). While it is more probable for non disjunction to occur during crossing over at meiosis I (anaphase 1)  due to improper homologous associations during prophase 1 and metaphase 1, in anaphase II or mitosis, such a criteria is not that mandatory and it is enough if the centromere splits properly. Nondisjunction at meiosis I is normally caused due to the failure to form the tetrad in anaphase I. 

According to Hassold, T.et al 2001, “These meiotic nondisjunction events accounts for almost half of the miscarriages seen during the first trimester of pregnancy, since these aneuploid zygotes generally fail to survive. Sometimes, however, aneuploid zygotes actually give rise to viable, but partially defective, embryos.”(Hassold, T.et al 2001). For example, embryos with a third copy of chromosome 21 (trisomy 21) develop into offspring with Down syndrome, a condition associated with mental retardation and altered physical appearance, and is the most common human birth defect. Indeed, studies of Down syndrome according to the National Institute of Health (www.nichd.nih.gov)  have found that the majority of the cases observed resulted from trisomy 21, with 88% of nondisjunction events coming from the female germline, thus, making the case for the studying of female meiotic nondisjunction a significant area of concern and future research (Lamb, N.E et al 2005).

Ford et al (1959) showed that individuals with Turner syndrome had a single X chromosome and were females, while patients with Klinefilter syndrome were shown to have two X chromosomes and a Y and were males (Jacobs and Strung, 1959). Turner syndrome in human beings especially females is caused due to 44 autosomes + 1X. Autosomes monosomic complements are lethal and die in utero. There are exceptions as in the case of Down’s syndrome which is caused by trisomy 21 or the extra copy of chromosome 21, where it is likely to survive well after adulthood. Trisomy 13, on the other hand produces Patau syndrome, whereas Trisomy 18, produces Edward’s syndrome. The 47, XXY karyotype is also a viable trisomy in humans called the Klinefelter syndrome. The sex chromosomes have a much wider range of viable aneuploidy than do the autosomes. The incidence of 45, X (Turner syndrome) individuals seems to be independent of maternal age. Experimental evidence has shown that 70 to 80% of patients with 45,X retain the maternal X chromosome (Lorda-Sancheezt al, 1992; Mathur et al, 1991). On the other hand, the incidence of 47, XXX and 47, XXY individuals increases with maternal age. In these cases non-disjunction apparently occurs mainly in maternal meiosis.

Individuals with three X chromosomes (47, XXX) do not seem to form a well-defined syndrome. They are often mentally retarded and are fertile (Polani PE, 1977). Females with more than three X chromosomes(e.g. 48,X XXX) suffer from severe mental retardation and several somatic anomalies, yet their sex development is usually normal (Nielsen et al, 1977). In the group of sex chromosome abnormalities with a male phenotype the 47, XXY and 47, XYY conditions occur approximately equal at birth. Individuals with 47, XXY karyotype form a well-defined syndrome (Klinefelter syndrome). Klinefelter syndrome is a condition in which affected men are sterile, have very small testes, sometimes have gynecomastia and may be mentally retarded( Connor and Ferguson-Smith,1991). Men with a 47,X YY sex chromosome constitution have been reported to be higher among penal institutions for the mentally abnormal (2/1000) and in mentally deficient adult males (3/1000) (Connor and Ferguson-Smith, 1991). Males with one Y chromosome and more than five X chromosomes are mentally retarded and display various other symptoms.

Conclusion:

Chromosomal mutations are brought about by changes in chromosomal structure and number. Depending on the extent of such changes, the resulting genotype is bound to alter the phenotype of an individual. Mental retardation, leukemia, sterility, altered physical appearance, abortions and still birth and hordes of other deleterious effects can be encountered in such mutations, while very rarely they bring about beneficial effects pertaining to evolutionary change. 

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