Chromosome translocation

In genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. It is detected on cytogenetics or a karyotype of affected cells. There are two main types, reciprocal (also known as non-Robertsonian) and Robertsonian. Also, translocations can be balanced (in an even exchange of material with no genetic information extra or missing, and ideally full functionality) or unbalanced (where the exchange of chromosome material is unequal resulting in extra or missing genes).

Of all the structural chromosome rearrangements, the most clinically significant is a translocation. Translocation involves two nonhomologous chromosomes (e.g., chromosome 2 and chromosome 6). Following a break in each of the chromosomes, and subsequent reunion, a segment of chromosome 2 becomes attached to chromosome 6 and vice versa.



Fig. 1.4. Translocation

In most cases, there is no loss or gain of chromosomal material during the exchange process. It is estimated that 1 in 500 individuals are normal translocation carriers.

On occasion, apparently balanced translocation carriers (on karyotype studies) are clinically abnormal. One explanation for this finding is that the break may have occurred in the middle of a gene which then results in the formation of an abnormally short, nonfunctional protein.

Individuals and families have been described with a translocation chromosome abnormality and a concurrent genetic condition; the genetic condition occurring because the chromosome breakpoint is in the midst of a gene. Research studies of these informative families have led to the localization of specific genes to specific chromosome segments (e.g., Duchenne muscular dystrophy on Xp21, neurofibromatosis on 17q, retinoblastoma on 13q14, etc.).

When an apparently balanced translocation is found on amniocentesis, chromosome studies are done on both parents. If the translocation is familial (one of the parents is a normal translocation carrier), then it is safe to assume that the fetus carrying a similar translocation is going to be normal. If the translocation is de novo (the parents are not translocation carriers), then the fetus has a 10% empiric risk of having a possible genetic abnormality. It may seem surprising that the risk is not much greater. This is because only 10% of the genome (genetic constitution of an individual) carries coding sequences for genes. The other 90% of the genome consists of noncoding sequences.

As previously noted, people who carry balanced translocations have a high risk of miscarriages and abnormal offspring because they are more likely to create unbalanced gametes. In meiosis, the normal chromosomes and the translocated chromosomes pair up by creating a cross figure (quadrivalent) (Figure 1.5). The chromosomes are distributed to the daughter cells by the centromeres which are attached to spindle fibers. The spindle fibers contract and draw the attached chromosome to each of the poles (segregation).

Fig. 1.5. Balanced reciprocal translocation. This figure shows the generation of a balanced reciprocal translocation between chromosomes 2 and 6, and formation of a quadrivalent in meiosis: t = translocated segment, c = centric segment, solid circle = chromosome 2 centromere, open circle = chromosome 6 centromere. Homologous centromeres are both solid or both open circles and nonhomologus centromeres are one solid and one open circle (see text).

There are three ways the chromosome pairs segregate. Adjacent 1 segregation occurs when adjacent chromosomes with nonhomologous centromeres move to daughter cells. Adjacent 2 segregation occurs when adjacent chromosomes with homologous centromeres move to daughter cells. Alternate segregation occurs when alternate chromosomes with nonhomologous centromeres move to daughter cells.

As shown by the six possible products in Figure 1.6, adjacent 1 and adjacent 2 segregation leads to unbalanced gametes, whereas alternate segregation leads to balanced gametes with a normal chromosome complement (in this illustration a normal 2 and a normal 6) or a balanced translocation complement (chromosome 2 with a piece of 6, and chromosome 6 with a piece of 2). Again from the illustration, two of the six possible gametes will lead to normal offspring, and four of the six gametes will result in chromosomally abnormal offspring. However, the actual risk for an abnormal offspring is highly variable, depending on the chromosomes involved and the size of the segments that are trisomic or monosomic.


Fig. 1.6. Six possible gametes following meiotic segregation of a reciprocal translocation

Noted above is the usual 2:2 segregation in meiosis where two chromosomes, of the original four chromosomes, are distributed to each of the two daughter cells. On occasion, a 3:1 segregation occurs in meiosis where three chromosomes go to one daughter cell and one chromosome goes to the other daughter cell; in effect an aneuploidy nondisjunction affecting translocation chromosomes.

The risk of abnormal offspring in translocation families depends to some extent on how the family was ascertained. If the family is being seen because they have a child with a chromosome abnormality, then obviously the unbalanced translocation karyotype is compatible with life. In such families empiric data suggests that the risk of recurrence is approximately 15%. If, on the other hand, the family comes to clinical attention because of multiple miscarriages, or as part of an infertility workup, then the unbalanced translocation karyotype is probably not compatible with life and the risk of abnormal offspring is probably low, around 1.5%.

A Robertsonian translocation is a particular type of translocation involving the reciprocal transfer of the long arms of two of the acrocentric chromosomes: 13, 14, 15, 21 or 22. On rare occasions, other non-acrocentric chromosomes undergo Robertsonian translocation, a reciprocal transfer of the whole long or short arms close to the centromere. A relatively common Robertsonian translocation is between chromosome 14 and chromosome 21. In meiosis, a trivalent is formed.

Fig. 1.7. Robertsonian translocation. This figure shows the formation of a Robertsonian translocation involving chromosomes 14 and 21, and the formation of a trivalent in meiosis. Alternate segregation results in gametes having either a normal (14 and 21) or balanced translocation (t(14;21)) chromosome complement. Adjacent segregation results in unbalanced gametes, either disomy (14 and t(14;21), or 21 and t(14;21)) or nullisomy (14 or 21).

With adjacent 1 and adjacent 2 segregation, the gametes produced result in trisomy 14, monosomy 14, trisomy 21 or monosomy 21 following fertilization. With alternate segregation, the resulting gametes have a balanced translocation 14;21 or the normal chromosome complement following fertilization. There are six possible gametes: one normal, one balanced translocation, and four unbalanced translocation complements.

Of the latter four, only trisomy 21 can come to term.

Theoretically, a person who carries a 14;21 translocation has a 1/3 chance of having a normal child, a 1/3 chance of having a child who carries a balanced translocation, and a 1/3 chance of having a child with Down syndrome. However, the actual risk for Down syndrome is much smaller because many of the trisomy 21 fetuses are spontaneously aborted. The empiric risk of having a child with Down syndrome is around 3 to 5% if the father is the carrier, and 10 to 15% if the mother is the carrier. The male translocation Down syndrome patient will have a karyotype of 46,XY,t(14;21) inferring that there are two normal chromosome 21s plus a third 21 that is attached to chromosome 14. The carrier mother of such a patient will have a karyotype of 45,XX,t(14;21) inferring a single chromosome 21, and a second 21 attached to chromosome 14.

Reciprocal Translocation: Philadelphia Chromosome

Can you identify the abnormal chromosome in this karyotype? This person has 46 chromosomes with a translocation of material between chromosome 9 and chromosome 22 (commonly known as the Philadelphia chromosome). Detailed studies of the Philadelphia chromosome show that most of chromosome 22 has been translocated onto the long arm of chromosome 9. In addition, the small distal portion of the short arm of chromosome 9 is translocated to chromosome 22. This translocation, which is found only in tumor cells, indicates that a patient has chronic myelogenous leukemia (CML). In CML, the cells that produce blood cells for the body (the hematopoietic cells) grow uncontrollably, leading to cancer.

The connection between this chromosomal abnormality and CML was clarified by studying the genes located on the chromosomes at the sites of the translocation breakpoints. In one of the translocated chromosomes, part of a gene called abl (pronounced A-ble) is moved from its normal location on chromosome 9 to a new location on chromosome 22. This breakage and reattachment leads to an altered abl gene. The protein produced from the mutant abl gene functions improperly, leading to CML.

Molecular biology

The exact chromosomal defect in Philadelphia chromosome is translocation. Parts of two chromosomes, 9 and 22, swap places. The result is that part of the BCR ("breakpoint cluster region") gene from chromosome 22 (region q11) is fused with part of the ABL gene on chromosome 9 (region q34). In agreement with the International System for Human Cytogenetic Nomenclature (ISCN), this chromosomal translocation is designated as t(9;22)(q34;q11). Abl stands for "Abelson", the name of a leukemia virus which carries a similar protein. The result of the translocation is a protein of p210 or sometimes p185(p simply stands for "protein"; the numbers represent the apparent molecular weight of the mutant proteins in kDa). The fused "bcr-abl" gene is located on the resulting, shorter chromosome 22. Because abl carries a domain that can add phosphate groups to tyrosine residues (tyrosine kinase) the bcr-abl fusion gene is also a tyrosine kinase. (Although the bcr region is also a serine/threonine kinase, the tyrosine kinase function is very relevant for therapy, as will be shown.)

The fused bcr-abl protein interacts with the interleukin 3beta(c) receptor subunit. The bcr-abl transcript is constitutively active, i.e. it does not require activation by other cellular messaging proteins. In turn, bcr-abl activates a number of cell cycle-controlling proteins and enzymes, speeding up cell division. Moreover, it inhibits DNA repair, causing genomic instability and potentially causing the feared blast crisis in CML.

identity of chromosome

Nucleosomes are the fundamental repeating units of eukaryotic chromatin, ^[1] with the exception of mature sperm. ^[2] They package DNA into chromosomes inside the cell nucleus and control gene expression. They are made up of DNA and four pairs of proteins called histones, and resemble "beads on a string of DNA" when observed with an electron microscope. The nucleosome hypothesis proposed by Don and Ada Olins^[3] and Roger Kornberg^[4][5] in 1974, was a paradigm shift for understanding eukaryotic gene expression. The proteins that make up the nucleosome are called histones. Histones H2A, H2B, H3 and H4 are part of the nucleosome while histone H1 is the linker DNA between the two nucleosomes.

Structure of the core particle

The crystal structure of the nucleosome has currently been determined with a resolution better than 2.0 Å,^[6] but most of the important features were known by 1997 with the publication of its structure at a resolution of 2.8 Å.^[7]

The nucleosome repeats, with some variations and exceptions, roughly every 200 base pairs (bp) throughout eukaryotic chromatin. The nucleosome core particle shown in the figure consists of about 146 bp of dsDNA wrapped in 1.65 left-handed superhelical turns around four identical pairs of proteins individually known as histones and collectively known as the histone octamer. The remaining 50 bp of the repeating unit consists of "linker DNA", dsDNA which separates the core particles.

Each of the four histones (H2A, H2B, H3, and H4) shares a very similar structural motif consisting of three alpha helices separated by loops. In solution, histones form pairs with identical copies of themselves and are referred to as dimers or histone-fold pairs. In the case of the H3 and H4 histones, they assemble further into tetramers, an association of two H3-H4 dimers, whereby buried charged groups of the same alpha helix on both of the H3 histones hydrogen bond to each other. The assembly of a nucleosome core particle occurs first by the attachment of the H3-H4 tetramer onto the dsDNA with the later association of two separate H2A-H2B dimers, a process that is likely to occur in a cooperative manner (i.e. both H2A-H2B dimers assemble onto the tetramer at once).

According to the crystal structure, the histone octamer likely interacts with the dsDNA around it roughly every 10 bp. Each of the four histone dimers contain three regions of interaction with the dsDNA. The central interaction site for each dimer is formed by an alpha helix from each histone in the pair pointing at a single phosphate group on the dsDNA to which they hydrogen bond. At positions 10 bp away on either side, a loop from both histones in the pair converge to hydrogen bond to other single phosphate groups. See the figure on the right for a visual representation. Two other interactions (for a total of 14) occur through the interaction of histone tails from each of the H3 histones. These interactions occur at the entry and exit points of the dsDNA wrapping around the nucleosome and help to clamp these regions onto the core particle.

Analysis of the structure of dsDNA wrapped around the histone octamer suggests that it is predominantly B-form, although more tightly constrained than free DNA due to its interaction with the octamer. Curvature into the superhelix comes primarily when either the minor or the major groove faces the octamer and therefore occurs in spurts of roughly 5 bp. Major groove bending around the octamer occurs smoothly. Minor groove bending is facilitated by arginine side chains inserted into the groove and occurs smoothly around the H3-H4 tetramer, but is kinked around the H2A/H2B dimer regions. The DNA is most tightly constrained in regions where it interacts with the double loop structures of the histone dimers mentioned above, which implies that there is more variability in how the DNA interacts with the double alpha helix structures of the histone dimers in order to accommodate the binding of different sequences.^[8]

Many proteins bind only to specific DNA sequences. Although nucleosomes tend to prefer some DNA sequences over others, they are capable of forming on just about any sequence. It has been shown that water molecules roughly double the number of histone-DNA interactions by acting as intermediates between atoms which would otherwise be too far apart to Hydrogen bond.^[9] It is the flexibility in the formation of these water-mediated interactions which allows for the histone octamer to wrap a very wide variety of DNA sequences.

Legend:

Nucleosome: Subunit of chromatin composed of a short length of DNA wrapped around a core of histone proteins.

The human genome contains about 3 billion nucleotide pairs organized as 23 chromosomes pairs. If uncoiled, the DNA contained by each of those chromosomes would measure between 1.7 and 8.5 cm (0.67 to 3.35 inches) long. This is too long to fit into a cell. Moreover, if chromosomes were composed of extended DNA, it is difficult to imagine how the DNA could be replicated and segregated into two daughter cells without breaking down.

In fact chromosomal DNA is packaged into a compact structure with the help of specialized proteins called histones. The complex DNA plus histones in eucaryotic cells is called chromatin.

The fundamental packing unit is known as a nucleosome. Each nucleosome is about 11nm in diameter. The DNA double helix wraps around a central core of eight histone protein molecules (an octamer) to form a single nucleosome. A second histone (H1 in the illustration) fastens the DNA to the nucleosome core. The total mass of this complex is about 100,000 daltons.

Nucleosomes are usually packed together, with the aid of a histone (H1,) to form a 30nm large fiber. As a 30nm fiber, the typical human chromosome would be about 0.1cm in length and would span the nucleus 100 times. This suggests higher orders of packaging, to give a chromosome the compact structure seen in a typical karyotype (metaphase) cell.

Genetic disorder

A genetic disorder is a condition caused by abnormalities in genes or chromosomes. While some diseases, such as cancer, are due to genetic abnormalities acquired in a few cells during life, the term "genetic disease" most commonly refers to diseases present in all cells of the body and present since conception. Some genetic disorders are caused by chromosomal abnormalities due to errors in meiosis, the process which produces reproductive cells such as sperm and eggs. Examples include Down syndrome (extra chromosome 21), Turner Syndrome (45X0) and Klinefelter's syndrome (a male with 2 X chromosomes). Other genetic changes may occur during the production of germ cells by the parent. One example is the triplet expansion repeat mutations which can cause fragile X syndrome or Huntington's disease. Defective genes may also be inherited intact from the parents. In this case, the genetic disorder is known as a hereditary disease. This can often happen unexpectedly when two healthy carriers of a defective recessive gene reproduce, but can also happen when the defective gene is dominant.

Currently about 4,000 genetic disorders are known, with more being discovered. Most disorders are quite rare and affect one person in every several thousands or millions. Cystic fibrosis is one of the most common genetic disorders; around 5% of the population of the United States carry at least one copy of the defective gene. Some types of recessive gene disorder confer an advantage in the heterozygous state in certain environments.^[1]

Genetic diseases are typically diagnosed and treated by geneticists. Genetic counselors assist the physicians and directly counsel patients. The study of genetic diseases is a scientific discipline whose theoretical underpinning is based on population genetics.

human chromosome

Chromosomes are organized structures of DNA and proteins that are found in cells. Chromosomes contain a single continuous piece of DNA, which contains many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. The word chromosome comes from the Greek χρῶμα (chroma, color) and σῶμα (soma, body) due to their property of being stained very strongly by some dyes.

Chromosomes vary extensively between different organisms. The DNA molecule may be circular or linear, and can contain anything from tens of kilobase pairs to hundreds of megabase pairs. Typically eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without nuclei) smaller circular chromosomes, although there are many exceptions to this rule. Furthermore, cells may contain more than one type of chromosome; for example mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosomes.

In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin. This allows the massively-long DNA molecules to fit into the cell nucleus. The structure of chromatin varies through the cell cycle, and is responsible for the organisation of chromosomes into the classic four-arm structure during mitosis and meiosis.

"Chromosome" is a rather loosely defined term. In prokaryotes, a small circular DNA molecule may be called either a plasmid or a small chromosome. These small circular genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origins. The simplest chromosomes are found in viruses: these DNA or RNA molecules are short linear or circular chromosomes that often lack any structural proteins.

his is a brief history of research in a complex field where each advance was hard won, and often hotly disputed at the time.

Visual discovery of chromosomes. Textbooks have often said that chromosomes were first observed in plant cells by a Swiss botanist named Karl Wilhelm von Nägeli in 1842.^[1] However, this opinion has been challenged, perhaps decisively, by Henry Harris, who has freshly reviewed the primary literature.^[2] In his opinion the claim of Nägeli to have seen spore mother cells divide is mistaken, as are some of his interpretations. Harris considers other candidates, especially Wilhelm Hofmeister, whose publications in 1848-9 include plates which definitely show mitotic events.^[3][4] Hofmeister was also the choice of Cyril Darlington.

The work of other cytologists such as Walther Flemming, Eduard Strasburger, Otto Bütschli, Oskar Hertwig and Carl Rabl should definitely be acknowledged. The use of basophilic aniline dyes was a new technique for effectively staining the chromatin material in the nucleus. Their behavior in animal (salamander) cells was later described in detail by Walther Flemming, who in 1882 "provided a superb summary of the state of the field".^[5][6] The name chromosome was invented in 1888 by Heinrich von Waldeyer. However, van Beneden's monograph of 1883 on the fertilised eggs of the parasitic roundworm Ascaris maglocephala was the outstanding work of this period.^[7] His conclusions are classic:

• Thus there is no fusion between the male chromatin and the female chromatin at any stage of division...
• The elements of male origin and those of female origin are never fused together in a cleavage nucleus, and perhaps they remain distinct in all the nuclei derived from them. [tranl: Harris p162]

"It is not easy to identify who first discerned chromosomes during mitosis, but there is no doubt that those who first saw them had no idea of their significance... [but] with the work of Balbiani and van Beneden we move away from... the mechanism of cell division to a precise delineation of chromosomes and what they do during the division of the cell." ^[8]

Van Beneden's master work was closely followed by that of Carl Rabl, who reached similar conclusions. ^[9] This more or less concludes the first period, in which chromosomes were visually sighted, and the morphological stages of mitosis were described. Coleman also gives a useful review of these discoveries.^[10]

Nucleus as the seat of heredity. The origin of this epoch-making idea lies in a few sentences tucked away in Ernst Haeckel's Generelle Morphologie of 1866.^[11] The evidence for this insight gradually acumulated until, after twenty or so years, two of the greatest in a line of great German scientists spelt it out. August Weismann proposed that the germ line was separate from the soma, and that the cell nucleus was the repository of the hereditary material, which he proposed was arranged along the chromosomes in a linear manner. Furthermore, he proposed that at fertilisation a new ombination of chromosomes (and their hereditary material) would be formed. This was the explanation for the reduction division of meiosis (first described by van Beneden).

Chromosomes as vectors of heredity. In a series of outstanding experiments, Theodor Boveri gave the definitive demonstration that chromosomes were the vectors of heredity. His two principles were:

The continuity of chromosomes

The individuality of chromosomes.

It was the second of these principles which was so original. He was able to test the proposal put forward by Wilhelm Roux, that each chromosome carries a different genetic load, and showed that Roux was right. Upon the rediscovery of Mendel, Boveri was able to point out the connection between the rules of inheritance and the behaviour of the chromosomes. It is interesting to see that Boveri influenced two generations of American cytologists: Edmund Beecher Wilson, Walter Sutton and Theophilus Painter were all influenced by Boveri (Wilson and Painter actually worked with him). In his famous textbook The Cell, Wilson linked Boveri and Sutton together by the Boveri-Sutton theory. Mayr remarks that the theory was hotly contested by some famous geneticists: William Bateson, Wilhelm Johannsen, Richard Goldschmidt and T.H. Morgan, all of a rather dogmatic turn of mind. Eventually complete proof came from chromosome maps – in Morgan's own lab!