Last Updated: 5/31/2001
The Muscular Dystrophy Foundation of South Africa is a non-profit organisation which supports people affected by muscular dystrophy and neuromuscular disorders.
WHAT IS GENETICS?
Genetics is the study of how traits/conditions/disorders are passed from one generation to the next. When we look at families some disorders seem to follow a pattern of being passed from generation to generation. This is due to the fact that genes (see 4) are located on chromosomes (see 3) and are passed from parents to their children. Since these genes carry the message of, for instance, a fault in a gene causing muscular dystrophy (MD), the MD is passed from parents to their children along with the gene. When conditions are passed from generation to generation, they are said to be inherited, or heritable.
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The human genome is all the genetic material that is present in a cell. Inside the nucleus of a human cell there are 46 chromosomes (see 3) that are divided into 23 pairs. The chromosomes are numbered (in their pairs) from large to small – thus chromosome (chr) 1 being the largest and chr 22 being the smallest. The 23rd pair of chromosomes is the sex chromosomes. The human genome is analogous to a set of encyclopaedia, with each set of chromosomes represented by a volume in the set. Therefore, our genome is equivalent to a set of encyclopaedia, with 23 volumes. |
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The nucleus is called an organelle of the cell. There are many organelles in the cell and each organelle has a unique function. Only one other organelle, the mitochondrion (also known as the power house of the cell - as it manufactures most of its energy), also contains DNA (see 12). This type of DNA is called mitochondrial DNA (mtDNA). If we add the mitochondrial genome to our set of encyclopaedia, we see that we actually have 24 volumes in the set. Human genomic DNA thus includes both nuclear DNA and mtDNA.
Each nucleus in the body contains 46 chromosomes, divided into two sets - 23 pairs of chromosomes in each set. One set (23 chrs) is inherited from the father and one set (23 chrs) is inherited from the mother. The 46 chromosomes are thus comprised of two copies of chromosome 1, two copies of chromosome 2, etc. (In the picture above, only three sets of chromosomes are indicated for illustrative purposes, but we know that there are 23 sets of chromosomes in the cell nucleus.) These sets are numbered 1-22 and are known as the autosomes, with the extra set referred to as the sex-chromosomes. The sex chromosome set can either be a true pair XX (in females) or an odd pair XY (in males). A female individual would thus have the following chromosomes: 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 6, 7, 7, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 13, 13, 14, 14, 15, 15, 16, 16, 17, 17, 18, 18, 19, 19, 20, 20, 21, 21, 22, 22, X, X - a total of 46 chromosomes.
The chromosomes are made of DNA (see 5), which is packaged in a particular way. In general human chromosomes have two arms: a long arm (designated as the q arm) and a short arm (designated as the p arm). The arms are separated by a region we call the centromere.
In the example of the set of encyclopaedia (the human genome), each volume in the set represents a pair of chromosomes. The set of encyclopaedias thus consists of 23 volumes. Each volume has many (500 – 1000) pages that contain information that is written in sentences (genes, see 4), and each sentence contains many words (DNA code, see 5).
WHAT IS A GENE?
Genes are the so-called "units of heredity", thus the "unit" in which genetic information is passed from one generation to the next. When the four building blocks (or molecular letters) of DNA (see 5) are strung together in a specific order, we call this "specific order of DNA bases" a gene. A gene is therefore a piece of DNA that carries a specific message. This specific message codes for a specific protein, or part of a protein, with proteins being the basic building blocks of life. Since we have two copies of each of our autosomes we have two copies of each of our autosomal genes. If we imagine our two copies of chromosome 1 lying next to each other, we see that at equivalent positions on the two copies of chromosome 1 we have the same genes. For instance, if we moved equal distances down the length on each of the copies of chromosome 1, we might find a gene that we will call M. Gene M will thus be present on each copy of chromosome 1 - so we have two copies of gene M (one inherited from mother and father respectively). Since the two copies in each set of chromosomes carry the same genes - we have two copies of each gene. This opens up the possibility for three types of situations.
One can have (if we define, N as the normal gene and M as the mutant gene):
- two copies of a normal/healthy gene: NN,
- one copy of the normal gene and one copy of the gene carrying a mutation (or gene fault – which is the equivalent of a spelling mistake in one word in our set of encyclopaedia): NM,
- a mutation in both of our copies of the gene, thus two copies of the mutation: MM.
Some disorders require only one faulty gene (equivalent to what we see in b) to result in a disorder - which is called dominant inheritance. Other disorders require two faulty genes (what we see in c) to be expressed (be present) - which is called recessive inheritance. (See 8-11 for more information on specific patterns of inheritance)
In our example of the set of encyclopaedias, a gene is equivalent to a sentence (on a page in one volume of the set). If we had to search for a specific gene, we would have to look through the entire set, page by page and sentence by sentence. Alternatively, if we know in which volume the page is located (on which chromosome the gene is) we only have to look in that particular volume – still a daunting task in a book with 500-1000 pages.
Genes are, amongst other things, responsible for directing the production of proteins. Proteins are generally of two classes: structural or functional. Structural proteins help build the structures within the cell such as cell membranes. The functional proteins are the "worker bees" of the cell and perform functions that are essential to normal cell function. For instance the functional proteins can help to shuttle chemicals around in the cell, or to activate certain chemical compounds. Proteins are therefore the building blocks of life that are coded for by genes that are found on chromosomes
WHAT IS DNA?
DNA is the abbreviation for deoxyribonucleic acid, which is the name of the chemical structure that our genetic material is packaged in. DNA is made up of the its four building blocks, also known as DNA’s four molecular letters: A (adenine), C (cytosine), G (guanine) and T (thymine). We refer to these building blocks as a DNA bases or nucleotides. The length of a piece (or fragment) of DNA is therefore measured as the number of nucleotide bases it contains, ie. 150 000 bases, or 150 kilo bases (kb), indicating how many bases are strung together to form the fragment of DNA. The four DNA bases are unique in that two bases are always complementary to the other two. A is complementary to T, and C is to G. Complementary means that the chemical structures of the A and T bases can "pair" (form bonds) between each other, in the process forming what is referred to as a "base pair" (bp). A fragment of DNA can thus "pair" with another fragment that has a complementary sequence (order of bases). This complementary feature means that we generally find DNA as a double stranded structure, with complementary bases joined by chemical bonds. Thus bases of DNA that are strung together are called strands of DNA, and two complementary strands can form a double stranded structure. The chemical structure of DNA is such that the complementary bases ultimately form, not only a double stranded structure, but a double stranded structure with a specific shape - a helix. DNA is therefore said to have a double helix structure.
When the four molecular letters are strung together in a specific order, we call this "specific order of DNA bases" a gene. More importantly, a gene is therefore a piece of DNA made up of a specific order of DNA bases that carries a specific message. These messages (sequence of bases) are said to "code" for proteins. This code refers to the fact that the message in the DNA is read in groups of three letters at a time, called a codon, like a sentence consisting of three letter words.
As an example:
Consider the following sequence of molecular letters: thecatatetherat
This is a sentence that, like DNA, is read three letters at a time in order to make sense: the cat ate the rat.
In the example of the encyclopaedia, the DNA message (code) is thus the sentences (gene/s) written on the pages of each of the volumes (chromosomes) in the set (genome), and each sentence is made up of words (codons). If we thus had to find a particular word (code) in the genome, it would be easier to find if we knew exactly which sentence (gene) on a particular page in a specific volume (chromosome) to look at. Alternatively we would have to read through the entire set just to find one word! Scientist face a similar situation when looking for a specific mutation (spelling mistake) in one codon (word) of a particular gene (sentence) on a specific chromosome (volume) in the human genome.
WHAT IS A MUTATION?
In DNA a mutation refers to an alteration in the DNA – an alteration being a fault or spelling mistake in the DNA. For instance if the normal DNA code is "CCG", a change of the first C to "TCG" would represent a mutation. Not all mutations cause a dramatic change in proteins - thus leading to a disorder. Sometimes a mutation can be located in a region of the gene that does not code or control the gene, or does not affect the code of the DNA in a deleterious manner. Some of these non-harmful mutations are called polymorphisms and account for some of the genetic diversity we observe in populations. There are many types of mutations, some of which involve deletions (a fragment of DNA is missing), insertions (a fragment of DNA is inserted) or substitutions (a fragment of DNA is substituted with another). In some cases the altered fragments of DNA in question are long - and these large rearrangements of DNA can then alter the meaning of the message in the DNA. In some instances the altered DNA fragment is short, only one base - in which case the mutation is referred to as a point mutation. If we consider only this type of mutation
(alteration of one base) we see that the following can occur if one base in a code is altered:As an example, assume the normal code is the sentence: the cat ate the rat
Deletion: Here one base (point mutation), for instance the third "t" (in ate) is deleted. Now the sentence reads "the cat aet her at…". Since we can only read this sentence in words of three letters each - the original meaning of the sentence is lost and there is no sense in the meaning of the sentence after the mutation (deletion).
Insertion: One base is inserted, for instance a "t" in the first word (code), just before the "e". Now the sentence reads "tht eca tat eth era t…". Again there is no sense in the meaning of the sentence after the mutation (insertion).
Substitution: One base can be replaced/substituted with another. If the "e" (in ate) is substituted with "r" the sentence now reads "the cat arr the rat".
Point mutations can further be classified as:
Missense mutation: If the second "t" (in cat) is altered to "r" (via a substitution), the sentence reads "the car ate the rat"! This changes the meaning of the sentence, but it still makes sense. However, it now means something different than the original (normal code) sentence. The CAR now ate the rat, not the CAT. Sometimes in DNA the last letter of the code is not very important, the protein will look a little different, but will still function. This implies that this type of mutation is not always deleterious.
Nonsense mutation: If the first "t" of the sentence is missing (via a deletion) the meaning of the sentence is lost completely. The sentence now reads "hec ata tet her at..". Let us assume that the word 'ata' in our DNA language means END (it is actually called a stop codon in DNA)). In other words the enzyme reading the message will stop at the word 'ata' since it has, according to the DNA rules, reached the end of the message. Now the sentence is only two words long "hec ata". This means that the protein that will be made from this message will be shorter, which in turn means that it will not be able to function properly. Here we have a type of mutation that can have a dramatic effect on the protein and therefore on the function of proteins.
Frameshift mutation: Since the DNA sentence is read in letters of three (also called a frame – with three letters in each codon or frame), the original sentence makes sense. The cat ate the rat - we know what that means. If, however, the frame is changed due to a deletion (or insertion) the code looks quite different, as we saw in the example of "deletions" above: "the cat aet her at…". This means that the protein will look the similar to the original one, until after the code of 'cat'. After this position the protein will look different, and this type of mutation can have a dramatic effect.
If we consider the example of the encyclopaedia, we see that a mutation can be the equivalent of a typing mistake (one letter instead of another) in a sentence. One of the words (codons) contains an alteration in one of the letters (bases), if the mutation is a point mutation.
Let us now consider our mutation information together with some specialised knowledge of what we know about genes. A gene is just a specific order of the four bases of DNA strung together. Genes also contain special messages, apart from the information of which protein or part of protein they code for. For instance at the start and end of genes there are specific sequences of message that indicates "start here" and "end here". If a mutation affects, let's say the "end here" message, it is possible for a fragment of DNA (that was not originally part of the gene) to be read as 'part of the gene'. The DNA message will therefore be much longer than in the normal copy of DNA. This DNA message (this gene) might have coded for a small protein that did a specific job in the cell, ie. due to its small size it could fit perfectly into a particular slot in the cell. However, the mutation now moved the "end here" signal, and the gene is incorrectly read as being much longer. The protein it codes for is consequently much bigger and can no longer fit where it was supposed to - thus cannot perform its original function. Another possibility is that the mutation might create an "end here" signal much earlier in the gene. Now the resultant protein will be smaller (truncated), due to the fact that the "end here" signal is much closer to the beginning. From these examples it is clear that whilst mutations can have mild effects, but they can sometimes have devastating effects.
WHAT IS A SPONTANEOUS MUTATION?
New mutations can occur in any of the types of inheritance patterns described below. When a new mutation occurs, the affected individual may be the first one affected in the family, and thus no family history of the disorder will exist. It means that the mistake (mutation) has been introduced earlier in the sperm or egg that formed the individual or one of his parents. While it is usually not known what causes a new mutation to arise, environmental factors such as irradiation (ie. X-rays) and chemicals have been implicated.
WHAT IS INHERITANCE?
Inheritance refers to the fact that traits or disorders are passed from one generation to the next. Traits like eye and hair colour, or muscular dystrophy, are said to be inherited.
WHAT IS AUTOSOMAL INHERITANCE?
When a gene is located on one of the autosomes (not the sex chromosomes), this gene is said to be inherited in an autosomal manner. Autosomal inheritance thus indicates the inheritance of genes/mutations that are located on chromosomes 1-22.
In this form of inheritance an affected parent passes on the defective gene. Only one copy of the faulty gene is sufficient for the disorder to manifest (be present). With each pregnancy there is a 50% chance of passing on the defective gene to the child. The child and the affected parent can be of either sex.
WHAT IS AUTOSOMAL RECESSIVE INHERITANCE? (See figure 2)
In this form of inheritance both parents pass one copy of the defective gene on to the child. An affected child thus requires two copies of the faulty gene before the disorder will be present (manifest). Both parents, each with one copy of the faulty gene, are so-called "carriers" – they carry the faulty gene but are not affected by it. Carriers also have a normal copy of the gene that compensates for the faulty copy, and will thus not have any problems.
With each pregnancy there is a 25% chance of having an affected child. If only one of the parents passes on a copy of the defective gene, the child will be a carrier – like the parent. Carriers are generally not symptomatic.
WHAT IS X-LINKED INHERITANCE?
This form indicates that the gene in question is located on the one of the sex-chromosomes, the X chromosome. Such genes are said to follow an X-linked pattern of inheritance. Females have two X chromosomes, and for this reason if one of the X chromosomes carries a defect (fault/mutation) the other X chromosome (normal) can compensate for it - if the defect is not a dominant trait. Males have only one X chromosome with no "backup" copy of a normal gene on a second X chromosome. For this reason, when a male inherit an X-linked disorder (mutation) he will be affected.
A woman with this rare type of disorder can pass it to both her sons and her daughters. An affected father passes it on to all his daughters, but to none of his sons as they inherit his Y chromosome.
WHAT IS X-LINKED RECESSIVE INHERITANCE? (See figure 3)
A woman with one copy of an X-linked mutation usually does not show signs of the X-linked disorder. However, she is a carrier and can pass this mutation on to her sons. Her sons will each have a 50% chance of inheriting the X-linked mutation and being affected. Her daughters will each have a 50% chance of inheriting the mutation and thus being a carrier of the disorder. All the daughters of an affected male would be carriers since they all inherit an X chromosome from their father. However, none of his sons would be affected since they inherit his Y chromosome.
| WHAT IS MITOCHONDRIAL INHERITANCE?
This refers to inheritance of the DNA that is located in the mitochondria (mtDNA). This type of DNA is also a double stranded helix, but is circular and is found in the cytoplasm of the cell and not in the nucleus (see 2). This is the only circular DNA in the human genome. Mitochondria are organelles in the cell that are responsible for energy production in the cell, and are the only organelles (besides the nucleus) that house DNA. |
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During fertilisation the egg is huge in size, when compared to the sperm. The sperm carries only enough mitochondria to produce energy for propelling itself. By comparison, the cytoplasm of the egg is full of mitochondria. This implies that after fertilisation, almost all the mtDNA originate from the mother. The mtDNA contributed by the father is negligible by comparison. Thus we say that mtDNA is inherited exclusively from the mother (maternal parent). Mitochondrial disorders, due to defects in mtDNA, are thus inherited exclusively from the mother.
However, the inheritance of mitochondrial disorders is complex since the number of mutant mitochondria is not constant from cell to cell. Also, during cell division the number of mutant mitochondria is not distributed evenly to all the daughter cells. In addition, some mitochondrial disorders display a threshold effect, requiring a certain percentage of mutant mitochondria before the disorder will be present. This threshold (%) is not constant for all the mitochondrial disorders. Since mitochondria are responsible for energy production of the cell, when the mtDNA is compromised (ie. via mutations) tissue that have a high energy requirement are generally first affected, such as skeletal muscle, eye, brain, heart muscle, etc.
WHY IS IT TAKING SUCH A LONG TIME TO FIND ALL THE MD MUTATIONS?
The different types of muscular dystrophies are not all due to the same type of mutation. In some cases large rearrangements interfere with the message in the DNA, whilst in others the alterations are point mutations. Let us consider the case of a point mutation that causes one type of muscle disorder called Malignant Hyperthermia (MH), and assume we are faced with the entire encyclopaedia of the human genome (all the genetic material) in which we have to find the mutation. We do not know in which volume to look, we do not know on which page to look, and we also do not know what type of point mutation to look for. Where do we start? It would be easier to look for the mutation if we knew in which volume, or even which page it was located on (it would save us the effort of looking through all 24 volumes)! We therefore need a good strategy to find the gene or the mutation. It is also possible that every patient, or group of patients, may have a different error/mutation, which complicates the process of mutation screening in future.
Candidate gene approach: One strategy could be to use knowledge that we already have - we know (we won't answer the question HOW we know, for the moment) that in volume 19 there is a chapter (region on the chromosome) with many pages containing many sentences (genes) that are involved in muscle function. We have decided to look for a specific gene involved in muscle function, so we have picked a candidate, or "possible", gene from all the other genes in the genome. Now we can decide to look at volume 19 (chromosome 19), in the chapter (region on the chromosome) where many of the muscle function genes (sentences) are located. This narrows down our search a great deal. We no longer have to look through the entire set of 24 volumes of the human genome. We only need to read through a few pages, to hopefully find the sentence (gene) containing the mutation, and recognise the mutation (spelling error). At the DNA level this strategy can take years, because we first have to find a way to KNOW that the muscle function genes are on chromosome 19. In MH, we knew this because the animal model (the pig) has a region on porcine chromosome 6 that is analogous to the human region on chromosome 19. This was known in 1986. We also knew that MH in pigs was due to a mutation in a specific gene (called RYR1) that is located within this region. The actual pig mutation was reported in 1991. In human MH the analogous (called syntenic between different species) region is located on chromosome 19q and the RYR1 gene was also localised, in 1991. Even though an animal model was available, it took five years to find the equivalent human gene and mutation. Despite earlier thoughts that this mutation would explain all human MH, as it did in the pig model, we now know that more than 20 mutations in the RYR1 gene on chromosome 19q, and at least six other genes in the human genome cause MH. A seventh locus was only reported in 1997. Today (in the year 2000), some 14 years after it was first investigated, the genetic basis of human MH is still not completely understood. However, those mutations that are currently known can be detected via a direct detection strategy. This means that the presence of the mutation is detected directly using molecular genetic techniques.
Genome screen: Not all muscular dystrophies have animal models from which to learn, however, and often one has to scan through the entire set of chromosomes (all volumes of the encyclopaedia of the human genome) in search of the genetic alteration. This approach is called a genome screen, and DNA markers are used to cover the entire genome (all the volumes) at regular intervals. One feature of this type of DNA marker is that we know exactly where in the human genome these markers are located. All we need to find out is which of the many available DNA markers is closest to the mutation. The markers are spaced at intervals, the same as saying we will look in each 50th page of the all the volumes of the human genome. Fortunately DNA is inherited from one generation to the next. Often when the DNA marker and the mutation are located close to each other, the two are inherited together. The closer these two are on the chromosome, the more we would expect to see them being inherited together. Here our analogy of the set of encyclopaedia does not work as well - although we can make the following assumption to allow for "inheritance" in the encyclopaedic world.
Let us assume that there is one set of encyclopaedia for every person on the planet, and that whenever we look at a set where the mutation is present, a page is coloured in blue IF the mutation is located close to a specific DNA marker. Here we will use our imagination and imagine that some DNA marker (that we will call marker B) has the ability to change the colour of the page containing the mutation to blue. To find the mutation we now have to look at many sets of encyclopaedia, the same as we would scan many families or individuals affected by the particular disorder with DNA markers. We would have to find sets where one of the volumes has a blue page, the same as we would scan the DNA from members of the family to find a marker that is always present in affected individuals. If we find many sets of encyclopaedias where the blue page is in volume 19, chapter 13, we can assume that it is due to the fact that the disease-causing mutation and marker B are located in close proximity on chromosome 19. We would therefore scan the DNA of affected (and other) family members to where the DNA marker is segregating with the disorder. The marker and the disorder is now said to be "linked" and we would infer from this that the disease-causing gene is located close to the marker. We can now search for genes in that region, and then determine the sequence of the gene – by looking at the code and compare it to the normal code, in order to identify the disease-causing mutation. Many mutations have been detected successfully with this type of strategy, but it can take years to identify a disease-causing gene as we only know the approximate area where the gene is located.
Other strategies: There are many different strategies to follow in our search for the mutations and genes that cause muscular dystrophy. The above indicate why these strategies often take a long time. To date the longest time it ever took to find a mutation, after having identified the region on the chromosome where it should be located, is for one of the muscular dystrophies. In 1990 a region on chromosome 4q was identified to contain the gene responsible for Facioscapulohumeral Muscular Dystrophy (FSHD). To date (in the year 2001), no specific mutation has been identified although it is known that a 3.2 kb repeat on chromosome 4q35 is linked to FSHD. The longer the repeat deletion, the more severe the FSHD individual is affected. Thus diagnosis of FSHD at the DNA level is done via an indirect detection method. The mutation is not detected (it is not known yet) but the repeat is linked to the mutation, and we thus detect deletions of the repeat and infer the mutation status from it. As we can see from the above, the answers that we want for all the muscular dystrophies (in terms of their genetic roots) will take many years and a lot of research effort.
THE ROLE OF THE MUSCULAR DYSTROPHY FOUNDATION (MDF) IN SOUTH AFRICA
The MDF supports individuals affected by muscular dystrophy and their families by offering emotional support, information - including a series of fact sheets, referrals to genetic counselling and other clinics, formation of support groups, assistance with special equipment, when possible, as well as financial support for research projects in muscular dystrophy in South Africa. Creating public awareness for muscular dystrophy is also an important aspect of our work, since the MDF relies solely on contributions from its members and other donors to provide an on-going support service. Through our newsletter members are kept informed of all the activities and receive national and international research updates. Please contact any office of the MDF if you require information about any of our activities or programmes.
SUPPORT GROUP OR CONTACT PERSON
For more information on Genetics as it relates to MD, please contact any of our offices and ask to be referred to a genetic counselling clinic.
WHERE CAN WE FIND ASSISTANCE?
Please contact your local MDF office for further information.
National Office Cape Branch
P.O. Box 1535, Pinegowrie, 2123 P.O. Box 13449, Mowbray, 7705
Tel: (011) 789-7634, Fax: (011) 789-7634 Tel: (021) 448-8766, Fax: (021) 448-8766
email: national@mdsa.org.za email: cape@mdsa.org.za
Gauteng Branch Kwazulu-Natal Branch
P.O. Box 1535, Pinegowrie, 2123 P.O. Box 290, New Germany, 3620
Tel: (011) 789-7635, Fax: (011) 781-3935 Tel: (031) 701-3801, Fax: (031) 701-3801
email: gauteng@mdsa.org.za email: kzn@mdsa.org.za
Local Clinics: Please contact your local MDF branch for further information.
General Information: Independent Living Centre (ILC): (011) 482-5476
Disability Info and Care (DIC): (011) 917-3284
Parking Concessions: Application to the Traffic Department of our your Local Authority.
Criteria: (a) if you need the extra width as provided by the special parking bays, or
(b) if you have a problem with walking long distances.
Special Equipment: Phone either ILC or DIC for information on where to get special equipment.
MDF Website: Please visit our MDF website (www.mdsa.org.za) for muscular dystrophy news updates.
PLEASE NOTE
The treatments and drugs mentioned in this fact sheet are for information purposes ONLY. Please consult your physician or other health care specialist for information regarding the use of any of the above. The MDF encourages duplication of this fact sheet, under the following condition: that it is duplicated in its entirety - including the MDF logo and full text. Only individuals authorised by the MDF may make changes to this fact sheet (the information "updated by" and "last update" should be completed). Alterations to this fact sheet by any other party are strictly prohibited.
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Compiled by: MDF-Gauteng Branch Updated by: MDF-Gauteng Branch
Approved and Released by: National Office of the MDF Last update: 30 June 2001
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