Question

Fragile X syndrome, Huntington disease and Friedrich Ataxia are all examples of

trinucleotide expansion disorders.

a.Explain why it takes over 200 trinucleotide repeats in Fragile X gene

before a patient

shows symptoms, while individuals with Huntington disease display full penetrance only

after 40 trinucleotide repeats in the huntingtin gene.

b. Present an explanation of why such high repeat numbers are possible in Friedreich

Ataxia.

c. Trinucleotide repeat examples in human disease are far more numerous than

tetranucleotide repeats. Give two possible reasons as to why.

Answer 1

a) Fragile X syndrome:-

Fragile X syndrome occurs due to an expansion of the CGG triplet repeat within the Fragile X mental retardation 1 (FMR1) gene on the X chromosome. Fragile X mental retardation protein (FMRP), is required for normal development of the connection between neurons.By genetic testing it is determined that the number of CGG repeats in the FMRI gene is Normal between 5 and 40 repeats, fragile X syndrome occurs when the repeat becomes more than 200.

most commonly an increase in the number of CGG trinucleotide repeats in the 5' untranslated region of FMR1. Mutation at that site is found in 1 out of about every 2000 males and 1 out of about every 259 females .FXS can also occur by point mutations which affect FMR1.

In unaffected individuals, the FMR1 gene contains 5–44 repeats of the sequence CGG, most commonly 29 or 30 repeats. Between 45-54 repeats it is considered in a "grey zone", with a premutation allele generally considered to be between 55 and 200 repeats in length. Individuals with fragile X syndrome have a full mutation of the FMR1 allele, with over 200 CGG repeats. In these individuals with a repeat expansion greater than 200, there is methylation of the CGG repeat expansion and FMR1 promoter, leading to the silencing of the FMR1 gene and a lack of its product.

This methylation of FMR1 in chromosome band Xq27.3 occurs due to constriction of the X chromosome which appears 'fragile' under the microscope at that point, this phenomemom gave the syndrome its name. One study found that FMR1 silencing is mediated by the FMR1 mRNA. The FMR1 mRNA contains the transcribed CGG-repeat tract as part of the 5' untranslated region, which hybridizes to the complementary CGG-repeat portion of the FMR1 gene to form an RNA·DNA duplex.

male carriers pass on their premutation to all of their daughters, with the length of the FMR1 CGG repeat do not increase during meiosis.

males with a full mutation only pass premutations to their daughters, females also with a full mutation are able to pass this full mutation on, so there is a 50% chance that a child will be affected.

The length of the CGG repeat frequently increase during meiosis in female premutation carriers due to instability basing on the length of their premutation, they may pass on a full mutation to their children who will then be affected.

Huntington's disease (HD), also known as Huntington's chorea, is an inherited disorder which leads to death of brain cells.

HD is inherited from a person's parents,in 10% of cases due to a new mutation.The disease is caused by an autosomal dominant mutation in either of an individual's two copies of a gene called Huntingtin. chilild of an affected person typically has a 50% chance of inheriting the disease. Expansion of CAG (cytosine-adenine-guanine) triplet repeats in the gene coding for the Huntingtin protein gives an abnormal protein, which gradually damages cells in the brain.

The Huntington's disease mutation is genetically dominant and fully penetrant: mutation of either of a person's HTT alleles causes the disease. It is not inherited according to sex, but the length of the repeated section of the gene and hence its severity can be influenced by the sex of the affected parent

HD is one of several trinucleotide repeat disorders, caused by the length of a repeated section of a gene exceeding a normal range.The HTT gene is located on the short arm of chromosome 4at 4p16.3. It contains a sequence of three DNA bases—cytosine-adenine-guanine (CAG)—repeated multiple times (i.e. ... CAGCAGCAG ...), known as a trinucleotide repeat.

CAG is the 3-letter genetic code (codon) for the amino acid glutamine, so a series of them leads to production of a chain of glutamine known as a polyglutamine tract (or polyQ tract), and the repeated part of the gene, the PolyQ region. Generally, people have less than 36 repeated glutamines in the polyQ region which results in production of the cytoplasmic protein Huntingtin.But, a sequence of 36 or more glutamines results in the production of a protein which has different characteristics. This altered form is called mutant huntingtin (mHTT), it increases the rate of decay of certain types of neurons. Generally, the number of CAG repeats is related to how much this process is affected, and it shows about 60% of the variation of the age of the onset of symptoms.The remaining variation is due to environment and other genes that modify the mechanism of HD. 36–39 repeats gives a reduced-penetrance form of the disease, with a much later appearance and slower progress of symptoms. With very large repeat counts, HD has full penetrance and can occur under the age of 20.

Huntington's disease has autosomal dominant inheritance, that is an affected individual typically inherits one copy of the gene with an expanded trinucleotide repeat (the mutant allele) from an affected parent.

Trinucleotide CAG repeats over 28 are unstable during replication, and this instability increases with the number of repeats present. This leads to new expansions as generations pass (dynamic mutations) instead of reproducing an exact copy of the trinucleotide repeat. leads to the number of repeats to change in successive generations,in such a way that an unaffected parent with an "intermediate" number of repeats (28–35), or "reduced penetrance" (36–40), may pass on a copy of the gene with an increase in the number of repeats that produces fully penetrant HD.Such increases in the number of repeats (leads to earlier age of onset and severity of disease) in successive generations is known as genetic anticipation.Instability is greater in spermatogenesis than oogenesis; maternally inherited alleles are usually of a similar repeat length,but in case of paternally inherited ones , they have a higher chance of increase in length. Rarely Huntington's disease caused by a new mutation, where neither parent have over 36 CAG repeat.

B) Friedreich's ataxia is an autosomal recessive inherited disease which causes progressive damage to the nervous system. It shows its initial symptoms of poor coordination like gait disturbance; it can also lead to scoliosis, heart disease and diabetes, but does not affect cognitive function.

The particular genetic mutation (expansion of an intronic GAA triplet repeat in the FXN gene) leads to reduced expression of the mitochondrial protein frataxin.

Friedreich's ataxia is an autosomal recessive disorder that occurs when the FXN gene contains amplified intronic GAA repeats (an example of Trinucleotide repeat expansion). The FXN gene codes the protein frataxin. GAA repeat expansion reduces frataxin levels . Frataxin is an iron-binding protein responsible for forming iron–sulphur clusters. One result of frataxin deficiency is mitochondrial iron overload which can cause damage to many proteins. The gene is located on chromosome 9.

The mutant gene contains expanded GAA triplet repeats in the first intron, in a few pedigrees, point mutations have been detected. Because the defect is located in an intron (which is removed from the mRNA transcript between transcription and translation), this mutation do not lead to production of abnormal frataxin proteins. Instead, the mutation causes gene silencing (i.e., the mutation decreases the transcription of the gene) through induction of a heterochromatin structure in a manner similar to position-effect variegation.

normal individuals have upto 38 GAA repeats, FRDA patients have approximately 70 to more than 1000 triplets, most commonly 600–900 GAA triplets on both alleles of the FXN gene ,the larger the number of GAAs, the earlier is the disease onset and the quicker is the decline of the patient. As a functional consequence of GAA repeat hyperexpansion, the FRDA patients have a marked deficiency of FXN mRNA and also protein levels, thought to be due to reduced FXN gene transcription.

C) Trinucleotide repeat disorders occurs due to extensive duplication of a single codon, due to trinucleotide expansion up to a repeat number above a certain threshold level.

Triplet expansion occurs due to slippage during DNA replication. Due to the tandem repeats in the DNA sequence and the instability of the sequence in these regions, 'loop out' structures formed during DNA replication while maintaining complementary base pairing between the parent strand and the daughter strand synthesized, a nick in one side of the DNA strand is caused by cleavage by endonuclease whereby the repetitive triplet is extended and sealed by DNA polymerase and DNA ligase. If the loop out structure is formed from sequence on the daughter strand this will result in an increase in the number of repeats. However, if the loop out structure is formed on the parent strand, a decrease in the number of repeats occurs. It is observed that expansion of these repeats is more common than reduction. In general, the larger the expansion the more likely they are to cause disease or increase the severity of disease. This property shows the characteristic of anticipation seen in trinucleotide repeat disorders. Anticipation is the tendency of age of onset to decrease and severity of symptoms to increase through successive generations of an affected family due to the expansion of these repeats.