Medical Genetics Wk 10 Flashcards
Mutations Caused by Expandable DNA Repeats
At the end of 20th century researchers discovered that some mutant genes contain an expansion of trinucleotide repeat sequences—specific short DNA sequences repeated many times. Normal individuals have a low number of repetitions of these sequences; however, individuals with over 20 different human disorders appear to have abnormally large numbers of repeat sequences—in some cases, over 200—within and surrounding specific genes. Examples of diseases associated with these trinucleotide repeat expansions are:
Fragile-X syndrome
Friedreich ataxia
Huntington disease
Mutations Caused by Expandable DNA Repeats /cont./
The mechanisms by which the repeated sequences expand from generation to generation are of great interest. It is thought that expansion may result from either errors during DNA replication or errors during DNA damage repair.
➢When trinucleotide repeats such as (CAG)n occur within a coding region, they can be translated into long tracks of glutamine. These glutamine tracks may cause the proteins to aggregate abnormally. When the repeats occur outside coding regions, but within the mRNA, it is thought that the mRNAs may act as “toxic” RNAs that bind to important regulatory proteins, sequestering them away from their normal functions in the cell.
➢Another possible consequence of long trinucleotide repeats is that the regions of DNA containing the repeats may become abnormally methylated, leading to silencing of gene transcription.
Fragile X syndrome /FMR1 gene/
Fragile X syndrome is a genetic condition that causes a range of developmental problems including learning disabilities and cognitive impairment. Usually, males are more severely affected by this disorder than females. About one-third of individuals with fragile X syndrome have features of autism spectrum disorders that affect communication and social interaction. Prominent characteristics of the Fragile-X syndrome include an elongated face, large or protruding ears, and low muscle tone.
Fragile-X syndrome /Martin-Bell Syndrome/
The FMR1 gene - Cytogenetic Location: Xq27.3 Mutations Caused by Expandable DNA Repeats.
200 CGG repeats in cells that develop into eggs.
➢an elongated face,
➢large or protruding ears, and
➢low muscle tone.
provides instructions for making a protein called FMRP
Friedreich ataxia
Friedreich ataxia is a genetic condition that affects the nervous system
and causes movement problems.
People with this condition develop impaired muscle coordination (ataxia) that worsens over time. Other features of this condition include the gradual loss of strength and sensation in the arms and
legs; muscle stiffness (spasticity); and impaired speech, hearing, and vision. Individuals with Friedreich ataxia often have a form of heart disease called hypertrophic cardiomyopathy, which enlarges and weakens the heart muscle and can be life-threatening. Some affected individuals develop diabetes or an abnormal curvature of the spine.
Mutations in the FXN gene cause Friedreich ataxia. This gene provides
instructions for making a protein called frataxin. Although its role is not fully understood, frataxin is important for the normal function of mitochondria. One region of the FXN gene contains a segment of DNA known as a GAA trinucleotide repeat. In people with Friedreich ataxia, the GAA segment is repeated 66 to more than 1,000 times. The length of the GAA trinucleotide repeat appears to be related to the age at which the symptoms of Friedreich ataxia appear, how severe they are, and how quickly they progress. The FXN gene is found on chromosome 9.
HUNTINGTON DISEASE (HD Mutation) Autosomal Dominant
MAJOR PHENOTYPIC FEATURES
• Age at onset: Late childhood to late adulthood • Movement abnormalities
• Cognitive abnormalities
• Psychiatric abnormalities
Huntington disease (HD) is a panethnic, autosomal dominant, progressive neurodegenerative disorder that is caused by mutations in the HD gene (also known as HTT gene). The prevalence of HD ranges from 3 to 7 per 100,000 among western Europeans. The HD gene product, huntingtin, is ubiquitously expressed.
The function of huntingtin remains unknown. Disease-causing mutations in HD usually result from an expansion of a polyglutamine- encoding CAG repeat sequence in exon 1; normal HD alleles have 10 to 26 CAG repeats, whereas mutant alleles have more than 36 repeats.
HUNTINGTON DISEASE
Expression of mutant huntingtin causes neuronal dysfunction, generalized brain atrophy, changes in neurotransmitter levels, and accumulation of neuronal nuclear and cytoplasmic aggregates. Ultimately, expression of mutant huntingtin leads to neuronal death; however, it is likely that clinical symptoms and neuronal dysfunction precede the development of intracellular aggregates and neuronal death. The mechanism by which expression of this expanded polyglutamine tract causes HD remains unclear.
Organisms Use DNA Repair Systems to Counteract Mutations
DNA repair systems are essential to the maintenance of the genetic integrity of organisms and, as such, to the survival of organisms on Earth. The balance between mutation and repair results in the observed mutation rates of individual genes and organisms. The Nobel Prize in Chemistry was awarded to Tomas Lindahl, Paul Modrich, and Aziz Sancar for their ground-breaking insights into the ways that cells detect and repair DNA damage—specifically the processes of base excision repair, mismatch repair, and nucleotide excision repair. The most common DNA repair mechanisms, that organisms use to counteract genetic damage are following:
Proofreading and Mismatch Repair
Postreplication Repair and the SOS Repair
Photoreactivation Repair
Base and Nucleotide Excision Repair
Double-Strand Break Repair
Proofreading and Mismatch Repair
Some of the most common types of mutations arise during DNA replication when an incorrect nucleotide is inserted by DNA polymerase. The major DNA synthesizing enzyme in bacteria (DNA polymerase III) makes an error approximately once every 100,000 insertions. Fortunately, DNA polymerase proofreads each step, catching 99 percent of those errors. If an incorrect nucleotide is inserted during polymerization, the enzyme can recognize the error and “reverse” its direction. It then behaves as a 3 to 5 exonuclease, cutting out the incorrect nucleotide and replacing it with the correct one.
To cope with errors such as base–base mismatches, small insertions, and deletions that remain after proofreading, another mechanism, called mismatch repair (MMR), may be activated. During MMR, the mismatches are detected, the incorrect nucleotide is removed, and the correct nucleotide is inserted in its place. In E. coli, after replication, the nitrogenous base adenine acquires a methyl group (CH3); the parental DNA strand will have methyl groups, whereas the newly synthesized strand lacks them. In humans, mutations in genes that code for DNA MMR proteins are associated with the hereditary nonpolyposis colon cancer. MMR defects are commonly found in other cancers, such as leukemias, lymphomas, and tumors of the ovary, prostate, and endometrium. Cells from these cancers show genome-wide increases in the rate of spontaneous mutation. The link between defective MMR and cancer is supported by experiments with mice. Mice that are engineered to have deficiencies in MMR genes accumulate large numbers of mutations and are cancer-prone.
Postreplication Repair
Another type of DNA repair system, called postreplication repair,responds after damaged DNA has escaped repair and has failed to be completely replicated. As illustrated in Figure, when DNA bearing a lesion of some sort (such as a pyrimidine dimer) is being replicated, DNA polymerase may stall at the lesion and then skip over it, leaving an unreplicated gap on the newly synthesized strand.
To correct the gap, RecA protein directs a recombinational exchange with the corresponding region on the undamaged parental strand of the same polarity (the “donor” strand). When the undamaged segment of the donor strand DNA replaces the gapped segment, a gap is created on the donor strand. The gap can be filled by repair synthesis as replication proceeds. Because a recombinational event is involved in this type of DNA repair, it is considered to be a form of homologous recombination repair.
SOS Repair System
Another postreplication repair pathway, the E. coli SOS repair system, also responds to damaged DNA, but in a different way. In the presence of a large number of unrepaired DNA mismatches and gaps, the bacteria can induce expression of about 20 genes (including lexA, recA, and uvr) whose products allow DNA replication to occur even in the presence of DNA lesions. This type of repair is a last resort to minimize DNA damage, hence its name. During SOS repair, DNA synthesis becomes error-prone, inserting random and possibly incorrect nucleotides in places that would normally stall DNA replication. As a result, SOS repair itself becomes mutagenic—although it may allow the cell to survive DNA damage that would otherwise kill it.
Photoreactivation Repair: Reversal of UV Damage
UV light introduces mutations by the creation of pyrimidine dimers.
Uvinduced damage to E. coli DNA can be partially reversed if, following
irradiation, the cells are exposed briefly to visible light, especially in the blue
range of the visible spectrum. The process is dependent on the activity of a
protein called photoreactivation enzyme (PRE) or photolyase. The
enzyme’s mode of action is to cleave the cross-linking bonds between
thymine dimers (Figure). Although the enzyme will associate with a thymine
dimer in the dark, it must absorb a photon of blue light to cleave the dimer.
In spite of its ability to reduce the number of Uvinduced mutations,
photoreactivation repair is not absolutely essential in E. coli; we know this
because a mutation creating a null allele in the gene coding for PRE is not
lethal. The enzyme is also detectable in many organisms, including other
bacteria, fungi, plants, and some vertebrates—though not in humans.
Humans and other organisms that lack photoreactivation repair must
rely on other repair mechanisms to reverse the effects of UV
radiation.
Base and Nucleotide Excision Repair
A number of light-independent DNA repair systems exist in all bacteria and eukaryotes. The basic mechanisms involved in these types of repair-collectively referred to as excision repair or cut-and-paste mechanisms-consist of the following three steps.
The damage, distortion, or error present on one of the two strands of the DNA helix is recognized and enzymatically clipped out by an endonuclease. Excisions in the phosphodiester backbone usually include a number of nucleotides adjacent to the error as well, leaving a gap on one strand of the helix.
A DNA polymerase fills in the gap by inserting nucleotides complementary to those on the intact strand, which it uses as a replicative template. The enzyme adds these nucleotides to the free 3’-OH end of the clipped DNA. In E. coli, this step is usually performed by DNA polymerase I.
3. DNA ligase seals the final “nick” that remains at the 3’-OH end of the last nucleotide inserted, closing the gap.
There are two types of excision repair:
1.base2.nucleotide excisior. excision repair repair
Base excision repair (BER)
Base excision repair (BER) corrects DNA that contains incorrect base pairings due to the presence of chemically modified bases or uridine nucleosides that are inappropriately incorporated into DNA or created by deamination of cytosine. The first step in the BER pathway involves the recognition of an inappropriately paired base by enzymes called DNA glycosylases. There are a number of DNA glycosylases, each of which recognizes a specific base. For example, the enzyme uracil DNA glycosylase recognizes the presence of uracil in DNA ( Figure). DNA glycosylases first cut the glycosidic bond between the target base and its sugar, creating an apyrimidinic (or apurinic) site. The sugar with the missing base is then recognized by an enzyme called AP endonuclease. The AP endonuclease makes cuts in the phosphodiester backbone at the apyrimidinic or apurinic site. The gap is filled by DNA polymerase and DNA ligase. Although much has been learned about the mechanisms of BER in E. coli, BER systems have also been detected in eukaryotes from yeast to humans. Experimental evidence shows that both mouse and human cells that are defective in BER activity are hypersensitive to the killing effects of gamma rays and oxidizing agents.
Base excision repair (BER) accomplished by uracil DNA glycosylase, AP endonuclease, DNA polymerase, and DNA ligase. Uracil is recognized as a noncomplementary base, excised, and replaced with the complementary base (C).
Nucleotide excision repair (NER)
Nucleotide excision repair (NER) pathways repair “bulky” lesions in DNA that alter or distort the double helix. These lesions include the UV-induced pyrimidine dimers and DNA adducts discussed previously. The NER pathway (Figure) was first discovered in 1964 by Paul Howard-Flanders and coworkers, who isolated several independent E.coli mutants that are sensitive to UV radiation. One group of genes was designated uvr (ultraviolet repair) and included the uvrA, uvrB, and uvrC mutations. In the NER pathway, the uvr gene products are involved in recognizing and clipping out lesions in the DNA. Usually, a specific number of nucleotides are clipped out around both sides of the lesion. In E. coli, usually a total of 13 nucleotides are removed, including the lesion. The repair is then completed by DNA polymerase I and DNA ligase, in a manner similar to that occurring in BER. The undamaged strand opposite the lesion is used as a template for the replication, resulting in repair.
Nucleotide excision repair (NER) of a UV-induced thymine dimer. During repair, 13 nucleotides are excised in bacteria, and 28 nucleotides are excised in eukaryotes.
