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122 Cards in this Set
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- Back
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Name the stages of gene expression |
1. Transcription 2. Splicing 3. Translation 4. Post translational modification |
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Describe the stages of gene expression |
1. Transcription - DNA copied onto a messenger molecule called mRNA via enzyme DNA polymerase and this occurs in nucleus 2. Splicing - mRNA leaves nucleus and travels to splicosome, where introns (non coding parts DNA) removed and exons (coding parts DNA) joined together to form mature mRNA - different patterns of splicing occur to allow for different proteins to be made from same gene 3. Translation - mature mRNA travels to the ribosome, where the tRNA carries amino acids to the ribosome. First tRNA attaches to the start codon (always AUG and first aa is always Methionine) and the tRNA and mRNA continue to form codon - anti codon bonds, as the ribosome moves along the mRNA, forming a protein and then eventually stop codon reached, and the ribosome releases the polypeptide and protein formed 4. Post translational modification - protein travels to golgi apparatus, where lipids and glucose molecules may be added to form lipoproteins, glycoproteins |
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What is Mendelian inheritance |
Mendelian inheritance is inheritance based on the transmission of a single gene, either on an autosomal dominant, autosomal recessive or X linked pattern |
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Name 3 typical mendelian inheritance patterns |
- Autosomal dominant - Autosomal recessive - X linked (normally recessive, but can be X linked dominant) |
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Name 4 atypical mendelian inheritance patterns |
- Genetic anticipation - Gonadal mosaicism - Pseudo-dominant inheritance - Mitochondrial inheritance |
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Autosomal dominant conditions |
- Huntingtons disease - Achondroplasia - Autosomal dominant polycystic kidney disease (ADPKD) - Inherited breast and bowel cancer predisposition syndromes - Neurofibromatosis - 1 |
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Features of autosomal dominant conditions |
- Vertical pattern - affects multiple generations - Both males and females affected - Only require one copy of the gene to be mutated, since they are autosomal dominant |
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What is variable expressivity/variable expression? |
Variable expressivity is a concept which explains why different members in a family which are affected by the same mutation and thus the same condition are affected to varying extents and severities. It is due to MODIFIER GENETIC VARIANTS - these are gene sequences either close to the original gene sequence or millions of base pairs away, which have changes in their sequences (but to a lesser extent than the original gene mutation causing the disease) and they can interact with the gene causing the disease They can act as silencers, so decrease transcription of the gene causing disease and thus decrease severity or they can act as enhancers, increasing the transcription of the gene causing the disease and thus increase the severity |
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Provide an example of a condition in which variable expression may be seen |
FGFR2 variants in BRCA2 mutations (breast cancer) |
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Explain the meaning of penetrance |
Penetrance - chance that an individual who has inherited the mutated copy of a gene will show clinical traits of that condition Penetrance can be; a) complete penetrance - achondroplasia b) incomplete penetrance - inherited breast and bowel cancer predisposition syndromes |
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What is meant by incomplete penetrance |
Incomplete penetrance - individual has inherited the mutated copy of the gene which is capable of causing the disease but they do not show any clinical traits of that condition, even though it is autosomal dominant However, they will still have a 50% chance of passing the mutated copy on to their offspring |
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Autosomal recessive conditions |
- Cystic fibrosis - Phenylketonuria (PKU) - Congenital adrenal hyperplasia - Spinal muscular atrophy |
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Describe 3 features of autosomal recessive conditions |
- Horizontal pattern - affect single generation - Affect both males and females - Require both copies of the gene to be mutated, since they are autosomal recessive |
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Explain the meaning of the term heterozygous compound |
If parents both have a mutation in the same gene, however it is different mutations, then the child will inherit two different mutated copies of the same gene. The child will be compound heterozygous and will still show traits of an autosomal recessive condition (same as homozygous - individual has identical mutations on 2 different copies of the same gene) |
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X linked inherited conditions |
- Beckers muscular dystrophy (less severe) - Duchenne muscular dystrophy (more severe) |
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Features of X linked recessive conditions |
- No male to male transmission (if there is male-male transmission, it cannot be X linked since males can only pass Y chromosome onto sons) - Only males affected - Knights move (males affected via unaffected females eg son affected and his uncle affected) |
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Why are males only really affected by X linked recessive conditions, but females aren't? |
Males only have one X chromosome, and if they have a gene mutation on X chromosome, then they will be affected However, females have two X chromosomes and thus, two copies of the gene, and the normal copy of the gene normally masks the mutated form, so the condition will not show |
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Describe the term of manifesting carriers |
Manifesting carriers are females who carry the X linked recessive condition, but show symptoms of this condition and it is due to SKEWED X INACTIVATION - In females, one x chromosome is normally inactivated in every cell to match the one X chromosome of males - however, this inactivation may become skewed and may mean that more of the normal X chromosomes are removed instead of the abnormal X chromosomes, and thus if more cells in the body contain the abnormal X, then the female will show traits of the condition, however she will be affected to a milder effect than males |
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X linked dominant conditions |
- Vitamin D resistant rickets - Rett syndrome - Incontintia pigmenti * rare to see males affected by incontintia pigmenti and rett syndrome, as they are severely affected and normally spontaneous abortion occurs |
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Features of X linked dominant conditions |
- Vertical pattern of inheritance - Both males and females affected - No male-male transmission, since males only pass Y chromosome to males |
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Explain genetic anticipation |
Genetic anticipation is the concept in which there is an increased severity of the condition and earlier age of onset in successive generations. It occurs due to increasing mutation size in successive generations when passed on Eg - first mutation has 40 repeats of the CAG and it codes for one amino acid and this is glutamine - then in the next generation, the mutation becomes larger with 43 repeats and third generation has 47 repeats |
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Name 3 conditions which show genetic anticipation |
- Huntingtons disease - Myotonic dystrophy - Fragile X syndrome |
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Explain pseudo-dominant inheritance |
Pseudo-dominant inheritance occurs when an autosomal recessive condition appears to be an autosomal dominant condition - it occurs due to a high population carrier frequency or due to consanguinity (1st cousins marrying each other - high chance that both will be carriers of autosomal recessive conditions) |
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Name a condition in which shows pseudo-dominant inheritance |
Gilberts syndrome - unconjugated hyperbilirubinaemia (deficiency of the enzyme UDP glucoronly transferase) |
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Explain mitochondrial inheritance |
- Syndromes associated with mitochondrial inheritance affect brain, muscle and eye - Mitochondrial DNA only inherited from mum, so if mum has mutation in her mitochondrial DNA it will be passed onto the children - Mitochondrial DNA is different to chromosomal DNA, in that it is only 16.6kbp and contains 37 genes - It is circular in shape, unlike chromosomal DNA which is linear in shape - It contains no introns (non coding parts of DNA) |
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Summary of AD conditions |
- Both males and females affected to equal frequency and severity - Vertical pattern of inheritance -multiple generations affected - Disease expressed in heterozygotes (only need 1 copy of gene to be mutated since its AD) - Variable expressivity - Incomplete penetrance |
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Summary of AR conditions |
- Both males and females affected by equal frequency and severity - Horizontal patterns of inheritance - single generation affected - Disease expressed in homozygotes or compound heterozygotes - Expressivity more constant within same family - Importance of consanguinity |
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Name the 4 capabilities for a cell to develop into cancer |
1. Proliferative signalling 2. Avoidance of apoptosis 3. Bypassing replicative senescence 4. Insensitivity to anti growth factors |
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1. Proliferative signalling |
Driving force which is present in the cell and it keeps the cells dividing, to cause uncontrolled cell division |
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2. Avoidance of apoptosis |
Apoptosis is programmed cell death and it will occur when there is high levels of DNA damage which cannot be repaired - it is triggered by P53 (expressed by TP53 gene) - cells with extensive DNA damage need to be able to survive this to turn into cancer cells |
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3. Bypassing replicative senescence |
Cells only divide a limited no of times and this is called replicative senescence - it is controlled by protective repeats at the end of the chromosomes called TELOMERES - as the chromosomes divide in mitosis, the telomeres become shorter and when they become short enough, they will cause apoptosis |
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4. Insensitivity to anti growth factors |
TGF-beta is an anti growth factors and it prevents cells from dividing, so for a cell to turn into cancer, it has to become resistant to these signals |
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What is the cell cycle |
The cell cycle is a series of events which occur within cells, and lead to the cells division and production of 2 new daughter cells The cell cycle is controlled by checkpoints and these are controlled by proteins (eg P53) |
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Describe stages of the cell cycle |
G1 - first stage of cell growth and synthesis of organelles S - DNA replication G2 - second stage of cell growth and synthesis of organelles M - Mitosis (cell division) |
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Describe features of tumour suppressor genes (TSGs) - brakes on cell division |
- Around 90 of them eg BRCA 1,BRCA 2, TP53 - 3 main functions; a) Inhibit progression through cell cycle b) They promote apoptosis (protective mechanism to prevent DNA damage in cells and prevent cancer formation eg TP53) c) Some function as stability genes in which they produce proteins to repair DNA eg BRCA 1 and BRCA 1 - They are recessive, requiring both copies of the gene to be mutated before condition develops - Loss of function mutation - unable to put brakes on cell division, so uncontrolled division |
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Describe features of proto-oncogenes - promote cell division |
- Example of proto oncogenes; RET (familial thryoid cancer), RAS, MYC -Participate in normal cellular response to growth factors - ie if cut yourself, growth factors released and they switch on proto oncogenes and this results in the production of proteins which stimulate cell proliferation for wound healing - Proto oncogenes are non mutated form and oncogenes are mutated form - Proto oncogenes are dominant and only require one copy of gene to become mutated before the disease develops - Gain of function mutation - overstimulation of cell division, so uncontrolled cell division |
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What are the two main cancer types |
a) Sporadic (95%) - most common - non inherited and occur spontaneously - single primary tumour which can metastasize and they normally have a later onset b) Familial (5%) - less common - inherited - multiple primary tumours which can metastasize and they normally have an earlier onset |
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Describe Knudsons two hit hypothesis |
Most inherited cancer predisposition syndromes are inherited in an autosomal dominant pattern and are due to a mutation in TSG rather than proto oncogene. However, the inheritance of one muated copy of TSG is not enough to cause a disease, since it is recessive and both copies of TSG need to be mutated before disease develops, so the normal copy (wild type copy) would need to mutate too. This is called Knudsons two hit hypothesis - first hit to a TSG is inherited and predisposes the person to cancer, however it isn't enough to cause the cancer since both copies need to be mutated, but the second hit can occur later in life, and lead to the development of the cancer 1st hit - normally a point mutation that is inherited 2nd hit - either point mutation that arises de novo or copied across from other chromosome (gene conversion), or it could be a deletion of that gene |
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What would you look for in familial cancers? |
- Individual history (multiple primary tumours, early age of onset) - Family history (more than one person in same family with cancers at same site or at related sites, early onsets) |
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What can be done at genetic clinics? |
- Drawing family tree to confirm diagnosis - Screening methods; mammography etc - Discuss risk factors ie COCP use, HRT use - Preventive methods; prophylactic bilateral mastectomy, bilateral oophoroectomy |
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Familial breast cancer |
- 95% sporadic - 5% inherited (BRCA 1, BRCA 2, TP53, PALB2, PTEN) *BRCA 1 - associated with ovarian cancer *BRCA 2 - associated with male breast cancer
BRCA mutation - actual risk depends on the family tree (if there are multiple people affected then there will be high penetrance which would mean higher risk) - mainly due to the effects of the modifier genetic variants |
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How do BRCA 1 and 2 genes work as stability genes? |
BRCA 1 and 2 genes are stability genes - they produce proteins which repair DNA BRCA 1/2 genes repair DNA by homologous recombination of double strand breaks |
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Describe genetic testing in breast cancer |
Genetic testing is carried out via next generation sequencing and is carried out in people who have at least a 10% chance of having mutated BRCA 1 or 2 genes - it is important to store DNA from blood of those in the family who were affected by the mutation to allow for future mutation analysis |
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Should a young girl be tested for breast cancer risk and a BRCA 1 or 2 gene mutation? |
No, she will not have developed breasts yet so there is no risks and also, we should wait to she is 16, to make her own decision |
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What are the options for young women worried about inheriting a BRCA 1/2 gene mutation or if she has tested positive for BRCA 1/2 mutation |
- Prophylactic bilateral mastectomy - Bilateral oophorectomy - Advice on risk factors ie COCP and HRT use - Advice on regular screening via mammography ie every 2 years from 50-70 - Advice on self assessment for lumps |
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Ovarian cancer |
Ovarian cancer normally due to a mutation in BRCA 1 gene However, may be linked to HNPCC is mutation in MLH1 or MSH2 |
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Treatment of ovarian cancer; PARP inhibition |
PARP inhibition is a relatively new method in treating ovarian cancer caused by BRCA 1/2 mutation, in which selectively kills the ovarian cancer cells Due to BRCA mutation, the DNA in the cell cannot be repaired, so the cells uptake a new mechanism of DNA repair, which uses an enzyme called PARP, however this is innacurate If we inhibit the PARP enzyme, then the ovarian cancer cells will not be able to repair their DNA, so apoptosis will be triggered and the cancer cells will die |
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Colon cancer |
- 95% sporadic - 5% inherited Most of them are inherited in an autosomal dominant pattern, except for MYH (autosomal recessive) |
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What is hereditary non polyposis colorectal cancer (HNPCC) - Lynch syndrome |
This is a bowel cancer, which results in a few polyps (<10) and it is associated with ovarian, uterine, stomach, urothelial cancers It occurs due to mutations in MLH1 and MSH2 genes mainly It occurs due to inheritance of mutations in the DNA mismatch repair system (MMR) genes * DNA polymerase carries out DNA replication and it has a proof checking mechanism, in which DNA exonuclease will remove any errors found and polymerase will extend the chain. However, if mutations occur in the MMR system genes, then they will not be able to remove any mutated sections of DNA, and this will lead to accumulating genetic changes, leading to cancer |
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What tests are carried out in those at high risk of HNPCC |
- 2 yearly colonoscopy from 25 - 2 yearly upper GI endoscopy from 50 * Patients testing positive for HNPCC, can undergo prophylactic colectomy |
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What is familial adenomatous polyposis syndrome (FAP)? - autosomal dominant pattern |
This is characterized by many polyps (>200) in the bowel and also associated with congenital hyperplasia of the retinal pigmented epithelium (CHRPE) The polyps are benign, however if they are left untreated, they will transform into colon cancer |
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What is MYH polyposis - autosomal recessive pattern? |
This is characterized by polyps in the bowel (15-200) and this is due to mutations in the MUTYH gene - which encodes enzymes involved in base excision repair (BER) |
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WHat is Li Fraumeni syndrome |
- Rare autosomal dominant cancer predisposition syndrome, which causes multiple primary cancers; - leukaemia - sarcomas - brain - breast |
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How does Li Fraumeni syndrome occur? |
Li Fraumeni syndrome occurs due to mutations in TP53 gene, which encodes for the protein P53 (guardian of the genome). Normally, P53 can trigger apoptosis if the cell has accumulated extensive DNA damage which cannot be repaired However, if P53 is mutated, then it cannot trigger apoptosis, so the DNA damage will accumulate and this will result in the accumulation of genetic mutations, which allow cancer cells to develop |
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What is Huntingtons disease? |
Autosomal dominant condition which shows genetic anticipation Onset of Huntingtons ; 30-50, but can be any time Symptoms of Huntingtons; - progressive chorea (involontary movements) - weight loss (cant eat due to movements and also moving burns more calories than they take in) - dementia - psychiatric symptoms |
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Describe the genetics of Huntingtons disease |
The pathogenic mechanism is due to repeats of the codon CAG (normally encodes for amino acid glutamine), so repeats of CAG will result in the formation of a polyglutamine tract, which will form insoluble protein aggregates and cause neurotoxicity, leading to the symptoms Due to genetic anticipation, this sequence is prone to expansion during meiosis, when it is passed on from fathers >40 repeats - complete penetrance |
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Describe genetic testing in Huntingtones disease |
Presymptomatic testing can be carried out to test unaffected relatives who havent shown signs of the condition yet |
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Would a young boy be tested for Huntingtons if his mum had just been diagnosed with it etc? |
No, a young man would not be tested until he was old enough to make own decisions, because; - Adult onset condition - Non curable - No benefit of child knowing at this age - Only 50% chance it would be passed on - The repeat sequences in the condition are more prone to expansion in meiosis when passed on from the father |
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What is myotonic dystrophy? |
Autosomal dominant inherited condition, which shows genetic anticipation Signs and symptoms; - Myotonia (failure of relaxation after contraction eg unable to stop gripping) - Cataracts |
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Describe the genetics of myotonic dystrophy |
The repeat nucleotide sequence in myotonic dystrophy is CTG and it occurs in the 3' region (downstream of the DMPK gene - non coding part of the gene) - thus the DNA transcribed to make mRNA, however the mRNA cannot be translated to protein as it was in the untranslated region of DMPK gene (repeat sequences in myotonic dystrophy are more prone to expansion in meiosis when passed on from mother) The DMPK mRNA which forms is abnormal and it can affect the splicing of other genes - it mops up the muscle blind protein, and thus disrupts the normal splicing mechanism of this protein It disrupts splicing of the chloride ion channel gene (CLCN1 gene) - myotonia and also the insulin receptor gene - diabetes mellitus |
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What is cystic fibrosis (CF)? |
- Autosomal recessive condition - 1 in 20-25 carriers in population - Signs and symptoms; lung infections (thick secretions) and exocrine pancreatic insufficiency (difficult secreting enzymes into gut) |
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What is the diagnosis of CF? |
- Immunoreactive Trypsin - antibodies to Tryspin in the newborn blood - Genetic tests - Look for mutations in CFTR gene |
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What is the genetics of CF? |
Mutation occurs in the CFTR gene, and this gene encodes for CFTR protein (acts as a chloride ion channel, allowing chloride ions to leave the cells and hydrate secretions) However, if mutations occur in the CFTR gene, then defective Cl ion channel and chloride ions cannot hydrate secretions, so this will result in thick secretions - leading to lung infections and exocrine pancreatic insufficiency The mutation which occurs most commonly is f508del mutation and it is an in frame deletion (deletion of 3 nucleotides, and this is deletion of codon which encodes for the amino acid phenylalanine, and the protein will not fold properly, and this defective Cl ion channel will not be inserted into the plasma membrane |
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What is Neurofibromatosis-1? |
- Autosomal dominant condition - Signs and symptoms; cafe au lait macules (coffee coloured spots), lisch nodules (brown spots in eyes), multiple neurofibromas, learning difficulties |
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What are the possible complications of Neurofibromatosis -1? |
- Scoliosis - Tibial fractures - Sarcomas - Phaeochromocytomas - Hypertension |
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What are the conditions Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD)? |
These are both X linked recessive conditions, in which DMD causes a more severe form of the disease and BMD causes a less severe form of the disease Signs and symptoms; Weak muscles around spine and breathing difficulties (due to diaphragm problems) * DMD gene is the largest gene in the body, however it doesnt produce the largest proteins - it contains the largest introns (non coding parts of proteins) - DMD gene makes the protein dystrophin, which interacts with actin in muscle cells, however in the mutation, the dystrophin is abnormal and cant form this interaction so the muscles are weak |
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What is the genetics of Becker muscular dystrophy (BMD) |
- BMD is the less severe form of muscular dystrophy (occurs due to IN FRAME DELETION) - IN FRAME DELETION - deletion of 3 nucleotides and lose one amino acid, thus it doesnt affect the amino acid after the deletion - Thus, since it causes the less severe form, it normally occurs in teenagers and doesnt need a wheelchair |
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What is the genetics of Duchenne muscular dystrophy (DMD) |
- DMD is the more severe form of muscular dystrophy (occurs due to OUT OF FRAME DELETION) - OUT OF FRAME DELETION - deletion of 4 nucleotides etc, thus deletion is not a multiple of 3 nucleotides, so it will affect all amino acids after the deletion - Thus, since it causes the more severe form, it normally occurs in children, and they normally need a wheelchair by teenage years |
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What is the diagnosis of BMD and DMD? |
- Boys will have extremely high levels of serum creatinine kinase (SCK) from birth before any other symptoms present - protein which leaks out the damaged muscle and becomes extremely high in the blood |
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What is meant by the term 'genotype-phenotype correlation'? |
This is the connection between the genotype and the resulting phenotype - it is seen in BMD and DMD, since the more severe genetic mutations result in the more severe clinical phenotype |
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What is Fragile X syndrome ? |
- X linked revessive condition - Most common cause of inherited learning difficulties - (Due to repeats in the5’ UTR region of FMR1 gene) |
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What is down syndrome? (Trisomy 21) |
Down syndrome is the presence of all or part of chromosome 21 in all cells How do trisomies occur? - Due to maternal non disjunction at meiosis (trisomies more common with increasing age) |
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What are the 3 ways in which down syndrome can be inherited |
1. Non disjunction (94%) 2. Translocation (5%) 3. Mosaicism (1%) |
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1. Non disjunction (94%) |
Most common form, where a pair of homologous chromosomes or sister chromatids fail to separate after meiosis 1 or 2 respectively, and this results in one gamete with 2 copies of chromosome 21 and another gamete with no copies Thus, when the gamete with 2 copies of chromosome 21 fertilises with gamete of other sex, then this will result in the zygote containing 3 copies of chromosome 21 |
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2. Translocation |
Translocation occurs when a part of chromosome 21 breaks off and attaches to a different chromosome, most commonly chromosome 14 (robertsonian translocation - chr 14 and 21) The person with this mutation will be aysmptomatic and will be unaware, and they will be BALANCED RECIPROCAL TRANSLOCATION CARRIERS (even though they have 45 chromosomes - one chromosome attached to another) However, when the gamete with the robertsonian translocation fertilises a gamete of opposite sex, then this result in the zygote containing 3 copies of chromosome 21 in every cell |
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3. Mosaicism |
This is extremely rare, and occurs due to non disjunction which occurs in mitosis after zygote formation. It is the least severe form of DS, as only some cells will contain three copies of chromosome 21 and some wont |
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Symptoms of DS |
- Congenital heart defects eg VSD - Single palmar crease - Increased risk leukaemia - More prone to obesity - Increased risk of Alzhemiers - Duodenal atresia - Learning difficulties - Increased nuchal fold - Cataracts - Recurrent otitis media - Hypothryoidism - Protruding tongue - Epicanthic folds - Brushfield spots - Slanted eyes - Small nose and ears |
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What is Trisomy 18? |
Edwards syndrome - small chin - clenches hands with overlapping fingers - malformations of heart, kidneys and other organs - if survive first year, then have learning difficulties |
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What is Trisomy 13? |
Patau syndrome - congenital heart disease - cleft lip and palate - abnormal ears - post axial polydactyly (extra little finger) - most die within one month and if they survive first year, will have learning difficulties |
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What is genetics? |
Study of single genes |
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What is genomics? |
Study of all or parts of an organisms genes (mitochondrial - 16,6kbp and chromosomal DNA - 3 billion base pairs) |
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Describe the genome |
Genome is 3 billion base pairs in size - refers to all DNA in a haploid set of chromosomes in a gamete Only 1.5% of the genome is coding and this is in forms of exons. However, the rest is non coding and is either found in a) Intergenic regions - between genes b) Within genes - in the form of introns (non coding parts of DNA) |
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What is a point mutation? |
A point mutation occurs when one nucleotide has been switched for another, and this results in a different codon which will encode for a different amino acid, so it will change the structure and function of the protein |
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What is a frameshift mutation? |
A frameshift mutation occurs when nucleotides are either inserted into or deleted from a strand of DNA and this will result in elongation or shortening of the DNA strand |
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Name 3 types of DNA detection methods |
1. Detection of point mutations 2. Detection of sub microscopic deletions or duplications 3. Detection of aneuploidies (abnormal chromosome number that isnt a multiple of 23, eg trisomy 13,18,21) |
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1. Detection of point mutations |
a) Next generation sequencing - can sequence many genes at once ie single genes, gene panel, exome, genome - Compared both forward and reverse strands of patient DNA to reference DNA and looks for any abnormalities b) Allele specific PCR (ARMS) - Form of PCR, which used to hone in on where mutation should be eg f508 del mutation in CF |
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2. Detection of sub microscopic deletions or duplications |
a) MLPA - Carried out when we know exactly where to look b) Array comparative genomic hybridisation (aCGH) - Carried out when we do not know exactly where to look - Mixes DNA from patient with reference DNA and should be equal (more reference DNA - deletion, more patient DNA - duplication) |
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3. Detection of aneuploidies (abnormal chromosome no that isnt a multiple of 23 eg trisomy 13,18,21) |
Quantitative fluorescent PCR (QF-PCR) - It looks at the markers on chromosomes and the signals which are given off from each chromosome pair and should always be 2, however if it was 3, it would suggest trisomy DNA to carry out QF-PCR from amniocentesis/CVS - multiple amniocentesis/PCR is a risk factor for miscarriages |
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Name 2 chromosomal based methods |
1. Karyotyping - Looking down light microscope and identify which chromosome is which and their partner and also can detect translocations 2. FISH (Fluorescent In Situ Hybridisation) - Use of molecular probes made with DNA and a fluorescent tag. They are used to identify specific chromosomes and then they look at specific parts of chromosomes (ie signal on one chromosome but not on other in pair would mean deletion of that gene/part of chromosome) |
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What is meant by the term 'Variant of uncertain significance'? |
Variants of uncertain significance are variants of a gene, which have been identified through genetic testing and they are not thought to be disease causing. It is thought they occur due to single nucleotide polymorphisms (SNPs), which are single point mutations occurring in a gene, causing variations of that gene, but they do not cause disease |
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What is personalised medicine? |
Personalised medicine is the concept of choosing medicine after the analysis of genetic markers in that persons DNA, and this improves the treatment options and makes them more effective |
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What is pharmacogenetics? |
This is the study of inherited genetic differences in drug metabolism and how can this effect an individuals response to a drug * There are many polymorphisms in the genes which encode the CYP450 enzymes which metabolise Warfarin - thought that in the future, we will be able to choose drugs after analysis of SNPs |
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Give 3 examples of specific drugs used in some known mutations |
1. Ivacaftor (specific CFTR gene mutation which results in the blocking of CFTR protein and ivacaftor changes configuration of protein to allow CF to leave cell and hydrate secretions 2. Trastuzumab/Herceptin (used for Her2 mutations in breast cancer) 3. Gefitinib (EGFR mutations in non small cell lung cancers) |
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What is non invasive pre natal diagnosis (NIPD)? |
NIPD is a new technique which can be used to look for achondroplasia (FGFR3 mutation - which has come from father) In NIPD, a blood sample is taken from the father and tested for any mutations which could have arisen from the father - foetal cells can enter the maternal circulation and they will contain paternal DNA which can be analysed |
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Describe gene therapy and give an example |
Gene therapy - making changes to structure or function of genes An example is exon skipping - exon skipping is a splicing mechanism which is used to skip over mutated sections of DNA, and leads to a shorter but functional protein) Antisense oligonucleotide therapy - form of exon skipping used in converting DMD to BMD - it converts it from out of frame deletion to in frame deletion, by deleting a multiple of 3 nucleotides, which was not the case before and part of protein will be lost but the protein will still be functional |
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What is pre implantation diagnosis (PGD)? |
This is genetic testing which is done pre implantation and it is carried out at 3 days, where there are 6-10 cells. However, only one or two cells are examined so it doesnt affect future development of embryo - done by PCR of FISH advantages - allows implantation of unaffected embryos disadvantages - can take while for results - not available to all women |
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Describe the assessment and screening of genetic disorders |
Assessment of patient - History - Family history - Exam (height, weight, occipital frontal circumference, dysmorphic features) - Investigations (echo, renal ultrasound) |
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Describe the dysmorphic features which could be found in an exam |
- Head size (macro/micro cephaly) - Eyes (hypertolerism - increased spacing) - Ears (anterior/posterior rotation, low set) - Nose, nostrils - Philtrum (smooth in FAS) - Mouth, teeth, lips, gum - Limbs - Skin (pigmentation) - Hands and feet (syndactyly, polydactyly, polysyndactyly) |
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Describe the 3 periods of screening in pregnancy |
1. Prenatal 2. Neonatal 3. Postnatal |
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1. Neonatal |
10-14 weeks; CUBS (combined ultrasound and blood screening) - Ultrasound; increased nuchal transluceny - Blood screening; hCG, alpha feto protein 20 weeks; anomaly scan (ultrasound scan to check for range of conditions) |
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2. Neonatal |
Heel prick and guthrie card -CF (immuneassay) -PKU (mass spectrometry) -Sickle cell disorder (chromatography) |
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3. Postnatal |
- Tay sachs disease, Thalassaemia |
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Name the two genetic diagnostic tests in pregnancy |
1. Chorionic villus sampling (CVS) - take cells from placenta - 10-12 weeks - 1 in 50 chance miscarriage 2. Amniocentesis - take cells from amniotic fluid - 16-18 weeks - 1 in 100 chance miscarriage |
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Would two different gene mutations on the same copy of a gene cause AR disorder? |
NO, both copies of a gene (gene in each chromosome) need to be mutated for an AR condition to develop. Having two different gene mutations in the same copy of a gene - IN CIS MUTATION |
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Name 3 ways in which a female could show traits of an X linked recessive condition |
a) Skewed X inactivation - Manifesting carrier b) Turners syndrome - 45X0 c) Father is affected and mother is a carrier |
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Summary of DNA detection methods |
a) Detection of point mutations - Next generation sequencing - ARMS (allele specific PCR) b) Detection of sub microscopic duplications or deleitons - MLPA c) Detection of aneuploidies - Quantitative fluorescent PCR (QF-PCR) |
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Name a condition in which doesnt show genetic anticipation |
Neurofibromatosis-1(autosomal dominant condition) |
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Name a condition in which always shows complete penetrance |
Achondroplasia (autosomal dominant condition) |
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What is a nonsense mutation? |
This occurs in which mutations which produce a stop codon, which will result in the protein stopping being made at the stop codon, so it will form a smaller protein (some drugs can readthrough stop codons) |
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What is a missense mutation? |
It is a single point mutation, in which one nucleotide has been switched for another and this results in a different codon, which will encode for an amino acid and this will then result in a different protein structure and function |
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What is gonadal mosaicism? |
This is when a mutation arises in the gonads of the parents and they pass it on , however, all their other cells are normal and some cells in gonad contain mutation and some dont This explains why healthy parents can give rise to individuals with the same genetic disorder |
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What is sensitivity?
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This is the ability of a test to identify those with a disease
(How much people who have disease get positive result on screening) Everyone testing positive with disease/everyone with disease |
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What is specificity? |
This is the ability of a test to identify those without disease Everyone testing negative without disease/everyone without disease |
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What is true positive? |
Everyone affected with positive result |
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What is false positive? |
Everyone unaffected with positive result |
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What is true negative? |
Everyone unaffected with negative result |
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What is false negative? |
Everyone affected with negative result |
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What is equation for sensitivity? |
TP/(TP + FN) |
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What is equation for specificity? |
TN/(FP + TN) |
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Explain how siblings could be affected by mitochondrial disorders to different extents? |
In each cell, there are lots of mitochondria and each mitochondria contains several molecules of circular DNA. There is a mixture of mitochondria that contain the mutation and some mitochondria that do not - this is HETEROPLASMY. For each disease, there is a threshold (no of mitochondrial mutants needed to get the disease) The mother passes on mutant mitochondria to child in varying extents - in egg cell passed to child 1, she might pass on more mutant mitochondria to child 1 than in the egg cell she passes to child 2 and this explains why siblings can be affected to different extents by mitochondrial disorders - HETEROPLASMY |
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List principles of a screening test |
- Must be a clearly defined disorder - Advantage to early diagnosis - Few false positives - Few false negatives - Benefits outweighs risks - Must be acceptable to population - Must be latent period of disease - Must know natural history of disease - Must be resources and finances available - Must be diagnostic test - Must be treatment |
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Why would HD not have a screening test? |
It is an incurable condition and there is no advantage to early diagnosis |
