6 - human genetics Flashcards
how do nucleotide repeats induce disease?
nucleotide repeats are very unstable
the repeats impair the structure of the protein → loss of function
there is no correlation between number of repeats and onset of disease
how can positional cloning be used to identify human disease-associated genes?
positional cloning is a laboratory technique used to locate the position of a disease-associated gene on a chromosome
the gene of interest is mapped very precisely
used for SINGLE-GENE DISORDERS
how do multifactorial gene disorders arise
variants often benign on their own
when multiple variants exist, they can combine and become deleterious, resulting in disease
how do you identify genes associated with multifactorial disorders?
GENOME WIDE ASSOCIATION STUDIES
observational study of a genome-wide set of genetic variants across different individuals
identify variants associated with a particular trait/disease
what is aneuploidy
an organism or cell having a chromosome number that is not an exact multiple of the haploid
list the structural abnormalities that result in chromosomal disorders
inversions insertions deletions duplications translocations
T or F
mutations in different genes always have different phenotypic effects
FALSE
mutations in different genes can have the same phenotypic effects
what are the consequences of missing an X chromosome in a germ cell VS a somatic cell
GERM CELL:
loss of X-chromosome is inherited by all cells
leads to Turner syndrome (short stature, delayed puberty, learning disabilities, heart defects)
SOMATIC CELL:
loss of X-chromosome leads to mosaic
complementation from normal cells means symptoms of Turner syndrome aren’t as penetrant
how do chromosomal translocations create gene fusion products?
use the philadelphia chromosome as an example
breakpoints at:
– bcr (housekeeping gene) on chromosome 22
– c-abl (tyrosine kinase which regulates cell-cycle proteins) on chromosome 9
start (amino terminus) of bcr fuse to end (carboxyl terminus) of c-abl to create gene fusion product
this particular gene fusion product is called the Philadelphia chromosome → very common in leukaemia
start of bcr gene contains promoter → gene product regulated by bcr ∴ is constantly expressed in white blood cells
gene product also has activity of c-abl → cells constantly proliferate → cancer
how do chromosomal translocations create insertions?
use berkitts lymphoma as an example
berkitts lymphoma occurs between chromosome 8 and chromosomes 2, 14 or 22 (most commonly 8 and 14)
breakpoints between 8 and 14 result in the c-myc gene inserting onto the end of chromosome 14
c-myc becomes under control of Ig promoter and enhancers = constant expression
Ig gene produces antibodies:
– expressed heavily by white blood cells
– very important for cell cycle
– constant expression = constant proliferation
what is the difference between tumour suppressor genes and oncogenes
give examples for both
tumour suppressor genes = when mutated, fail to suppress pathways for tumour development
e.g. p53, BRCA1
oncogenes = when mutated, actively promote pathways for tumour development
e. g. RAS, MYC
* note: non-mutated oncogene = proto-oncogene
what happens when cancer genes are mutated in germ-line cells?
when cancer genes* are mutated in germ-line, there is a predisposition to develop cancer
- mutation is most often loss of function in tumour suppressor gene
two-hit hypothesis:
cancer develops only if a second mutation in somatic cells knocks out the function of the remaining wild-type allele
in gleevac effective in treating leukaemia
gleevac blocks the active site on the abl protein in the bcr-abl complex
abl is now unable to phosphorylate/activate proteins
∴ no production of myeloid leukaemia
are B-raf inhibitors effective in treating leukaemia
no
B-raf inhibitors do prevent cell proliferation but only act on a single pathway
∴ tumour able to develop using a different pathway
effective drugs target multiple pathways
how can mutation databases be used to identify cancer-associated genes?
within individual cancer there are often mutation in the same pathway (similar genes affected)
COSMIC databases collates mutations to see which genes / what types of mutations are commonly effected in different cancers
can learn how different types of cancers function
what is bioinformatic analysis used for?
bioinformatics is the study of genetic and other biological information using computer and statistical techniques
used to analyse: – sequence mutations – sequence motifs found in other disease-associated genes – gene co-expression – transcriptional networks – functional genomics
how can bioinformatic predictions be validated?
use protein or RNA analysis to determine when and where a gene is expressed
1) use antibodies to stain for particular proteins/RNA OR add GFP to gene sequence at carboxy or amino terminal (depends on structure). these methods allow one to characterise exact location and distribution of proteins throughout cell
2) insert strong promoter into gene of interest so it is overexpressed. observe phenotype or monitor RNA expression to discern effects
3) use RNA interference to block gene expression (RNA cleavage or transcript degradation). observe phenotype when gene doesn’t function to discern effects
describe process of somatic gene therapy
bone marrow stem cells are taken
a retroviral vector is prepared containing WT copy of mutant gene
retrovirus infects stem cells —> WT incoorperated into host genome
still have copy of defective gene, but now have copy of functional gene
cells are grown and expanded in lab until sufficient quantity is reached
cells injected back into patient
describe process of germline gene therapy
guideRNA targets enzyme to particular location in genome
creates ds break in DNA at location
non-homologous repair often results in small insertion or deletion —> random mutation
mutation used to knock-out target gene
can also use donor DNA to insert at location i.e. a transgene or WT copy
must be inserted at very particular time in development (too late, result in mosaic)
pros and cons of germline vs somatic gene therapy
GERMLINE pros: • corrects every cell in body • no mutant allele present if successful cons: • ethical issues re. genetic engineering • very expensive • doubts on effectiveness
SOMATIC: pros: • can treat patients at any age cons: • mutant allele remains in some cells • may not recover full physiological function • chance of death
