Precision medicine Flashcards
what is precision medicine?
medical care designed to optimize efficiency or therapeutic benefit for particular groups of patients, especially by using genetic or molecular profiling.
within the last 10-20 years we have moved towards targeting the therapy to those that will get the most benefits – by examining their characteristics in detail, giving them a personalised treatment where the benefits are most significant and side effects are minimised
what is the traditional approach to treatment?
The traditional approach is treating the population with the same treatment – a mixed group but we target the average patient – measure if it is effective by looking at our outcomes, some would develop no outcome, other negative but normally on average most would have improved outcome
whatshe difference between precision medicine and personalised medicine?
- Precision matched to a population characteristics (genome, environment, population health data then data analytics)
- Not interchangeable – personalised is unique to the individual (and is unlikely to work for anyone else)
Enabled by new omics technologies, Mostly genetic markers currently, Particularly now pharmacogenetics (the study of how a persons genetic makeup (genome) influences their response to treatment)
how many drugs currently use pharmacogenomic markers?
Currently > 200 drugs have label information regarding pharmacogenomic markers* Incl. CVD, lung disease, HIV, cancer, arthritis, high cholesterol and depression
how can we target mutated proteins?
Next gen sequencing allows identification of single gene mutations which cause disease. Gene therapies can rectify these mutations in some cases.
If genetic mutations can be rectified, it is possible to target mutated proteins with small molecule inhibitors or Mabs
give some examples of precision medicine used for cancer currently
- Monoclonal antibodies like Herceptin is a form of precision medicine - HER2 mutation Different responses and outcomes to Cx and radiotherapy
- Drugs targeting BCR-ABL via tyrosine kinase (TK): Small molecule TKI)
- Anti-angiogenesis treatments, EGFR inhibitors, BRAF inhibitors (if + test for BRAF mutations)
- EGFR inhibitors (test for mutations), ALK inhibitors, ROS1, KRAS, NTRK, BRAF, MET, RET, anti-angiogenesis
- Mabs against CD20, CD19, CD79b, often conjugated to chemotherapy drugs
- BRAF, MEK, KIT inhibitors after identifying mutations
precision medicine may help by/when…
(3)
- Stratifying diseases to identify improved std therapeutic outcomes
- Targeting via known mechanisms
- Targeting by location
what factors complicate the question are tumours self or non self?
- Some harbour microbes (viruses/bacteria/fungi)
- Some are caused by viruses
- Some lose HLA
- Expression of Tumour Associated Antigens (abnormal quantity, location or time) – e.g. NY-ESO-1, XAGE1, MAGE, CTA
- Expression of Tumour Specific Antigens (resulting from DNA mutations)
what is the spontaneous response to tumours?
which MHC class are tumour antigens expressed on
?
- Tumour proteins are endogenous and could be presented by tumour cells on HLA-I
- Dead tumour cells can be phagocytosed by APCs (Mf, DCs) as exogenous proteins on HLA-II, and presented on HLA-I (cross-presentation)
what is the principle of immunotherapy?
aim to enhance the natural anti-tumour response
give five examples of immunotherapies
- Immune checkpoint inhibitors (PD1/PDL1)
- T cell transfer therapy (CAR-T cells)
- Monoclonal antibodies (anti-TAA mAbs)
- Treatment vaccines (TAAs)
- Immune system modulators (cytokines, BCG/oncolytic viruses, immunomodulatory drugs e.g. angiogenesis inhibitors)
how can we boost the anti-tumour T cell response with immunotherapy?
immune checkpoin inhibitors like PDL1/PD1
they prevent t cell exhaustion, removing the checkpoint to allow t cells t o remain active against the tumour
what is the sceintific basis of immuhne checkpoint inhibitors?
- Acute stimulation of T cells requires rapid response, closely followed by immunomodulation (PD-1/PD-L1) to prevent uncontrolled T cell activity
- Chronic stimulation of T cells leads to “exhaustion” (PD-1 Tim-3, LAG on T cells)
- Tumours have an immunosuppressive/exhausted environment, incl. PD-L1 on tumour cells
- Immunotherapy interferes with PD-1/PD-L1 interaction, removing checkpoint
what are the types of mutations?
▪ Substitution (point mutations) :
Silent (same amino acid), nonsense (stop codon), missense (diff amino acid)
▪ Insertions
▪ Deletions
▪ Duplication
▪ Inversions
▪ Translocations
give an example of a mutation leading to loss of function
p53 is a driver gene
* Mutated p53 leads to LOF
* As function is repressing cell proliferation in response to stress, LOF predisposes to cancer
what causes tumours?
- DNA changes in Proto-oncogenes, Tumour suppressor genes, DNA repair genes lead to tumour growth
All driver genes - DNA changes induced by:
➢Environmental damage (UV, smoke, alcohol)
➢Inheritance
➢Viruses
➢Cell division errors
how are mutated proteins recognised?
T cells identify peptide from non self proteins but they have to be presented on host MHC
During infections, non-self is obvious – viral/bacterial proteins are inside the cell
On tumours, non-self could mean proteins not normally produced in adult tissues – CTAs, splice variants etc.
Or abnormal proteins arising from cancer-causing mutations
why dont we know what the t cell targets?
TIL analysis suggests 5-20 clonotypes in a tumour
* Neoantigens are peptides derived from mutated proteins capable of binding to MHC
* Cannot easily identify T cell targets as the TCR does not inform the MHC peptide
breifly describe whole genome sequencing using Illumina
DNA fragmented then
▪ Adaptor sequences ligated to ends (contain primer sequences)
▪ Size sorted (size depends on machine)
‘Bridge amplification’
▪ Single strand binds to sequences attached to a solid surface
▪ Free end binds to a nearby complimentary sequence (bridge)
▪ dNTPs (unmodified) are added to create a double strand
▪ Denatured to form 2 strands attached to the tile
▪ Repeated to form local clusters of copies of the same sequence
describe how the sequence is determined in illumina sequencing
▪ “Reversible terminators” instead of a mixture of dNTPs and ddNTPs
▪ Mutant DNA polymerase required to incorporate modified bases
▪ 3’ end free to incorporate next base
▪ All 4 modified dNTPs added
▪ Correct base incorporates
▪ Multiple bases cannot be incorporated due to blocking group
▪ Free bases washed away
▪ Laser excites fluorescent dye
▪ Base (colour of fluorescence) is detected
▪ Fluorescent dye and 3’ blocking group cleaved
▪ Cycle repeats
why does the quality decrease with as the sequence is read in illumina sequencing?
The dna pol that adds these adapted nucleotides (mutant dna pol needed bcos they are flourescent) it doesn’t have a high fidelity – so the quality at the start is quite high but less and less reliable as you go along the fragment
▪ Initially just ~35bp of sequence now up to 300bp
▪ Modified DNA polymerase required to attach reversible terminators, can lead to errors in incorporation
▪ Sequence quality reduces towards the end of the read, as clusters get out of sync
what is the most common implementation of illumina sequencing?
Paired–end Illumina sequencing
what is the most common implementation of illumina sequencing?
Paired–end Illumina sequencing
describe the principle of paired end sequencing?
▪ Both ends of a sequence fragment are sequenced in turn
▪ Pairs are identified by shared location on the chip, producing 2 sequence files eg. sample_1.fq and sample_2.fq that are in sync.
▪ Then aligned to the reference sequence
what is in a FASTQ file?
Illumina sequencing results
multiple paired end sequences are aligned over a reference sequence
the file indicates the quality, you have a string of bases that you belive to be the sequence and below each position is a character indicating the quality
▪ Four lines per sequence
1. Sequence identifier (starting with @)
2. DNA sequence
3. ’+’ optional sequence identifier
4. Corresponding quality scores (single ASCII character per base)
what is phred?
▪ Quality score (Phred) is an integer representing the probability that the corresponding base is correct
▪ Phred of 20 = 1 in 100 or 99% accuracy
what can be sequenced by Next Generation Seqencing for mutanome analysis?
▪ Whole genome sequence (WGS)
▪ Exome (Exome-seq)
▪ mRNA
▪ miRNA
▪ Methylation sequencing
▪ Chip-seq
▪ Ribo-seq
what is the exome?
An exome is the sequence of all the exons in a genome, reflecting the protein-coding portion of a genome (a subset of the genome, exons only). In humans, the exome is about 1.5% of the genome.
what percentage of mutations causing disease are located in the exome?
85%
estimated
why would you sequence the exome?
- Next Generation Sequencing makes sequencing the exome a cost effective strategy
- Can screen all genes at once
o ~20,000 genes (average of 8 exons per gene)
o >150,000 exons
o ~50Mb of sequence - Much cheaper and easier to interpret than whole genome
o Focuses on the part of the genome we understand best, the exons of genes - Applications:
o To discover variants involved in rare Mendelian and complex diseases
o To detect somatic mutations in cancer
o Molecular diagnostics
(costs £600 to sequence as opposed to £1000 for entire genome)
how do you do exome sequencing?
Still taking genomic dna and fragmenting it up
Hybridisation
Wasg
Incubating it up with beads that will binds to sequences that we already known and pull them down
Captured DNA is then sequenced and analysed
how is exome sequencing analysed?
Start with QC: (quality control)
Remove adaptor sequences (we know what they were so computational remove)
Remove low quality sequence information (look at the fastq file and remove all with low quality)
Align with reference genome and start to compare (computationally)
so you can say according to probability, whether the base inserted is correct for the sequence, whether it is hetero or homogenous
use database to compare
what are the three stages of exome sequencing analysis?
Alignment
- raw sequence reads
- human genome reference sequence
Variant calling
- ‘pile up’ reads
- identify variants
Annotation
- annotated output files
how many variants are found per sample?
~23000 variants per sample
what are the three parts to the annotation stage of exome sequence analysis?
- Gene and variant information (genes already known)
Gene name, Transcript ID, Exon number, Nucleotide change, amino acid change, Type of mutation; indel, missense, nonsense, silent - Cross reference with databases of known variants
(100000 genome project to show potential variation) - Functional prediction (is it involved in disease, could it be?)