Microsatellite Instability in
Sporadic Colorectal Cancer:
A Retrospective Study
Kimberley Slowther
Trainee Project
West Midlands Regional Genetics Laboratory
Colorectal Cancer


35,000 diagnosed per year
Treatment and prognosis depend upon
tumour stage
Causes of CRC
Sporadic
75%
Familial
19%
Rare
Syndromes
<1%
FAP
1%
HNPCC
5%
Genetic Pathways in CRC

Microsatellite Instability (MSI-H)

Deficiency in DNA mismatch repair (MMR)



Chromosomal Instability (CIN)


HNPCC (MSH2, MLH1, MSH6, PMS2)
15% sporadic (hypermethylation of MLH1 promoter)
APC, TP53, KRAS, Loss of Heterozygosity,
aneuploid DNA content
CpG Island Methylator Phenotype (CIMP)

Methylation of CpG islands in promoter regions of
tumour suppressor genes

Sporadic CRCs demonstrating MSI are a subset
Microsatellite Instability
Normal
Normal
Tumour
Tumour
Comparison of Sporadic MSI-H
with HNPCC Tumours

Both are MSI-H and proximally located

BUT Sporadic MSI-H:





Greater predilection for the proximal colon
Higher frequency of BRAF mutations
Lower frequency of KRAS mutations
More age related
More common in females
Originate from serrated polyps

HNPCC tumours originate from adenomas
RAS-RAF-MEK-ERK Pathway
Regulates growth, differentiation and apoptosis
KRAS
GTPase
90% mutations in codons
12 and 13
30-40% sporadic MSS CRC
40% HNPCC

BRAF
Serine-threonine specific
kinase
40% sporadic MSI-H
tumours
Not present in HNPCC
Common mutation (90%) is
V600E

MSI and Prognosis

MSI in CRC is a positive prognostic
indicator



Better five-year rate of overall survival than
MSS tumours
Less likely to metastasise
Why?


Enhanced mutation rate induces a burden not
compatible with tumour cell survival
Abnormal peptides produced elicit antitumour
immune responses that limit tumour growth
Treatment of CRC

Dictated by stage



Stage I: surgery alone
Stage II, III+IV: adjuvant therapy
Less clear cut with Stage II


Patients reviewed on individual basis
Chemotherapy Drugs


5-fluorouracil/folinic acid (5FU/FA) to
Stage II and III patients
5FU/FA and Oxaliplatin to Stage IV and fitter
Stage II and III patients
MSI and Chemotherapy

Resistance of MSI-H CRC to 5FU is well documented



MSS patients have increased survival with 5FU
MSI-H patients do not have improved survival following
treatment
Why have 5FU treatment if there is no benefit?

Oxaliplatin therapy not affected by loss of MMR

Information about MSI status could impact treatment:
Decision of whether or not to opt for adjuvant therapy

Allocation patients to oxaliplatin therapy
Project Aim
Investigate whether microsatellite
analysis of sporadic colorectal cancer
would be a valuable service to offer in
future in order to tailor patient
treatment
Methods

MSI status of patients with locally advanced, treatable sporadic
(absence of family history) CRC



7 microsatellite markers



41 – Department of Surgery Epithelial Research Group
32 – Department of Histopathology
MSI-H if instability present at 2 or more markers
MSI-L if instability present at 1 marker
Analysed MSI data in relation to:





Age at diagnosis
Gender
BRAF exon 15 mutation
KRAS codon 12 and 13 mutation (41/73)
Tumour site
Tumour differentiation
Results: Microsatellite Status and
Age
Total
MSIH
Mean age (y ± SD) 69±14 66±17
MSS + MSI-L
P
value
70±13
0.36
Plus-minus values are means ±SD.


On average patients demonstrating MSI in their
tumours were younger but this was not statistically
significant
Of the 14 patient samples demonstrating MSI, 7 were
older than 70 at diagnosis

Usually opt not to be treated with adjuvant therapy; MSI
status could be used to help make this decision
Results: Microsatellite Status and
Gender
Gender (%)

Total
MSIH
MSS + MSI-L
36 (49)
5 (7)
31 (43)
Female 37 (51)
9 (12)
28 (38)
Male
P
value
0.26
More female samples demonstrated MSI-H
but this was not statistically significant
Results: Microsatellite Status and
Tumour Differentiation
Differentiation
(%)

Total
MSIH
MSS + MSI-L
Poor
10 (15)
6 (9)
4 (6)
Moderate
49 (71)
7 (10)
42 (61)
Well
10 (15)
0 (0)
10 (15)
MSI-H tumours more poorly differentiated
than MSS or MSI-L tumours
Results: Microsatellite Status and
Tumour Site
Total
Cancer Site
(%)

MSI-H MSS + MSI-L
Proximal 34 (44) 11 (14)
Distal
43 (56)
4 (5)
P
value
23 (30)
39 (51)
0.011
MSI-H associated with localisation of tumour
to the proximal colon
Results: Microsatellite Status and
BRAF/KRAS Mutations
Total MSI-H MSS + MSI-L



KRAS mutation (%)
11
1 (9)
10 (91)
BRAF mutation (%)
2
2 (100)
0
KRAS mutations and BRAF mutations were mutually
exclusive
MSS and MSI-L tumours had a higher frequency of
KRAS mutations
MSI-H tumours higher frequency of BRAF mutations
Discussion

What are the benefits of MSI analysis for sporadic
CRC?

Improved patient care



? Cost Benefit


MSI-H not given unnecessary treatment (5-FU)
MSI-H allocated to more effective treatments (Oxaliplatin)
Reduced chemotherapy?
Perhaps not all tumours were sporadic

Ethical Implications

Possibility that HNPCC patients included


Only 14% MSI-H samples contained BRAF mutations
Recommend that patients are counselled
Future Developments

Prospective study

Treatment vs no treatment



? Unethical
Complicated by wide use of oxaliplatin
Oxaliplatin vs 5FU

In the future just offer oxaliplatin therapy to
MSI-H patients
Conclusion


Techniques available to put a system
into place to offer MSI analysis of
sporadic colorectal tumours
Tailor patient treatment depending
upon tumour stage, microsatellite status
and age
Acknowledgements

West Midlands Regional Genetics Laboratory




University of Birmingham Department of Surgery
Epithelial Research Group



Fiona Macdonald
Jennie Bell
Kerry Wall
Dion Morton
Germaine Caldwell
University Hospital of Birmingham Histopathology
Department



Philippe Taniere
Brendan O’Sullivan
Graham Caine