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21

NCCN Guidelines for Patients

®

:

Rectal Cancer, Version 1.2017

2

Treatment planning

Cancer cell tests

treatments for metastatic rectal cancer do not work

if the

RAS

genes are abnormal. Thus, testing for

mutations in

KRAS

and

NRAS

genes is advised for

metastatic disease.

BRAF mutation

The

BRAF V600E

mutation is also known to affect

some treatments. About 5 to 9 out of every 100 rectal

cancers have a mutated

BRAF

gene. Testing for

the

BRAF V600E

mutation is advised for metastatic

disease.

MMR and MSI

Normal MMR (

m

is

m

atch

r

epair) proteins correct

DNA errors that occur when copies of DNA are being

made. In some rectal cancers, MMR mutations

cause one or more MMR proteins to be absent. As a

result, DNA errors aren’t corrected and the number

of gene mutations increases. Doctors call this dMMR

(

d

efective

m

is

m

atch

r

epair).

The DNA errors caused by dMMR often occur in

microsatellites. Microsatellites are a tiny part of the

DNA code that is repeated many times in a row.

See Figure 8

. Due to dMMR, microsatellites may

be shorter or longer than normal. This is called MSI

(

m

icro

s

atellite

i

nstability).

Loss of MMR proteins and MSI are features of Lynch

syndrome. One or both features is present in over

90 of every 100 Lynch syndrome-related cancers

(>90%). However, these features can still occur in the

Figure 8

MMR system

A. The four types of MMR

proteins are present to correct

DNA errors when copies of DNA

are being made. An error has

been made in the C-G pair. G has

been replaced by T.

B. The MMR system is deficient.

Some MMR proteins are missing.

A DNA microsatellite has been

shortened in the bottom DNA

copy.

A T C G T C G T A T C T C G T A G

T A G C A G C A T A

T

A G C A T C

A C G T A C

T G C A T G

A T C G T C G T A T C T C G T A G

T A G C A G C A T A G A G C A T C

DNA

factory

MMR

MMR

MMR

MMR

A T C A C A C A C A C A C A T A G

T A G T G T G T G T G T G T A T C

A C G T A C

T G C A T G

A T

C A C A C

T A T C T C G T A G

T A

G T G T G

A T A G A G C A T C

DNA

factory

MMR

Copyright © 2017 National Comprehensive Cancer Network

®

(NCCN

®

).

www.nccn.org

A. Normal MMR

B. dMMR