The activated RAS proteins excite the mitogen-activated protein (MAP) kinase pathway
(RAS–RAF–MEK–ERK–MAP kinase pathway) by recruiting the cytosolic protein RAF (Downward,
2003). RAF gene family consists of three members (ARAF1, BRAF and RAF1), each encoding
serine/threonine kinases that are regulated by binding to RAS. RAS–RAF–MEK–ERK–MAP
kinase pathway plays a critical role in cell proliferation, and is frequently activated
in cancer cells. For example, approximately 10–20% of all human tumours contain mutated
versions of RAS proteins (Bos, 1989). Recently, Davies et al (2002) identified somatic
mutations of BRAF in 66% of malignant melanomas and at a lower frequency in a wide
range of human cancers. So far, all BRAF mutations have been reported within two kinase
domains (G-loop and activation segment domains), and the most common mutation is a
single substitution, V599E (Brose et al, 2002; Davies et al, 2002; Naoki et al, 2002;
Rajagopalan et al, 2002; Yuen et al, 2002; Pollock et al, 2003; Satyamoorthy et al,
2003). Mutated BRAF proteins have elevated kinase activity and transforming activity
in NIH3T3 cells (Davies et al, 2002). Furthermore, RAS function is not required for
the growth of cancer cell lines with the V599E mutation (Davies et al, 2002). Several
studies have reported a low incidence of Ras gene mutation in non-Hodgkin's lymphoma
(NHL) (Ahuja et al, 1990). Although screening of BRAF mutation in human tumours has
widely been performed, to date the data on BRAF mutation in NHL tissues is lacking.
In the present study, we investigated the occurrence of BRAF gene mutations in NHLs.
MATERIALS AND METHODS
Paraffin-embedded tissues of human NHL were obtained from 164 patients. These samples
were stained with haematoxylin–eosin, examined by immunohistology, and then classified
according to the Revised European-American Lymphoma (REAL) classification (Harris
et al, 1994). The NHLs analysed consisted of seven B-cell small lymphocytic lymphomas,
three mantle cell lymphomas, four follicular lymphomas, 49 mucosa-associated lymphoid
tissue (MALT)-type lymphomas, 67 diffuse large B-cell lymphomas, four precursor T-lymphoblastic
lymphomas, one T-cell chronic lymphocytic leukaemia, 14 peripheral T-cell lymphomas,
unclassified, 14 angiocentric lymphomas, and one intestinal T-cell lymphoma. Ethical
committee approval for the study was obtained. Through the microdissection technique,
we selectively procured tumour cells and corresponding normal cells from histological
sections of the 164 NHLs. Briefly, malignant cells were selectively procured from
haematoxylin and eosin-stained sections using a 30G1/2 hypodermic needle (Becton Dickinson,
Franklin Lakes, NJ, USA) affixed to a micromanipulator, as described previously (Lee
et al, 1998). We also microdissected normal cells and used them for corresponding
normal DNA. This microdissection technique used in this study has been proved to be
precise and effective for procurement of tumour cells without normal cell contamination
(Lee et al, 1998). DNA extraction was performed by a modified single-step DNA extraction
method, as described previously (Lee et al, 1998).
Genomic DNA each from normal cells or tumour cells was amplified with two primer pairs
covering exons 11 and 15 of BRAF gene, because all of the BRAF mutations have been
so far detected in exons 11 and 15 that encode the kinase domains in G-loop and the
activation segment of BRAF, respectively. Radioisotope was incorporated into the PCR
products for detection by autoradiogram. The PCR reaction mixture was denatured for
1 min at 94°C and incubated for 30 cycles. Other procedures of polymerase chain reaction
(PCR) and single-strand conformation polymorphism (SSCP) analysis were performed as
described previously (Shin et al, 1999). After SSCP, DNAs showing mobility shifts
were cutout from the dried gel, and reamplified for 30 cycles using the same primer
sets. Sequencing of the PCR products was carried out using the cyclic sequencing kit
(Perkin-Elmer, Foster City, CA, USA) according to the manufacturer's recommendation.
RESULTS
SSCP analysis of BRAF identified four aberrant bands (Figure 1
Figure 1
BRAF gene mutations in NHLs. SSCP (A–C) and DNA sequencing analyses (D–F) of DNA from
tumours (lane T) and normal tissues (lane N). Exon 11 (A, B) and exon 15 (C) of BRAF
were amplified. SSCPs of DNA from the tumours show wild-type bands and additional
aberrant bands (arrows) as compared to SSCP from normal cells from the same patients.
(D) Sequencing analysis from the aberrant band in (A). There is a G to C transversion
at nucleotide 1403 of BRAF (arrow) in tumour tissue as compared to normal tissue.
(E) Sequencing analysis from the aberrant band in (B). There is a G to C transversion
at nucleotide 1402 of BRAF (arrow) in tumour tissue as compared to normal tissue.
(F) Sequencing analysis from the aberrant band in (C). There is an A to G transition
at nucleotide 1778 of BRAF (arrow) in tumour tissue as compared to normal tissue.
Numbering of cDNA of BRAF was made in respect to the ATG start codon (GenBank).
). Enrichment and DNA sequence analysis of these aberrantly migrating bands led to
the identification of four BRAF mutations (2.4%) (Figure 1). All of the four BRAF
mutations were observed in diffuse large B-cell lymphomas (6.0% of the 67 cases).
Although BRAF mutations were detected only in diffuse large B-cell lymphomas, this
relationship was not statistically significant (P>0.05). Three of the four BRAF mutations
involved codon 468 (two G468R and one G468A) in the G-loop domain, and the remaining
one was found at codon 593 (D593G) in the activation segment domain (Table 1
Table 1
Summary of BRAF mutations identified in the NHLs
BRAF mutations
Nucleotide
Amino acids
Anatomical site
Histologic type
G1403C
G468A
Cervical lymph node
Diffuse large B-cell lymphoma
G1403C
G468A
Cervical lymph node
Diffuse large B-cell lymphoma
G1402C
G468R
Tongue mucosa
Diffuse large B-cell lymphoma
A1778G
D593G
Ileum
Diffuse large B-cell lymphoma
, Figure 1). None of the corresponding normal samples showed evidence of mutations
by SSCP (Figure 1), indicating the mutations detected in the specimens had risen somatically.
We repeated the experiments two times, including tissue microdissection, PCR, SSCP
and sequencing analysis to ensure the specificity of the results, and found that the
data were consistent (data not shown).
DISCUSSION
Whereas the malignant melanoma is the most common tumour with BRAF mutations (roughly
60%), this tumour is known to possess a much lesser frequency of RAS mutations. Such
differential occurrences of BRAF and RAS mutation in some human cancers led us to
analyse BRAF mutation in NHL in which RAS mutation is known to be an uncommon event.
We found that BRAF gene is somatically mutated in NHLs, indicating that RAS–RAF kinase
pathway in some NHLs may be regulated by somatic mutations of BRAF. Despite the low
frequency of BRAF mutation in NHL compared with that of malignant melanoma, our data
suggest that alteration of RAS–RAF kinase pathway by BRAF mutation may play an important
role in NHL carcinogenesis.
In the present study, none of the BRAF mutations involved the amino acid V599. The
data are quite contrast to those of malignant melanomas, where approximately 90% of
BRAF mutations involved V599, raising the possibility that the contribution of BRAF
mutations in the development of NHL might be different from that of malignant melanoma.
Additionally, three (two G468A and one G468R) of the four BRAF mutations in this study
involved the same amino acid (G468) that is located in the GXGXXG motif within the
G-loop of the kinase domain. The G468A mutation was proven to be an activating mutation
by the kinase assay and the transformation assay (Davies et al, 2002). For G468R,
a novel BRAF mutation, its functional implication is not known at this stage. In one
NHL, we also found D593G BRAF mutation that has also been detected in colon tumours
previously.
The most impressive examples of recent cancer therapies used protein kinase inhibitors
such as Imanitib (Gleevec) (Downward, 2003). Since RAS–RAF–MEK–ERK–MAP kinase pathway
is activated by protein kinase, therapies that target this signalling pathway would
therefore be very valuable in treating tumours that have activating mutations of BRAF.
In this respect, the present study may provide the possibility of therapy targeting
mutated BRAF in NHL.