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Advances in Brief |
Clinical Cancer Genetics and Human Cancer Genetics Programs, Comprehensive Cancer Center, and Division of Human Genetics, Department of Internal Medicine, The Ohio State University, Columbus, Ohio 43210 [O. G., M. A., H. D., C. E.]; Medizinische Universitätsklinik, Albert-Ludwigs-Universität, D79106 Freiburg, Germany [H. P. H. N.]; and Cancer Research Campaign, Human Cancer Genetics Research Group, University of Cambridge, Cambridge CB2 2QQ, United Kingdom [C. E.]
| ABSTRACT |
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| Introduction |
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10% are hereditary (1
, 2)
and may be
found in association with the MEN 23
syndromes or VHL
(3, 4, 5)
. Isolated familial forms of pheochromocytoma
without other associated clinical features occur even less commonly
(6)
. The etiology of most familial forms of
pheochromocytoma is well known. Germ-line mutations in the
RET proto-oncogene are found in >98% of MEN 2 patients
with pheochromocytoma (5
, 7)
, and germ-line mutations in
the tumor suppressor gene VHL are found in >98% of VHL
patients with pheochromocytoma (8
, 9)
. In contrast, little
is known about the etiology of the more common sporadic form of
pheochromocytoma. Somatic RET mutations have been found in
up to 10% of tumors and somatic VHL mutations in no more
than 2% (10, 11, 12, 13)
. Pheochromocytomas are also referred to as adrenal paragangliomas. Paragangliomas originate from the neural crest-derived paraganglia of the autonomic nervous system. They are identified throughout the body in both adrenal and extra-adrenal sites. Hereditary paragangliomas are highly vascularized tumors located in the head and neck. They are most commonly found at the bifurcation of the carotid artery in the neck, also referred to as the carotid body. Recently, germ-line mutations of SDHD were found in patients with hereditary paraganglioma (14) . SDHD, located on 11q23 (15) , encodes the small subunit (cybS) of cytochrome b in the succinate-ubiquinone oxidoreductase complex (mitochondrial complex II). This enzyme complex plays an important role in both the tricarboxylic acid cycle and the aerobic respiratory chain of eukaryotic cell mitochondria. SDHD comprises four exons and three introns spanning over 19 kb (16) .
We sought to determine whether SDHD plays a role in the development of sporadic cases of what has been defined as pheochromocytomas in the German-Polish Registry by analyzing the SDHD gene for sequence variants and microsatellite markers flanking the gene for LOH. Our results indicate that SDHD plays a role in the pathogenesis of pheochromocytoma.
| Materials and Methods |
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DNA extraction was performed using the QIAamp tissue kit (Qiagen, Santa Clarita, CA) according to the manufacturers instruction.
Mutation Analysis.
PCR amplification was carried out in 1 x PCR buffer
(Qiagen) that contained 200 µmol/liter deoxynucleotide triphosphate,
0.6 µmol/liter of each primer (14)
, 2.5 units of Taq
polymerase (Qiagen), and 100200 ng of tumor DNA template in a 50 µl
volume. PCR conditions were: one cycle of 15 min at 95°C; 35 cycles
of 1 min at 95°C; 1 min at 58°C; 1 min at 72°C; followed by one
cycle of 10 min at 72°C.
PCR amplicons were gel (Bio-Rad Laboratories, Hercules, CA) and column (Wizard PCR Prep; Promega, Madison, WI) purified and subjected to semiautomated sequencing using the above primers, dye terminator technology, and the ABI377xl or PE3700. If sequencing revealed a variant, the corresponding leukocyte DNA was examined in the same manner to determine whether the sequence variant was somatic or germ-line. Each sample with a sequence variant was subjected to repeat mutation analysis that used a separate PCR reaction.
LOH Analysis.
All of the PCR reactions were carried out using 0.6 µM
each of forward and reverse primer in 1 x PCR buffer
(Qiagen), 4.5 mM MgCl2 (Qiagen),
1 x Q-buffer (Qiagen), 2.5 units of HotStarTaq
polymerase (Qiagen), and 200 µM deoxynucleotide
triphosphate (Life Technologies, Inc., Gaithersburg, MD) in a
final volume of 50 µl. Reactions were subjected to 35 cycles of
94°C for 1 min, 5560°C for 1 min, and 72°C for 1 min followed
by 10 min at 72°C. LOH analysis was performed as described previously
(18)
using the two markers flanking SDHD,
D11S1347 (telomeric) and D11S1986 (centromeric). Both forward primers
were 5'-labeled with either HEX or 6-FAM fluorescent dye
(Research Genetics, Huntsville, AL). PCR reactions were carried out as
described above and separated by electrophoresis through 6% denaturing
polyacrylamide gels using an Applied Biosystems model 377 automated DNA
sequencer (Applied Biosystems and Perkin-Elmer Corp., Ridgeway,
CA). The results were analyzed by automated fluorescence
detection using the GeneScan collection and analysis software
(GeneScan; Applied Biosystems, Norwalk, CT). Scoring of LOH was
performed by inspection of the GeneScan analysis output. A double peak,
observed in the microsatellite marker that was amplified from DNA
extracted from the germ-line sample, indicated heterozygosity. A single
peak in DNA extracted and amplified from the corresponding tumor sample
indicated a loss of one allele. If normal cells were admixed with tumor
cells, a ratio of 1.5:1 or greater of the germ-line DNA peak:tumor DNA
peak was operationally defined as LOH.
| Results |
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LOH Analysis.
Overall, LOH (Fig. 2)
in at least one of
the two 11q23 markers was found in 13 tumors (72%). LOH of both
markers was found in 3 tumors (17%). The remaining 10 tumors showed
LOH of one or the other 11q23 marker. All of the tumors already
harboring intragenic SDHD mutations also showed LOH.
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| Discussion |
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The nonsense germ-line R38X mutation is almost certainly pathogenic because it would likely result in a very short nonfunctional protein. This mutation was also found by Baysal et al. (14) in the germ line of one family with hereditary paraganglioma. The germ-line IVS1+2T>G mutation occurs at one of the highly conserved consensus splice sequences and is almost certainly functionally significant inasmuch as it would be predicted to result in aberrant splicing. Unfortunately, despite great efforts, no RNA is available. This germ-line variant has not been found in 78 regional control alleles, and Baysal et al. (14) did not note the presence of this variant in 200 of their control chromosomes either. That this patient developed a carotid body tumor, almost certainly a paraganglioma, 4 years subsequent to his diagnosis of isolated pheochromocytoma and that the tumor had LOH of the remaining allele lend strong support that this germ-line variant is actually an occult germ-line SDHD mutation that predisposed to both the pheochromocytoma and the carotid body tumor. Finally, the G12S germ-line variant that occurred in a teenager with adrenal and extra-adrenal pheochromocytoma is somewhat of a dilemma because it has been found in one regional control who did not have any evidence of neuroendocrine tumors at the age of 72. Baysal et al. (14) , on the other hand, did not note G12S in their 200 normal control chromosomes. Nonetheless, we can postulate either that this is a pathogenic but low-penetrance germ-line missense mutation that predisposes to pheochromocytoma or that this variant could be a very rare polymorphism. Codon 12 is conserved between Homo sapiens and the domestic cattle (14) . In that position, an alteration from a very small to a larger neutral and polar amino acid might have some consequence; although without direct functional assay, it is open to speculation. Given that the corresponding tumor had LOH of the wild-type allele also lends some support that G12S may be somehow pathogenic. Nonetheless, the conservative estimate of occult germ-line SDHD mutations that are likely pathogenic would be 11% if we discount the G12S variant (see below).
The LOH frequency at the SDHD locus on 11q23 is worthy of
note, as well. LOH in pheochromocytomas has been reported on 1p
(4271%), 3p25 (1656%), 17p (24%), and 22q (3153%; Refs.
19, 20, 21, 22
). However, LOH on chromosome 11 in
pheochromocytomas has not been reported to date. We found LOH in at
least one of the two investigated markers on 11q23 in 72% of
pheochromocytomas, a significant frequency. Compared with this high
frequency of LOH, the frequency of functional mutations in
SDHD (17%) is relatively low. The 6% somatic mutation
frequency in SDHD, however, is comparable with that of
RET and VHL mutations. We suspect that in the
tumors with apparent hemizygous deletion, other epigenetic mechanisms,
e.g., methylation of CpG islands in this region, may also
play a role in silencing SDHD similar to the inactivation of
143-3
in breast cancer (23)
. Because the promoter of
SDHD has not been identified yet (16)
, no
further attempt to address this question has been undertaken. Another
explanation of our LOH data is that the LOH actually reflects deletion
of genes that neighbor SDHD and not SDHD itself.
The molecular basis for sporadic pheochromocytomas has been somewhat
elusive. Allelotyping studies have revealed regions of LOH (above).
Despite relatively high frequencies of LOH in the VHL gene
region (3p25), <2% of sporadic pheochromocytomas carry somatic
VHL mutations. Similarly, somatic RET mutations
have been shown to occur in 010% of sporadic pheochromocytomas
(10
, 12 , 24)
. Although our data need to be confirmed in a
larger series, they strongly argue for a role of SDHD in the
pathogenesis of sporadic pheochromocytoma. It was of particular
interest that we found not only a somatic mutation but also occult
germ-line variants, suggesting a familial predisposition. Among prior
series of apparently sporadic pheochromocytoma presentations, occult
germ-line mutations in VHL and RET were found
only in
5% and <2%, respectively (10
, 13)
. Thus, the
minimum estimate of occult germ-line SDHD mutation frequency
of 11% (17% if the variant of unknown significance is considered)
found in these patients is significant. This mutation frequency is
clinically important in the context that a preliminary survey of the
entire Registry, which comprises
100 nonfamilial pheochromocytoma
cases, also reveals an occult germ-line SDHD mutation
frequency of 10%.5
From this current study alone, it is not known whether germ-line
SDHD mutations in isolated pheochromocytoma cases would
predict for the development of extra-adrenal disease or whether they
could also predispose to a familial pheochromocytoma only syndrome. Of
interest, among the Registry cases as a whole, 70% of those with
germ-line SDHD mutations had intra-adrenal pheochromocytomas
only, and 30% had extra-adrenal disease.5
Families with pheochromocytomas only (6)
who do not have
germ-line mutations in RET and VHL should be
scanned for mutations in SDHD on a research basis in the
first instance. Additionally, if further study proves that occult
germ-line SDHD mutations occur with the frequency reported
here or higher among apparently sporadic pheochromocytoma cases, then
routine SDHD mutation analysis might be considered in all
such presentations as well. Whether the presence of germ-line
SDHD mutations can predict for extra-adrenal disease or for
the future occurrence of extra-adrenal tumors in cases that
present intra-adrenal pheochromocytomas should be the subject of
further study.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Partially supported by the National Cancer
Institute, Bethesda, Maryland (P30CA16058 to Ohio State University
Comprehensive Cancer Center) and by generous gifts from the Brown
family in memory of Welton Brown and the Abrams family (to
C. E.). ![]()
2 To whom requests for reprints should be
addressed, at Human Cancer Genetics Program, The Ohio State University,
420 W. 12th Avenue, Room 690C TMRF, Columbus, OH 43210.
Phone: (614) 292-2347; Fax: (614) 688-3582 or (614) 688-4245;
E-mail: eng-1{at}medctr.osu.edu ![]()
3 The abbreviations used are: MEN 2, multiple
endocrine neoplasia type 2; VHL, von Hippel-Lindau syndrome; LOH, loss
of heterozygosity. ![]()
4 H. P. H. Neumann, S. Gläsker, B. U.
Bender, T. W. Apel, R. Munk, O. Gimm, H. Dziema, C. Smigielski, K.
Zerres, A. Januszewicz, and C. Eng. High frequency of inherited
mutations causing pheochromocytoma in a registry-based cohort,
submitted for publication. ![]()
5 H. P. H. Neumann, S. Gläsker, B. U.
Bender, T. W. Apel, R. Munk, O. Gimm, H. Dziema, A. Januszewicz, and C.
Eng. Occult germline SDHD mutations in a registry-based
pheochromocytoma cohort, manuscript in preparation. ![]()
Received 6/ 9/00. Accepted 10/31/00.
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