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Advances in Brief |
Departments of Otolaryngology-Head and Neck Surgery [J. C., W. H. W., G. M., W. K., D. S.] and Pathology [W. H. W., R. C.], Johns Hopkins Hospital, Baltimore, Maryland 21204
| ABSTRACT |
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| Introduction |
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PCR based microsatellite marker analysis enables a determination of specific genetic changes that occur in lesions with diverse histopathology. This study correlates genetic changes and histopathological progression within recurrent premalignant and malignant head and neck lesions in individual patients. These results help to further establish the temporal order of genetic events in HNSC progression and provide insight into the underlying genetic changes and biological behavior that accompany clinical phenotypic progression.
| Materials and Methods |
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Selection of Loci for Microsatellite Analysis.
Ten chromosomal arms were chosen for this analysis on the basis of the
following criteria: (a) they identify a minimal area of loss
at a putative tumor suppressor gene locus; (b) they identify
a proto-oncogene amplicon detectable by microsatellite analysis; or
(c) they have displayed a substantial proportion of LOH
(>40%) in invasive lesions by allelotype analysis. The 9p21
chromosomal band contains the p16 (MTS1) gene, a
cyclin/cyclin-dependent kinase inhibitor involved in cell cycle
regulation and corresponds to an area of genetic loss common to many
solid tumors.(3
, 4) At this time, this locus constitutes
the region with the most frequent LOH in HNSC, and the p16 protein is
not expressed in >80% of HNSC.(5
, 6)
The chromosomal
band 11q13 includes the bcl-1/int-2 locus, an amplicon
carrying the proto-oncogene cyclin D1, one of
the few proto-oncogenes implicated in HNSC (7)
. LOH in
this region actually represents allelic imbalance via amplification of
cyclin D1, as confirmed by studies using
fluorescence in situ hybridization (8)
. The
tumor suppressor gene p53 is commonly mutated in HNSC
(9)
. The p53 gene is found on chromosomal arm
17p13, which also corresponds to an area of frequent LOH in HNSC
(5)
. Chromosomal arm 3p has been shown to contain at least
three putative HNSCC tumor suppressor loci (10
, 11)
.
Chromosomal band 13q21 contains an area with frequent LOH near the
Rb locus that is now thought to include a second, novel
tumor suppressor gene locus (12)
. Chromosomes 6p and 8
contain loci thought to contain putative tumor suppressor genes that
have not been precisely mapped and are as yet unidentified but are
included because these chromosomal arms are lost in a high percentage
of preinvasive and invasive lesions (2
, 5)
. Microsatellite
markers included in this study are: D3S1067, D3S1284, D3S1038,
D3S1007 (chromosomal arm 3p), D6S265, TCTE, D6S105
(chromosomal arm 6p), D8S261, D8S262, D8S257, D8S167, D8S273
(chromosomal arm 8), IFN-
, D9S736, D9S171 (chromosomal
arm 9p21), D11S873, INT-2, PYGM (chromosomal arm 11q13),
D13S170, D13S133 (chromosomal arm 13q21), TP-53,
and CHRNB-1 (chromosomal arm 17p13).
Tissue and DNA Extraction.
Tissue was obtained from archival, paraffin-embedded blocks from the
Johns Hopkins Hospital Department of Pathology or from freshly frozen
tissue, obtained with consent from Johns Hopkins Hospital patients.
Representative sections from tissue used for DNA extraction were
stained with H&E, and the diagnosis was confirmed for each lesion by a
pathologist (W. H. W.). Freshly frozen tissue from a biopsy
specimen was meticulously dissected on a cryostat to ensure that the
specimen contained at least 75% epithelial cells from the mucosal
lesion. Areas of homogeneous histopathological appearance were
dissected separately if more than one distinct area of
histopathological alteration existed. A control section was stained
before microdissection and after each interval of 12 sections to ensure
an adequate proportion of epithelial cells from the mucosal lesion of
interest. Approximately 1235 12-µm sections, depending on the
adequacy of the specimen, were then collected and placed in 1%
SDS/proteinase K (0.5 mg/ml) at 58°C for 24 h. Paraffin-embedded
tissue from directed mucosal biopsies was sectioned into 25 14-µm
sections. Each individual section was placed on a glass slide and
individually microdissected using a dissecting microscope to obtain
>75% epithelial cells. The samples were placed in xylene overnight to
remove the paraffin, pelleted in 70% ethanol, dried, and incubated in
SDS/proteinase K at 58°C for 72 h. Digested tissue from both
sources was then subjected to phenol-chloroform extraction and ethanol
precipitation as described previously (12)
. Normal control
DNA was obtained either by: (a) venipuncture and isolation
of lymphocyte DNA as described previously (12)
;
(b) microdissection of nonepithelial normal tissue in the
previously mentioned archival, paraffin-embedded biopsy specimens; or
if necessary, (c) isolation of DNA from nonepithelial,
paraffin-embedded tissue from archival paraffin blocks other than the
biopsy specimen blocks in the manner described above.
Microsatellite Analysis.
Microsatellite markers suitable for PCR analysis were obtained from
Research Genetics. Prior to amplification, 50 ng of one primer from
each pair were end labeled with [
-32P]ATP (20
MCi; Amersham) and T4 kinase (New England Biolabs) in a total
volume of 50 µl. PCR reactions were carried out in a total volume of
12.5 µl containing 10 ng of genomic DNA, 0.2 ng of labeled primer,
and 15 ng of each unlabeled primer. The PCR buffer included 16.6
mM ammonium sulfate, 67 mM Tris (pH 8.8), 6.7
mM magnesium chloride, 10 mM
ß-mercaptoethanol, 1% DMSO to which were added 1.5 mM
deoxynucleotide triphosphates and 1.0 unit of Taq DNA polymerase
(Boehringer Mannheim). PCR amplifications of each primer set were
performed for 3035 cycles consisting of denaturation at 95°C for
30 s, annnealing at 5060°C for 60 s, and extension at
70°C for 60 s as described (12)
. One-third of the
PCR product was separated on 8% urea-formamide-polyacrylamide gels and
exposed to film from 4 to 48 h as described. For informative
cases, allelic loss (or allelic imbalance in the case of the 11q13
locus) was scored if one allele was >40% decreased in tumor DNA when
compared with the same allele in normal control DNA.
| Results |
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Another patient (no. 4) was rebiopsied over a 1-month interval for a
leukoplakic lesion of the TVC, showing a matching pattern of loss at
five loci with an identical breakpoint at 3p in lesions that were both
diagnosed as dysplastic (Fig. 1)
. However, two loci, 8q and 13q21,
displayed LOH in the second biopsy specimen, indicating that a subclone
from a common progenitor cell developed additional genetic alterations
and acquired a growth advantage, producing a persistent dysplastic
lesion.
Finally, patient 5 is represented in Figs. 1
and 2
. This patient received serial bilateral
biopsies for persistent laryngeal leukoplakia demonstrating dysplasia
on six separate occasions over a 12-year period, culminating in
diagnosis of a left true vocal cord HNSC. At least four separate
genetic progression events can be discerned from this examination. We
noted that identical alleles were lost in each of the six loci of
interest when LOH occured, indicating a common clonal origin for each
lesion. We also noted that genetic alterations initially evident on
biopsies taken from the left TVC subsequently became manifest on both
vocal cords.
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| Discussion |
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The data presented in this study reinforce the paradigm of a genetic progression model for HNSC as presented previously. Certain genetic events (9p21 LOH, 3p LOH, and 17p13 LOH) tend to occur earlier on the progression pathway, but other, usually late-occurring events (13q11, 8), may also occur early in the time course of progression. For all patients, the accumulation of genetic events was associated with histopathological progression.
In addition, the last patient presented (no. 5) demonstrated that progression to malignancy occurs over a period of years. During this progression, a significant number of genetic events occur early on the progression pathway, whereas clinical expression of a malignant phenotype (symptoms or gross morphological changes) occurs later. For this patient, all detectable genetic alterations at the loci we tested were described 9 years before the clinical appearance of malignancy. This is consistent with our previous observation that the incidence of genetic alteration in dysplastic, premalignant lesions is greater than half the rate of genetic alteration found in invasive HNSC (2) . When coupled with the observation that the latency period between carcinogen exposure and appearance of malignancy may be as long as 25 years in HNSC, these data suggest that a significant amount of detectable genetic alteration may be present in affected mucosa years before an invasive phenotype is produced.
Finally, the last patient (no. 5) also afforded insight into the nature of clonal expansion for HNSC and human cancers in general. As successive genetic alterations were acquired by a dominant clonal population in this patient, a phenotypic progression to malignancy was seen, accompanied by a wave of clonal expansion to the contralateral TVC in the absence of an invasive phenotype. This demonstrates that clonal, epithelial populations, conferred with a significant growth advantage, may migrate to distances of several centimeters. Supporting evidence for this phenomenon is also provided by: (a) studies that indicate a clonal relationship between adjacent epithelial areas of diverse histopathlogical appearance (2 , 13) ; (b) studies that indicate that a second primary HNSC is usually clonally related to the initial primary HNSC, despite an anatomically distant site of origin (14) ; and (c) studies that demonstrate patches of normal appearing epithelia in the upper aerodigestive tract that are clonally related to regional metastases in patients with HNSC of unknown primary origin (15) .
These observations have several clinical implications. Clonal genetic alterations may precede the development of malignancy by a significant amount of time, at least several years. Clonal expansion and lateral migration of genetically altered clonal populations may involve a significant portion of the upper aerodigestive tract mucosa before the appearance of a malignant phenotype. Taken together, these observations indicate that there may be a prolonged latency period during which clonal genetic populations may be detected, but invasive progression has not yet taken place.
Characterization of genetic events in premalignant lesions may allow definition of those lesions that may display a more aggressive clinical behavior, perhaps warranting more aggressive treatment strategies. Genetic progression may therefore provide additional indications of clinical behavior when combined with histopathological appearance (16) . Chemopreventive strategies could be targeted to these early clonal cell populations. Finally, these populations of clonally expanded, genetically altered cells would be the ideal targets of a strategy aimed at early detection of HNSC before development of an invasive phenotype. A large clonal population of in the aerodigestive mucosa harboring early genetic changes should shed sufficient material to allow detection of genetic alterations in saliva. This hypothesis constitutes the basis of a novel approach for cancer detection in patients with HNSC (17) .
| FOOTNOTES |
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1 To whom requests for reprints should be
addressed, at Division of Head and Neck Cancer Research, Department of
Otolaryngology-Head and Neck Surgery, Room 818, Ross Research Building,
720 Rutland Avenue, Baltimore, MD 21205. ![]()
2 The abbreviations used are: HNSC, head and neck
squamous cell carcinoma; LOH, loss of heterozygosity; TVC, true vocal
cord. ![]()
Received 12/15/98; revised 2/11/99; accepted 2/12/99.
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