Histologische und molekulargenetische Untersuchungen von strahleninduzierten Schilddrüsentumoren im Mausmodell

Thyroid cancer derived from follicular epithelial cells is the most
common endocrine malignancy in man. An increased incidence of
predominantly papillary thyroid carcinomas (PTC) was found in
children exposed to radiation after the Chernobyl nuclear accident
in 1986. Therefore, in this study, the goal was to establish a
mouse model of thyroid carcinogenesis, based on a standardized
histological classification scheme for the murine thyroid tumors,
and complemented by molecular genetic analyses. In previous
studies, radioiodine (I131, 111 kBq) was injected into iodine
deficient fed mothers of various mouse strains (F1-hybrids and
backcrosses of C57/BL6, C3H, BALB/c, and JF1). The first injection
was applied during gestation and the second during lactation. The
necropsy tissue was submitted for the analysis in this study. A set
of 365 thyroid glands (203 irradiated and 162 control mice) was
histological examined following the current WHO classification of
human thyroid tumors (2004) for comparative purposes. The
irradiated mice showed 24 % of cases with simple hyperplasia (SH),
20 % with nodular hyperplasia (NH), 7 % with follicular thyroid
adenoma (FTA), and 5 % with follicular thyroid carcinoma (FTC)
whereas in the control group only 3 % SH, 3 % of NH, 1 % of FTA,
and 1 % of FTC were observed. Interestingly, no PTC was diagnosed
in the mice, which is the most frequent irradiation-related type of
thyroid cancer in human. Therefore, the histological type of the
radiation-associated thyroid tumors in mice differs from that in
human. However, some cases of murine FTC presented PTC-like
biological behavior. In addition to the significant increase of
hyperplasias in irradiated mice, most of the FTC (82 %) arose
amongst a background of hyperplastic nodules. Therefore, a
progression from NH to FTC, based on genetic instability, cannot be
ruled out. The following molecular methods were used: PCR-
(polymerase chain reaction-) based loss of heterozygosity (LOH),
comparative genomic hybridization (CGH), and
fluorescence-in-situ-hybridization (FISH). Since the CGH-study in
mice using formalin-fixed paraffin-embedded tissue (FFPE) is not
yet established, an important part of the study was dedicated to
evaluate this methodology. The LOH-study was performed with thyroid
gland tissue from 40 mice (seven normal thyroid glands, 12 SH, 10
NH, 10 FTA, and one FTC) using 36 microsatellites for nine
different loci. With the exception of an LOH with a single
microsatellite on chromosome 14 in 40 % of NH, LOH was found in 75
% of the irradiated male mice with H6F1-background on chromosomes
4, 5, 6, 11, 14, and / or 19. This suggests the existence of a
mouse strain specific genetic predisposition, which influence on
the genetic stability. One of the FTA (an atypical FTA) was highly
suspicious for a deletion of the tumorsuppressorgene Rb1 (supported
by intragenic FISH-analysis), which could play an important role in
the thyroid carcinogenesis. For the CGH-study, thyroid tissue
derived from 21 different mice (F2-hybrids) was analyzed (two
normal thyroid glands, one SH, 12 NH, two FTA, and eight FTC). In
46 % of the hyperplasias, small chromosomal gains and losses
located on different chromosomes were observed; suggesting that
there exists a genetic instability, which may lead eventually to
malignant progression. Regional polyploidies on chromosomes 4 and 5
were demonstrated in one of the FTAs, which could be a hint for the
location of oncogenes. Taken together, in FTA development there is
a broad spectrum of genetic alteration, and by inference
mechanisms. In contrast, the FTC exhibited a significant increase
of specific aneuploidies, mainly deletions of the chromosomes 4 (88
%), 9 (50 %), and 14 (38 %). Identical alterations of chromosomes 4
and 9 were also observed in the one case of an FTC from a
non-irradiated mouse. These data indicate that irradiation, most
probably, increases the frequency of genetic changes, but does not
change the type of genetic alterations, which play a crucial role
in thyroid carcinogenesis in mice. A better understanding of
molecular genetics involved in thyroid tumorigenesis in
standardized mouse models may give insight into the pathogenesis of
the various tumor types. Together with the results from human
pathology and in vitro studies, this may lead to a better knowledge
about the molecular pathways with diagnostic, prognostic, and
therapeutic relevance. The results of this study demonstrate a
morphological and genetical difference between human (PTC) and
murine (FTC) radiation-associated thyroid tumors, but a strong
similarity to the human follicular tumors. Therefore, this mouse
model serves as a good model of carcinogenetic mechanisms, tumor
induction, and progression in the human follicular tumors FTA and
FTC, resulting from the cooperative effect of radioiodine
exposition and iodine deficiency.