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1 Institut National de la Santé et de la Recherche Médicale (U555), IPHM,
2 Laboratoire de Biochimie et Biologie Moléculaire, ERT-MEIDIA, IFR Jean Roche,
3 UPRES EA 3287 Pathologie respiratoire liée à l’environnement and
4 Laboratoire de Pathologie, Faculty of Medicine, Mediterranean University, 27 Bd Jean Moulin, 13385 Marseilles cedex 5, 13385 France
(Requests for offprints should be addressed to J Di Cristofaro; Email: judicristofaro{at}hotmail.com)
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Abstract |
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 Top Abstract IntroductionMaterials and methodsResultsDiscussionReferences |
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1 rearrangement was correlated with qualitative changes in tpo mRNA (P < 0.01). These results confirmed the decrease of TPO expression in 97% of thyroid carcinomas regardless of histological type and the overexpression of shorter splice variants in follicular tumors. Both reduction in quantity of TPO and impairment of its maturation process could account for the atypical immunohistochemical reaction of MoAb47 with TPO.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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In 1991 we discovered that the anti-TPO monoclonal antibody MoAb47 reacted with less than 30% of thyroid carcinomas (De Micco et al. 1991). When present in malignant cells, immunocytochemical (ICC) staining showed an abnormal pattern. Because these differences were so pronounced in comparison with other anti-TPO antibodies (De Micco et al. 1994a, Mizukami et al. 1994), MoAb47 was investigated as a malignancy marker on thyroid fine needle aspiration (FNA) samples. Several independent studies reached similar conclusions. The sensitivity and specificity of this method was close to 100% in initial short series (De Micco et al. 1994a, Christensen et al. 2000) and around 98% and 80% respectively in larger series (Faroux et al. 1997, De Micco et al. 1999). Based on these findings, staining with MoAb47 has been routinely used in several laboratories and has proven to be particularly useful in the ‘grey area’ of follicular tumors (De Micco et al. 1994b, 1999).
The mechanisms underlying TPO anomalies in thyroid tumors remain obscure. Northern blot analysis of tpo mRNA in carcinoma has yielded discrepant results (Hoang-Vu et al. 1992, Elisei et al. 1994) and the hypothesis that an immunological alteration accounts for the variable affinity of TPO for different antibodies (Mizukami et al. 1994, Tanaka et al. 1996) has been questioned (Czarnocka et al. 2001). In pilot studies using immunohistochemistry and in situ hybridization, we observed more heterogeneity in TPO and tpo mRNA in malignant tumors (De Micco et al. 2000). RT-PCR showed that modifications of alternative splicing could account for these variations (Ferrand et al. 2001, Le Fourn et al. 2004).
In this study we measured the expression of complete and spliced variants of tpo mRNA in thyroid tumors using quantitative PCR (Q-PCR). We also studied alterations involving the ras, Braf, ret, or pax8 genes since they might be correlated with the modification in tpo mRNA expression.
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Materials and methods |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Patients were treated in the Department of Endocrine Surgery at the Timone University Hospital in Marseilles. Patient work-up included FNA biopsy. Tissues taken during surgery were kept frozen. Histological diagnosis was established according to the World Health Organization classification (De Lellis et al. 2004). Pieces of normal tissues (NT = 13) were taken near colloid nodules (CN). The benign tumors (BT) (n = 30) included seven CN and 23 follicular adenomas (FA). The sex ratio (women/men) was 1.31/1 and the mean age was 50.0 years (range, 23–74 years). The carcinomas (n = 62) included 11 minimally invasive follicular carcinomas (FCmi), ten widely invasive follicular carcinomas (FCwi), 20 classical papillary carcinomas (PCc) (13 with purely papillary architecture and seven with mixed, papillary and follicular architecture), 12 follicular variants of papillary carcinomas (PCfv) and nine oncocytic carcinomas (OC). The sex ratio (women/men) was 2.44/1 and the mean age was 46.4 years (range, 16–79 years). Tumor stage according to Pathological Tumor Node Metastasis (pTNM) classification is shown in Table 1
. The study protocol was approved by the Clinical Research Committee of Marseilles Public Hospital System. Informed individual consent to use residual pieces of tissues for research purpose was obtained.
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ICC for TPO was routinely performed concurrently with Giemsa staining on one or two slides of FNA biopsy. Smears for ICC were air-dried and immediately stored at 4 °C until staining. ICC consisted of overnight incubation at 4 °C with MoAb47 (1/100) (DAKO, Glostrup, Denmark), incubation with a streptavidin–biotin–peroxidase kit (LSAB; DAKO) and revelation of peroxidase activity with a 3,3' diaminobenzidine–H2O2 kit. FNA samples from surgically removed benign tissue was used as a positive control. The primary antibody was omitted from negative controls. The percentage of positive cells was recorded, excluding cells exhibiting abnormal staining with fading of perinuclear staining (De Micco et al. 1994a).
RNA isolation and cDNA synthesis
RNA isolation was performed using Trizol (Invitrogen, Cergy-Pontoise, France). RNA was extracted with 200 µl chloroform and precipitated with 350 µl isopropanol overnight at –207 °C. After centrifugation and washing, pellets were diluted in 20 µl diethyl pyrocarbonate-treated water. RNA concentration was estimated with an optical density absorption setting of 260 nm. One microgram of RNA was reverse-transcribed in a volume of 20 µl using random priming and Superscript II (Invitrogen).
Q-PCR
A two-step relative method was used to quantify tpo mRNA. First, expression of total tpo mRNA in each sample was normalized to expression of the tata box binding protein gene (tbp) and compared with the median of values from NT (Picard et al. 2006). Secondly, the ratios of tpo variants to total tpo were determined in each sample.
Primers were designed using Primer Express software (Fig. 1) (Applied Biosystems, Courtab
uf, France). Their characteristics are given in Table 2. Analysis of total tpo mRNA expression was performed with TPO primers amplifying a sequence in exons 5–6. Primers spanning the banks of deleted exons were designed for separate quantification of tpo variants (Vandenbroucke et al. 2001). The P1 primer pair amplified all variants, except those lacking exon 16 (tpo1, tpo2, tpo4, tpo5 and tpo2/4). The P2 primer pair amplified variants lacking exon 10 (tpo2, tpo6, tpo2/3 and tpo2/4). The P3 primer pair amplified variants lacking exon 16 (tpo3, tpo6 and tpo2/3). The P6 primer pair amplified only tpo6.
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Oncogene analysis
Primers used to amplify ret/papillary thyroid carcinoma (PTC)1, ret/PTC3 (Santoro et al. 2000), exon 15 of Braf (Nikiforova et al. 2004), exons 1 and 2 of pax8/ppar 1 (Nikiforova et al. 2002) and Nras (Vasko et al. 2003) have been described previously. Reactions were performed in 25 µl of a PCR solution containing 4 µl cDNA, 5 mM MgCl2, 1 x buffer, 4 ng/µl primer and 0.25 U Taq polymerase (Promega, Charbonnières, France). The amplification process included the following steps: denaturation at 94 °C for 5 min, amplification by 35 cycles of denaturation at 94 °C for 30 s, hybridization with Tm primers for 1 min and elongation at 72 °C for 1 min. PCR products were visualized by electrophoresis in 1.5% agarose gel. Products were submitted to direct sequencing (Genome Express, Grenoble, France). Mutations were confirmed by sequencing of antisense strands.
Statistical analysis
Frequencies of single variables were compared using the 
2 or Fisher exact test. Oncogene mutations and patient age were analysed by Student’s t-test. After checking for normal distribution of individual tpo mRNA values, mean values were analyzed by ANOVA and the Bonferroni test. P values less than 0.05 were considered as significant. Linear regression was studied to compare Q-PCR and ICC TPO values. All statistical tests were performed using the Statistical Package for Social Sciences (SPSS, Inc., Chicago, IL, USA).
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Results |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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All follicular cells from BT and NT showed diffuse, cytoplasmic labeling of TPO that was more pronounced in the perinuclear area, thus giving a characteristic ring-shaped pattern (Fig. 2). More than 80% positive cells were found in six of seven CN and 21 of 23 FA and mean values were significantly higher in FA and CN than all histological types of carcinoma (P < 0.0001) (Table 3). All carcinomas except two FC contained less than 80% positive cells. TPO expression was very low in PCc with a mean value significantly lower than in OC and FC (9.7% vs 37.8 and 42.9%; P = 0.015 and P < 0.001 respectively). PCfv (30%) and OC displayed intermediate mean values between PCc and FC. When present in malignant cells, staining was coarse and located at the periphery of the cytoplasm. The perinuclear ring was missing (Fig. 2).
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Mean total tpo mRNA was significantly lower in all histological types of carcinomas than in NT (111.2%) and BT (126.0%) (P < 0.01) (Table 3
). Expression in PCc was the lowest (4.9%), always below 30% of the normal value (Fig. 3) and significantly lower than in OC and FC (P = 0.015 and P < 0.001 respectively). Intermediate values were found in OC (36.4%) and PCfv (26%). The highest values in carcinoma were observed in FC (49.3%). Mean content in FCmi (60.6%) was significantly higher than in FCwi (37.1%; P = 0.04) and significantly lower than in CN and NT (P = 0.005 and P = 0.01 respectively). Mean FCmi content was also lower than in FA but this difference was not statistically significant (P = 0.6). Only one OC and one FCmi expressed tpo mRNA values in the normal range (118.1% and 120.2% respectively). Mean tpo expression was similar in FA and NT. It was higher in CN than NT but this difference was not statistically significant (P = 0.09).
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). Several PCc at stage T1N0 expressed less than 1% of normal values whereas the two tumors at stage T4 (one FC and one OC) expressed 47.14 and 83.51% of normal values respectively.
For each sample, we calculated the ratio of variants amplified with the P1, P2, P3 and P6 primers to the total tpo amplified with the TPO primers (Table 4).
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P2 variant expression in malignant tumors was higher than in NT. The mean ratio in FA was close to normal. Mean ratio in FC was significantly different from NT (P = 0.01). FCmi presented the highest mean P2 value that was significantly different from the mean P2 value in NT and BT (P = 0.003 and P = 0.001 respectively). The only FCmi expressing normal total tpo mRNA (120.2%) also expressed 105.3% of P2 variant.
All carcinomas had higher mean P3 ratios than NT. BT had a lower mean P3 ratio than NT. Ratios in PCc were close to normal. The highest rates were observed in FCmi and OC. The mean ratio in FCmi was significantly higher than in NT (P = 0.02) and BT (P = 0.001). The ratio in OC was significantly higher than in both BT (P = 0.001) and PCc (P = 0.01). The ratio in PCfv was intermediate between the ratios in FC and PCc.
Mean P6 variant expression in relation to NT was higher in OC and PCc and lower in FC, PCfv and BT. PCfv presented intermediate values between FC and PCc. However, the amount of P6 variant was highly variable from case to case and these differences were not significant.
ICC and Q-PCR correlation
The correlation was determined between TPO expression studied by ICC with MoAb47 and total tpo mRNA expression measured by Q-PCR. A linear regression curve was drawn using data from all samples. Findings showed a highly significant correlation (P < 0.0001) with a coefficient (R) equal to 0.59 (Fig. 4).
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Oncogene mutations and rearrangements are summarized in Table 5. All Nras mutations occurred at codon 61 (exon 2), with equal incidence of transition and transversion. Mutations were present in 38.1% of FC, 25% of PCfv, 10% of TB and no OC. Nras mutation was not correlated with tpo mRNA expression or clinical features, but was significantly more frequent in FC than in FA (P = 0.02).
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1 rearrangements were present in 42.8% of FC, 8.3% of PCfv and no OC, CN and FA. Eight out of ten rearrangements involved fusion between exon 9 of pax8 and exon 1 of ppar 1, with or without exon 8 of pax8. Two rearrangements involved fusion between exon 7 of pax8 and exon 1 of ppar 1.
Patients presenting FC with pax8/ppar 1 were significantly younger than patients presenting FC without the rearrangement (43.3 vs 58.1 years; P = 0.04). This difference was also observed in patients presenting PCfv with or without pax8/ppar 1 (32 vs. 43.8 years; P = 0.04). The incidence of pax8/ppar 1 was higher in FCmi (six of 11) than FCwi (three of ten) but this difference was not statistically significant. pax8/ppar 1 was not correlated with gender.
A significant increase in mean P1 variant expression was found in FC with pax8/ppar 1 as compared with FC without pax8/ppar 1 (104.1 ± 26.5% vs 78.1 ± 15.4%, P = 0.01).
Braf mutations were present in 75% of PCc and 8.3% of PCfv (P < 0.001). They were never observed in BT and OC. All Braf mutations involved T to A missense transversion at nucleotide 1799. One PCc presented a previously unreported mutation at nucleotide 1799–1800 (TG>AA) that also led to valine-to-glutamate substitution. No K601E mutation was found. Braf mutation was not correlated with gender but it was correlated with older age in patients with PCc (46.3 vs 29.8 years, P = 0.02). It was more frequent in PCc with pure papillary architecture (12 of 13) than in PCc with mixed papillary and follicular architecture (three of seven), but the difference was not statistically significant. Total tpo mRNA expression was lower in PCc with as opposed to without Braf mutation (3.4% vs 13.5%), but this difference was not statistically significant.
A ret/PTC rearrangement was found in five PCc, three PTC1 and three PTC3. One tumor exhibited both PTC1 and PTC3; it was at stage T3N1 with extra-glandular extension and multiple lymph-node metastasis. No other tumor type presented ret/PTC rearrangements. No correlation was found between ret/PTC and tpo expression or clinical features. Several tumors displayed more than one molecular alteration. Four FC (19%) exhibited both Nras mutations and pax8/ppar 1 rearrangements. These tumors displayed no special histological or clinical features. Two PCc exhibited both ret/PTC (one PTC1 and one PTC3) and Braf mutations. Both were at stage T3 and were characterized by multiple intra-thyroid tumor foci and extra-glandular extension.
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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1 rearrangements, Nras mutations and Braf mutations. Results of Q-PCR analysis of total tpo mRNA correlated closely with those of ICC using MoAb47 and confirmed a decrease in TPO expression in 97% of thyroid carcinomas, regardless of histological type. Reduction in TPO expression by malignant thyroid tumors has already been reported but previous studies concerned mainly PC (Hoang-Vu et al. 1992, Tanaka et al. 1996, Lazar et al. 1999, Huang et al. 2001). This reduction stems from a decrease in PAX8 expression and parallels the decrease of many other differentiated functions such as sodium iodide symporter expression (Fabbro et al. 1994, Lazar et al. 1999). Our study also includes a large series of benign and malignant follicular tumors. We found that mean tpo expression was significantly lower in PCc than in NT, FA and FC but also significantly lower in FC, OC and PCfv than in FA. In agreement with Lazar et al. (1999) we found similar tpo expression in FA and NT. Overlapping values were found in some individual cases of FA and FCmi exhibiting unusually low and high levels of tpo expression respectively. This overlapping was consistent with recent data suggesting the existence of an adenoma–carcinoma sequence in which a subset of FA might predispose to malignant transformation (Vasko et al. 2004, Sarquis et al. 2006). The finding that tpo is significantly lower in FCwi than FCmi and higher in PCfv than PCc confirms the strong correlation between tpo expression and follicular differentiation. The correlation with histological differentiation also appears in PCc since tumors with mixed papillary and follicular architecture expressed more TPO than tumors with pure papillary architecture. Conversely, clinical pTNM staging based mainly on tumor size and extra-thyroid invasiveness has little correlation with tpo expression as long as the tumor tissue remains differentiated. Regarding differentiation, it should be noted that dedifferentiation of FC is often correlated with invasiveness, which would explain why tpo expression was usually lower in highly invasive than encapsulated tumors.
We found that the relative proportion of spliced tpo variants was enhanced in carcinomas, especially FC and OC. The increase was significant for P2 variants in FC and P3 variants in OC. Involvement of the whole spectrum of differentiated follicular tumors but not of PCc suggests that disruption of tpo mRNA maturation is a characteristic feature of follicular thyroid carcinogenesis.
The increase in shorter TPO isoforms could account for several features of follicular tumors and explain the variations observed in ICC experiments using various anti-TPO antibodies. Since it has been shown that TPO2 is folded incorrectly, unable to bind to heme and enzymatically inactive (Niccoli et al. 1997), predominance of TPO2 over TPO1 might reduce iodine concentration activity in tumor tissue. In addition, TPO2 and TPO3 have a shorter half-life than TPO 1 (3 and 7 h vs 11 h respectively), and remain almost exclusively in the intracellular compartment instead of reaching the cell surface (Niccoli et al. 1997, Niccoli-Sire et al. 2001). Immunological profiles cannot explain the differences in reactivity between MoAb47 and other monoclonal antibodies since MoAb47 recognized both TPO2 and TPO3 under experimental conditions (Niccoli et al. 1997, Niccoli-Sire et al. 2001). Rapid turnover of TPO2 and TPO3 might provide a more plausible explanation for the weak reaction obtained with MoAb47 on malignant follicular tumors. Indeed, a special property of MoAb47 in comparison with other antibodies is to react with a native, unglycosylated TPO, having a short half-life, rather than with mature forms of the protein (Chazenbalk et al. 1993, Fayadat et al. 1998). In NT, the concentration of native TPO and tpo mRNA is higher in the nuclear envelope than in the endoplasmic reticulum (Pohl et al. 1993). In tumor tissue, overproduction of mistargeted variants with a short half-life and the quantitative decrease of TPO expression could explain both the reduction of immunostaining obtained with MoAb47 and its location in the cytoplasm rather than in the perinuclear area. The presence of abnormally located TPO in malignant follicular tumors still expressing a significant amount of the protein lowers the risk of overlooking the malignancy with ICC. For this reason, ICC appears to be more accurate for diagnosing malignancy than Q-PCR which only measures quantitative variations of tpo expression.
The high versus low incidence of Braf mutations and ret/PTC respectively found in PCc can be explained by the clinico–pathological features, i.e. patient age over 35 years in most cases, higher frequency of advanced tumor stage and purely papillary histology. These three features have been correlated with Braf mutations and ret/PTC previously (Cohen et al. 2003, Kim et al. 2004, Nikiforova et al. 2004, Trovisco et al. 2004, Di Cristofaro et al. 2005, Xing et al. 2004). The antagonism between oncogene activation of the ERK signaling cascade and expression of differentiated functions by thyroid cells has been confirmed by expression array analysis and may explain the inhibition of tpo expression in PC (Melillo et al. 2005). In our study, all PCc exhibited low tpo expression and PCc harboring Braf T1799A mutation were associated with even lower tpo values. This difference was not statistically significant probably because the number of PCc without Braf T1799A mutation was too small.
In FC, the rates of pax8-ppar 1 and Nras mutation were similar and within the same range as previously reported (Nikiforova et al. 2002, 2003, Dwight et al. 2003, Vasko et al. 2003). Unlike Nikiforova et al. (2003), we found overlapping in 19% (four of 21) of FC. This suggests that the two events are not mutually exclusive and that concurrence might contribute to the emergence of different phenotypes.
We found that pax8-ppar 1 has a direct impact on tpo maturation since a significant predominance of variants amplified by P1 was observed in tumors exhibiting pax8-ppar 1. Expression profile analysis of FC displaying pax8-ppar 1 fusion oncogene demonstrated impairment of ribosomal proteins and of translation-associated genes that could explain disruption of the protein maturation process (Lui et al. 2005). Further study will be needed to determine the exact effect of pax8-ppar 1 on tpo mRNA maturation.
Unlike the pax8-ppar 1 rearrangement, ras mutation was not correlated with tpo expression. This unexpected finding supports recent speculation that FC with pax8-ppar 1 and FC with ras mutation constitute distinct biological entities (Nikiforova et al. 2003, Lui et al. 2005). Experimental data in human thyroid epithelial cells show that mutant RAS induces self-limited proliferation without loss of differentiation (Gire & Wynford-Thomas 2000). This finding fits well with our observation that Nras mutation has a weak effect on tpo expression and that another more determinant event is necessary to explain its downregulation in FC.
None of the molecular mutations investigated here was found in OC. This suggests that the oncogenic process of OC and follicular tumors is different and that other molecular mutations remain to be discovered. In view of the similarities between OC and FC with regard to expression of differentiated function and splicing anomalies of tpo, it can be assumed that such mutations have overlapping pathways.
ICC of TPO using MoAb47 was the first marker procedure proposed to assist diagnosis of malignancy on thyroid FNA (De Micco et al. 1994a). Its value has been confirmed by several independent studies with sensitivity and specificity up to 97.4% and 82% respectively (De Micco et al. 1994a, 1999, Faroux et al. 1997, Christensen et al. 2000). To our knowledge, no other malignancy marker investigated in a large series of patients has presented sensitivity over 92% (Saggiorato et al. 2005). This quantitative analysis of tpo mRNA confirmed impairment of TPO expression in all histological thyroid carcinoma subtypes. In addition, we demonstrated that disruption of tpo mRNA maturation in follicular malignant thyroid tumors leads to overproduction of shorter TPO isoforms with different biological properties. This finding not only provides a plausible explanation for the TPO immunostaining anomalies observed using MoAb47 antibody in carcinomas but also lends further support to the diagnostic value of this marker in thyroid FNA.
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Funding |
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References |
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