The common deletion of mitochondrial DNA is found in goiters and thyroid tumors with and without oxyphil cell change

Molecular pathology of thyroid tumours of follicular cells: a review of genetic alterations and their clinicopathological relevance

Thyroid cancer is the most common endocrine malignancy. Knowledge of the molecular pathology of thyroid tumours originating from follicular cells has greatly advanced in the past several years. Common molecular alterations, such as BRAF p.V600E, RAS point mutations, and fusion oncogenes (RET-PTC being the prototypical example), have been, respectively, associated with conventional papillary carcinoma, follicular-patterned tumours (follicular adenoma, follicular carcinoma, and the follicular variant of papillary carcinoma/non-invasive follicular thyroid neoplasm with papillary-like nuclear features), and with papillary carcinomas from young patients and arising after exposure to ionising radiation, respectively. The remarkable correlation between genotype and phenotype shows how specific, mutually exclusive molecular changes can promote tumour development and initiate a multistep tumorigenic process that is characterised by aberrant activation of mitogen-activated protein kinase and phosphoinositide 3-kinase-PTEN-AKT signalling. Molecular alterations are becoming useful biomarkers for diagnosis and risk stratification, and as potential treatment targets for aggressive forms of thyroid carcinoma. What follows is a review of the principal genetic alterations of thyroid tumours originating from follicular cells and of their clinicopathological relevance.

Mitochondrial DNA "common deletion" in post-fine needle aspiration infarcted oncocytic thyroid tumors

Thyroid fine needle aspiration (FNA) can rarely induce morphological changes potentially hindering the histopathological diagnosis, especially in Hurthle cell tumors (HCTs), which may easily undergo post-FNA infarction or necrosis. HCTs contain mitochondrion (mt)-rich cells that may bear mtDNA mutations, the most frequent being the so-called common deletion (CD). The aim of this study was to determine the presence and extent of the mtDNA CD in a series of thyroid HCTs that underwent extensive infarction following FNA procedure in comparison with a control series of HCTs lacking post-FNA ischemic/hemorrhagic alterations. Of 257 HCTs with available matched FNA and surgical specimens, 8 cases showed extensive (≥80%) infarction or necrosis in the resected nodule (4 adenomas, 1 carcinoma, 3 HCTs undefined for malignancy). Noninfarcted tumors in the control series included 9 adenomas, 1 carcinoma, and 1 follicular tumor of uncertain malignant potential. These lesions were significantly (P = .03) larger than infarcted nodules. The mtDNA CD was identified using semiquantitative real-time polymerase chain reaction in 2 of 8 (25%) infarcted tumors. In HCTs lacking infarction/necrosis of the control series, the CD was significantly (P = .05) more common (8/11 cases, 72.7%). In 7 of the 10 deleted cases, the CD was present also in the adjacent nonneoplastic parenchyma. In conclusion, the rare oncocytic tumors undergoing extensive infarction are smaller than those lacking ischemic changes and bear the mtDNA CD in a significantly lower proportion compared with control noninfarcted HCTs. This may suggest that mtDNA deletion confers a survival advantage to oncocytic cells in stress conditions, including FNA procedures.

Oxyphil cell metaplasia in the parathyroids is characterized by somatic mitochondrial DNA mutations in NADH dehydrogenase genes and cytochrome c oxidase activity-impairing genes

Oxyphil cell transformation of epithelial cells due to the accumulation of mitochondria occurs often during cellular aging. To understand the pathogenic mechanisms, we studied mitochondrial DNA (mtDNA) alterations in the three cell types of the parathyroids using multiplex real-time PCR and next-generation sequencing. mtDNA was analyzed from cytochrome c oxidase (COX)-positive and COX-negative areas of 19 parathyroids. Mitochondria-rich pre-oxyphil/oxyphil cells were more prone to develop COX defects than the mitochondria-poor clear chief cells (P < 0.001). mtDNA increased approximately 2.5-fold from clear chief to oxyphil cells. In COX deficiency, the increase was even more pronounced, and COX-negative oxyphil cells had approximately two times more mtDNA than COX-positive oxyphil cells (P < 0.001), illustrating the influence of COX deficiency on mtDNA biosynthesis, probably as a consequence of insufficient ATP synthesis. Next-generation sequencing revealed a broad spectrum of putative pathogenic mtDNA point mutations affecting NADH dehydrogenase and COX genes as well as regulatory elements of mtDNA. NADH dehydrogenase gene mutations preferentially accumulated in COX-positive pre-oxyphil/oxyphil cells and, therefore, could be essential for inducing oxyphil cell transformation by increasing mtDNA/mitochondrial biogenesis. In contrast, COX-negative cells predominantly harbored mutations in the MT-CO1 and MT-CO3 genes and in regulatory mtDNA elements, but only rarely NADH dehydrogenase mutations. Thus, multiple hits in NADH dehydrogenase and COX activity-impairing genes represent the molecular basis of oxyphil cell transformation in the parathyroids.

Relevance of mitochondrial genetics and metabolism in cancer development

Cancer cells are characterized in general by a decrease of mitochondrial respiration and oxidative phosphorylation, together with a strong enhancement of glycolysis, the so-called Warburg effect. The decrease of mitochondrial activity in cancer cells may have multiple reasons, related either to the input of reducing equivalents to the electron transfer chain or to direct alterations of the mitochondrial respiratory complexes. In some cases, the depression of respiratory activity is clearly the consequence of disruptive mitochondrial DNA (mtDNA) mutations and leads as a consequence to enhanced generation of reactive oxygen species (ROS). By acting both as mutagens and cellular mitogens, ROS may contribute directly to cancer progression. On the basis of our experimental evidence, we suggest a deep implication of the supercomplex organization of the respiratory chain as a missing link between oxidative stress, energy failure, and tumorigenesis. We speculate that under conditions of oxidative stress, a dissociation of mitochondrial supercomplexes occurs, with destabilization of complex I and secondary enhanced generation of ROS, thus leading to a vicious circle amplifying mitochondrial dysfunction. An excellent model to dissect the role of pathogenic, disassembling mtDNA mutations in tumor progression and their contribution to the metabolic reprogramming of cancer cells (glycolysis vs. respiration) is provided by an often underdiagnosed subset of tumors, namely, the oncocytomas, characterized by disruptive mutations of mtDNA, especially of complex I subunits. Such mutations almost completely abolish complex I activity, which slows down the Krebs cycle, favoring a high ratio of α-ketoglutarate/succinate and consequent destabilization of hypoxia inducible factor 1α (HIF1α). On the other hand, if complex I is partially defective, the levels of NAD(+) may be sufficient to implement the Krebs cycle with higher levels of intermediates that stabilize HIF1α, thus favoring tumor malignancy. The threshold model we propose, based on the population-like dynamics of mitochondrial genetics (heteroplasmy vs. homoplasmy), implies that below threshold complex I is present and functioning correctly, thus favoring tumor growth, whereas above threshold, when complex I is not assembled, tumor growth is arrested. We have therefore termed "oncojanus" the mtDNA genes whose disruptive mutations have such a double-edged effect.

The biology and the genetics of Hurthle cell tumors of the thyroid

The biology and the genetics of Hürthle cell tumors are reviewed starting from the characterization and differential diagnosis of the numerous benign and malignant, neoplastic and nonneoplastic lesions of the thyroid in which Hürthle cell transformation is frequently observed. The clinicopathologic and molecular evidence obtained from the comparative study of the aforementioned conditions indicate that Hürthle cell appearance represents a phenotype that is superimposed on the genotypic and conventional histopathologic features of the tumors. Hürthle cell tumors differ from their non-Hürthle counterparts regarding the prevalence of large deletions of mitochondrial DNA (mtDNA), mutations of mtDNA genes coding for oxidative phosphorylation (OXPHOS) proteins (namely mutations of complex I subunit genes) and mutations of nuclear genes coding also for mitochondrial OXPHOS proteins. Such mitochondrial alterations lead to energy production defects in Hürthle cell tumors; the increased proliferation of mitochondria may reflect a compensatory mechanism for such defects and is associated with the overexpression of factors involved in mitochondrial biogenesis. The mitochondrial abnormalities are also thought to play a major role in the predisposition for necrosis instead of apoptosis which seems to be blocked in most Hürthle cell tumors. Finally, the results obtained in experimental models using cybrid cell lines and the data obtained from histopathologic and molecular studies of familial Hürthle cell tumors are used, together with the aforementioned genetic and epigenetic alterations, to progress in the understanding of the mechanisms through which mitochondrial abnormalities may be involved in the different steps of thyroid carcinogenesis, from tumor initiation to metastization.

Cellular model of Warburg effect identifies tumor promoting function of UCP2 in breast cancer and its suppression by genipin

The Warburg Effect is characterized by an irreversible injury to mitochondrial oxidative phosphorylation (OXPHOS) and an increased rate of aerobic glycolysis. In this study, we utilized a breast epithelial cell line lacking mitochondrial DNA (rho⁰) that exhibits the Warburg Effect associated with breast cancer. We developed a MitoExpress array for rapid analysis of all known nuclear genes encoding the mitochondrial proteome. The gene-expression pattern was compared among a normal breast epithelial cell line, its rho⁰ derivative, breast cancer cell lines and primary breast tumors. Among several genes, our study revealed that over-expression of mitochondrial uncoupling protein UCP2 in rho⁰ breast epithelial cells reflects gene expression changes in breast cancer cell lines and in primary breast tumors. Furthermore, over-expression of UCP2 was also found in leukemia, ovarian, bladder, esophagus, testicular, colorectal, kidney, pancreatic, lung and prostate tumors. Ectopic expression of UCP2 in MCF7 breast cancer cells led to a decreased mitochondrial membrane potential and increased tumorigenic properties as measured by cell migration, in vitro invasion and anchorage independent growth. Consistent with in vitro studies, we demonstrate that UCP2 over-expression leads to development of tumors in vivo in an orthotopic model of breast cancer. Genipin, a plant derived small molecule, suppressed the UCP2 led tumorigenic properties, which were mediated by decreased reactive oxygen species and down-regulation of UCP2. However, UCP1, 3, 4 and 5 gene expression was unaffected. UCP2 transcription was controlled by SMAD4. Together, these studies suggest a tumor-promoting function of UCP2 in breast cancer. In summary, our studies demonstrate that i) the Warburg Effect is mediated by UCP2; ii) UCP2 is over-expressed in breast and many other cancers; iii) UCP2 promotes tumorigenic properties in vitro and in vivo and iv) genipin suppresses the tumor promoting function of UCP2.

Genetic insights into OXPHOS defect and its role in cancer

Warburg proposed that cancer originates from irreversible injury to mitochondrial oxidative phosphorylation (mtOXPHOS), which leads to an increase rate of aerobic glycolysis in most cancers. However, despite several decades of research related to Warburg effect, very little is known about the underlying genetic cause(s) of mtOXPHOS impairment in cancers. Proteins that participate in mtOXPHOS are encoded by both mitochondrial DNA (mtDNA) as well as nuclear DNA. This review describes mutations in mtDNA and reduced mtDNA copy number, which contribute to OXPHOS defects in cancer cells. Maternally inherited mtDNA renders susceptibility to cancer, and mutation in the nuclear encoded genes causes defects in mtOXPHOS system. Mitochondria damage checkpoint (mitocheckpoint) induces epigenomic changes in the nucleus, which can reverse injury to OXPHOS. However, irreversible injury to OXPHOS can lead to persistent mitochondrial dysfunction inducing genetic instability in the nuclear genome. Together, we propose that "mitocheckpoint" led epigenomic and genomic changes must play a key role in reversible and irreversible injury to OXPHOS described by Warburg. These epigenetic and genetic changes underlie the Warburg phenotype, which contributes to the development of cancer.

Mitochondrial mutations in hepatocellular carcinomas and fibrolamellar carcinomas

Mitochondrial mutations are well documented in hepatocellular carcinoma, but their role in carcinogenesis remains unclear. To clarify their significance, a comprehensive analysis was performed of hepatocellular carcinomas (N=24), including quantifying the total mitochondrial DNA levels, quantifying the levels of mitochondrial DNA with the common deletion, and complete sequencing of the mitochondrial control region. In addition, these studies were expanded and reinforced by analysis of fibrolamellar carcinomas (N=15), a unique type of liver carcinoma that has increased numbers of mitochondria on electron microscopy. Overall, approximately 50% of hepatocellular carcinomas had lower levels of total mitochondrial DNA than paired non-neoplastic tissues. Interestingly, despite their increased numbers of mitochondria, primary fibrolamellar carcinomas had lower levels of total mitochondrial DNA. In contrast, metastatic fibrolamellar carcinomas had greatly increased mitochondrial DNA levels. Overall, deletions in the control region were associated with lower total DNA levels in typical hepatocellular carcinoma, but somatic single base pair mutations were not. In fact, almost all single base pair mutations were either reversions to the wild-type sequence or known population polymorphisms, strongly suggesting they are not directly oncogenic. Complete sequencing of the entire mitochondrial genome in fibrolamellar carcinomas identified several somatic mutations, but no consistent pattern of mutations was found. Overall, the levels of the common deletion were highest in tissues with lower total mitochondrial DNA. In conclusion, control region deletions, but not somatic mutations, may influence total DNA copy numbers. Somatic control region mutations in hepatocellular carcinoma are not directly oncogenic but instead may be adaptive.

Molecular features of thyroid oncocytic tumors

Thyroid oncocytic neoplasms are tumors composed of cells characterized by an aberrant increase of mitochondrial mass. They represent a subset of thyroid tumors whose classification and clinical features has been a matter of controversy for clinicians and pathologists alike. The prevalence of oncocytic tumors in the thyroid gland, the relevance of the issues debated, and the obvious cellular derangement of oncocytic cells, namely a complete deregulation of the mitochondrial mass and metabolism, have spurred many investigators to study the molecular mechanism underlying the genesis of this peculiar cancer phenotype. Their findings, which are unraveling the tumor pathobiology, are the subject of the present review.

Mitochondria and cancer

The authors review the role played by mutations in mitochondrial DNA and in nuclear genes encoding mitochondrial proteins in cancer development, with an emphasis on the alterations of the oxidative phosphorylation system and glycolysis.

The Thyroid Hürthle (Oncocytic) Cell and Its Associated Pathologic Conditions: A Surgical Pathology and Cytopathology Review

Context.—Hürthle cells are eosinophilic, follicular-derived cells that are associated with a variety of nonneoplastic and neoplastic thyroid lesions. The differential diagnosis of Hürthle cell lesions is quite broad.

Objective.—To review the pathologic conditions associated with Hürthle cells in the thyroid and to discuss pathology of thyroid lesions associated with oncocytic cytology.

Data Sources.—A variety of thyroid nonneoplastic (autoimmune thyroiditis, multinodular goiter) and neoplastic conditions (Hürthle cell adenoma, Hürthle cell carcinoma) are associated with Hürthle cell cytology. In addition, there are several thyroid neoplasms that should be considered when one observes a Hürthle cell neoplasm in the thyroid (oncocytic variant of medullary carcinoma, several variants of papillary thyroid carcinoma).

Conclusions.—Oncocytic cytology is seen in a variety of thyroid conditions that are associated with a broad differential diagnosis and care must be used for accurate diagnosis. Newer molecular-based techniques may be useful for further classification of thyroid neoplasms with oncocytic pathology.

Intragenic mutations in thyroid cancer

The close genotype-phenotype relationship that characterizes thyroid oncology stimulated the authors to address this article by using a mixed, genetic and phenotypic approach. As such, this article addresses the following aspects of intragenic mutations in thyroid cancer: thyroid stimulating hormone receptor and guanine-nucleotide-binding proteins of the stimulatory family mutations in hyperfunctioning tumors; mutations in RAS and other genes and aneuploidy; PAX8-PPARgamma rearrangements; BRAF mutations; mutations in oxidative phosphorylation and Krebs cycle genes in Hürthle cell tumors; mutations in succinate dehydrogenase genes in medullary carcinoma and C-cell hyperplasia; and mutations in TP53 and other genes in poorly differentiated and anaplastic carcinomas.

Mitochondrial DNA mutations in differentiated thyroid cancer with respect to the age factor

INTRODUCTION

Increased numbers of mitochondria in differentiated thyroid cancer and, most strikingly, mutations in human mitochondrial DNA (mtDNA) in older people have led to speculation that mtDNA mutations might contribute to aging or accumulate in postmitotic tissues with age. Mutation analyses of mtDNA in papillary (PTCs) and follicular (FTCs) thyroid carcinomas have been limited to date. The significance and frequency of mtDNA mutations in PTC and FTC are therefore controversial, as is age dependence.

METHODS

We analyzed eight sample pairs of PTC and six of FTC tissue with the corresponding normal thyroid tissue. DNA was extracted from frozen and formaldehyde-fixed tissue using the QIAmp Tissue Kit. Sequence differences in the mtDNA between tumor and normal tissue were detected using appropriate polymerase chain reaction (PCR) products for heteroduplex analysis in a denaturing high performance liquid chromatography (HPLC) Wave System (Transgenomic). Mutations were confirmed and identified by sequencing the PCR products of conspicuous chromatograms. The samples were obtained from 346 patients with PTC and 105 patients with FTC. We analyzed the whole mitochondrial genome from seven PTC and three FTC tumors along with the corresponding normal thyroid tissue. 3/7 PTC samples showed two heteroplasmic mutations and one polymorphism; all 3 FTCs showed homoplasmic and/or heteroplasmic mutations.

RESULTS

All but one of these tumors are well documented in the mitochondrial database MITOMAP. MtDNA mutations were found in all three patients aged 45 years and older. There was no correlation, however, in this small group to clinical prognostic factors for recurrence and especially for survival in differentiated thyroid carcinomas, such as histology, tumor size, lymph node metastases, distant metastases, and gender, most likely because of the short follow-up. While univariate analysis of the findings in the whole cohort of 346 patients with PTC suggested that age is a significant prognostic factor for survival (P = 0.0237) but not for recurrence (P = 0.65), this was not the case in the 105 patients with FTC.

CONCLUSIONS

Although we found accumulation of mutations in two older patients with PTC and one patient with FTC (all three patients older than 45 years had mtDNA mutations), the low frequency of these mutations in the small group of 10 analyzed patients did not correlate with statistically validated clinical prognosticators for recurrence or survival, especially not with age. The low power of our data are therefore not able to support or refute the hypothesis that these mtDNA mutations are related to age-dependent tumor progression in the thyroid or that they "may be involved in thyroid tumorigenesis."

GSTT1 and M1 polymorphisms in Hürthle thyroid cancer patients

Glutathione S-transferases (GST) are an important part of cell defense against numerous genotoxic compounds and ROS. In order to test the possibility of association between the GSTT1 and M1 null allele variant, and the risk of TCO (thyroid carcinoma with cell oxyphilia), a case-control study was carried out. The rationale for our study was that according to the important roles of GST enzymes in cells and association of GST null genotypes with many types of tumors, inactivating polymorphisms may be genetic susceptibility factors in the etiology of oxyphilic thyroid tumors characterized by mitochondrial dysfunction, increased ROS production and resistance to chemio- and radio-therapy. We found the frequency of GSTT1 null genotype of 19.2% in cases and 15.7% in controls, with an adjusted odds ratio (OR) of 1.4 (95% confidence interval (CI), 0.70-2.81), and a frequency of GSTM1 null genotype of 59% in cases with oxyphilic tumors and of 55.6% in controls (OR 1.24; 95% CI, 0.62-2.48), indicating that the GSTT1 and M1 null genotypes do not increase the risk of development of oxyphilic tumors.

How to define follicular thyroid carcinoma?

The appropriate diagnosis of follicular thyroid carcinoma (FTC) still depends on its histological discrimination from follicular adenoma (including the distinction of benign from malignant oncocytic variants), papillary thyroid carcinoma (particularly from the follicular variants) and poorly differentiated thyroid carcinoma. The use of immunohistochemical markers contributed only marginally to better defining FTC. The introduction of the micro array technique, however, may offer the possibility of getting a better insight into the natural history, as well as predicting the clinical course, of a given thyroid nodule. This review attempts to recapitulate common standards in the diagnosis of FTC, to summarise current molecular data available to distinguish FTC from other benign and malignant tumours and, finally, to outline future perspectives to define FTC on its specific genetic features.

Clinico-pathological features of a series of 11 oncocytic endocrine tumours of the pancreas

Oncocytic changes may occur very infrequently in neuroendocrine tumours. To estimate the prevalence, pathological features and clinical behaviour of oncocytic endocrine tumours of the pancreas, we reviewed a series of 227 lesions from two institutions. Eleven cases with predominant oncocytic features were selected, representing 4.85% of the whole series. The morphological features and immunophenotype of such tumours did not differ from conventional endocrine pancreatic tumours, except for the presence of abundant eosinophilic and granular cytoplasm. Anti-mitochondrial antigen was positive in all cases tested, and by electron microscopy, numerous mitochondria were observed in the cytoplasm of tumour cells. The majority of cases were nonfunctioning, and in most cases, pathologic signs of malignancy, leading to a diagnosis of endocrine carcinoma, were observed. In addition, the three nonmalignant cases matched the criteria of well-differentiated tumours of uncertain malignant potential. Nearly 50% of the cases were clinically aggressive, and lymph node and liver metastases were present at the time of diagnosis in a minority of cases. Therefore, oncocytic endocrine pancreatic tumours represent a peculiar morphological and clinical variant characterised by frequent hormonal inactivity and malignant behaviour, which should be considered in the differential diagnosis, especially when dealing with a metastatic lesion.

The C allele of theGNB3 C825T polymorphism of the G protein β3-subunit is associated with an increased risk for the development of oncocytic thyroid tumours

Carriers of the C allele of the common C825T polymorphism in the GNB3 gene of the G protein have been associated with the development of follicular thyroid adenomas. Since the C allele of this polymorphism is related to a lower signalling capacity, it was speculated whether the C825T polymorphism may play a particular role in oncocytic thyroid tumours, which are recognized for their reduced ability to synthesize thyroid-specific proteins and hormones, although they possess an intact thyroid-stimulating hormone receptor-adenylyl cyclase system. Using pyrosequencing, both the genotype distribution and the allele frequency of the C825T polymorphism were investigated in a series of 104 patients with oncocytic thyroid tumours of follicular cell origin [58 adenomas, 41 follicular thyroid carcinomas (FTCs), and five papillary thyroid carcinomas (PTCs)]; the results were compared with those obtained from 321 age and gender-matched healthy blood donors and a series of 327 non-oncocytic thyroid tumours of follicular cell origin (119 adenomas, 80 FTCs, and 186 PTCs). Analysis of the genotype distribution (comparing oncocytic with non-oncocytic tumours of the present series) revealed a significantly increased odds ratio (OR) for CC versus TT (OR = 4.22; p = 0.011) and CC versus CT (OR = 1.62; p = 0.049) carriers to develop an oncocytic thyroid tumour; ORs to develop an oncocytic thyroid tumour were also increased comparing the genotype distribution between the group of oncocytic tumours and healthy controls for CC versus TT (OR = 3.73; p = 0.017) and CC versus all T carriers (OR = 1.56; p = 0.034). Oncocytic thyroid tumours as a group showed a statistically significant increase of the C-allele frequency when compared with all non-oncocytic tumours (p = 0.0039) as well as healthy blood donors (p = 0.017). The results strongly suggest that the C allele of the GNB3 C825T polymorphism of the G protein beta3-subunit is associated with an increased risk for the development of oncocytic thyroid tumours. This polymorphism may thus be considered a (co)factor favouring the development of oncocytic thyroid tumours, although the biological mechanism(s) underlying this association remain obscure.

Selenoprotein expression in Hürthle cell carcinomas and in the human Hürthle cell carcinoma line XTC.UC1

Hürthle cell carcinomas (HTC) are characterized by mitochondrial amplification and enhanced oxygen metabolism. To clarify if defects in enzymes scavenging reactive oxygen species are involved in the pathogenesis of HTC, we analyzed selenium (Se)-dependent expression of various detoxifying selenoproteins in the HTC cell line XTC.UC1. Glutathione peroxidase and thioredoxin reductase activity was found both in cell lysates and conditioned media of XTC.UC1 cells and was increased by Na(2)SeO(3). Western blot analysis demonstrated the presence of thioredoxin reductase both in cell lysates and conditioned media and of glutathione peroxidase 3 in conditioned media. Type I 5'-deiodinase, another selenoprotein that catalyzes thyroid hormone metabolism, was detectable only in cell lysates by enzyme assay and Western blot, and responded to stimulation by both Na(2)SeO(3) and retinoic acid. A selenoprotein P signal was detected in conditioned media by Western blot, but was not enhanced by Na(2)SeO(3) treatment. In situ hybridization revealed glutathione peroxidase mRNAs in HTC specimen; glutathione peroxidase 3 mRNA levels were reduced. These data suggest adequate expression and Se-dependent regulation of a couple of selenoproteins involved in antioxidant defense and thyroid hormone metabolism in XTC.UC1 cells, so far giving no evidence of a role of these proteins in the pathogenesis of HTCs.

Large-scale mitochondrial DNA deletion mutations and nuclear genome instability in human breast cancer

Deletion mutations in mitochondrial (mt) DNA (mtDNA) as well as microsatellite instability (MSI) and loss of heterozygosity (LOH) in nuclear DNA (nDNA) exist in human cancer. We determined if: (1) large-scale mtDNA deletion mutations were present in cancerous and not in normal breast tissue, and (2) combining mt- and nDNA findings would provide complementary information to identify breast cancer. Thirty-nine matched breast cancer/histologically normal and 23 "true" normal tissue samples from women without breast cancer were microdissected and DNA extracted. 4977, approximately 3938, approximately 4388 and approximately 4576bp deletions were observed, with the 4576bp deletion being present in 0% of true normal, 13% of histologically normal specimens from a cancerous breast and 77% of breast cancers. The other three deletions were not specific to a breast containing cancer. LOH was found in 66.7% and MSI in 38.5% of samples. 38/39 (97.4%) tumors had at least one nDNA or 4576bp mtDNA alteration, suggesting that mt- and nDNA analysis provides complementary information in breast cancer detection. The 4576bp deletion appears to indicate cancer in the breast. The higher mtDNA copy number in cancer coupled with a mtDNA deletion mutation which appears specific for breasts which contain cancer may prove to be a good target to screen for cancer in the breast, including specimens of low and/or mixed cellularity.

Mitochondrial DNA and human thyroid diseases

Cells of the thyroid tissue, either diseased or normal, can accumulate altered mitochondrial genomes in primary lesions and in surrounding parenchyma. Depending on the experimental approaches and the extent of the mutational process, it has been possible to demonstrate the occurrence of homoplasmic or heteroplasmic point mutations, presence of a common deletion and random large-scale mtDNA aberrations in various pathological states. Point somatic mutations documented in 5-60% of thyroid tumors do not concentrate in obvious hotspots but tend to cluster in certain regions of the mitochondrial genome and their distribution may differ between carcinomas and controls. Large-scale deletions in mtDNA are quite prevalent in healthy and diseased thyroid; however, the proportion of aberrant mtDNA molecules accounts for a very small part of total mtDNA and does not seem to correlate with pathological characteristics of thyroid tumors. Common deletion is most abundant in Hurthle cell tumors, yet it also occurs in other thyroid diseases as well as in normal tissue. The principal difference between the common deletion and other deletion-type mtDNA molecules is that the former does not depend on the relative mtDNA content in the tissue whereas in a subset of thyroid tumors, such as radiation-associated papillary carcinomas and follicular adenomas, there is a strong correlation between mtDNA levels and prevalence of large-scale deletions. Relative mtDNA levels by themselves are elevated in most thyroid tumors compared to normal tissue. Distinct differential distribution and prevalence of mutational mtDNA burden in normal tissue and thyroid lesions are suggestive of the implication of altered mtDNA in thyroid diseases, especially in cancer.

Mitochondrial DNA somatic mutations (point mutations and large deletions) and mitochondrial DNA variants in human thyroid pathology: a study with emphasis on Hürthle cell tumors

In an attempt to progress in the understanding of the relationship of mitochondrial DNA (mtDNA) alterations and thyroid tumorigenesis, we studied the mtDNA in 79 benign and malignant tumors (43 Hürthle and 36 non-Hürthle cell neoplasms) and respective normal parenchyma. The mtDNA common deletion (CD) was evaluated by semiquantitative polymerase chain reaction. Somatic point mutations and sequence variants of mtDNA were searched for in 66 tumors (59 patients) and adjacent parenchyma by direct sequencing of 70% of the mitochondrial genome (including all of the 13 OXPHOS system genes). We detected 57 somatic mutations, mostly transitions, in 34 tumors and 253 sequence variants in 59 patients. Follicular and papillary carcinomas carried a significantly higher prevalence of non-silent point mutations of complex I genes than adenomas. We also detected a significantly higher prevalence of complex I and complex IV sequence variants in the normal parenchyma adjacent to the malignant tumors. Every Hürthle cell tumor displayed a relatively high percentage (up to 16%) of mtDNA CD independently of the lesion's histotype. The percentage of deleted mtDNA molecules was significantly higher in tumors with D-loop mutations than in mtDNA stable tumors. Sequence variants of the ATPase 6 gene, one of the complex V genes thought to play a role in mtDNA maintenance and integrity in yeast, were significantly more prevalent in patients with Hürthle cell tumors than in patients with non-Hürthle cell neoplasms. We conclude that mtDNA variants and mtDNA somatic mutations of complex I and complex IV genes seem to be involved in thyroid tumorigenesis. Germline polymorphisms of the ATPase 6 gene are associated with the occurrence of mtDNA CD, the hallmark of Hürthle cell tumors.

Microsatellite instability, mitochondrial DNA large deletions, and mitochondrial DNA mutations in gastric carcinoma

Mitochondrial DNA (mtDNA) large deletions and mtDNA mutations have been demonstrated in various types of human cancer. The relationship between the occurrence of such alterations and the nuclear microsatellite instability (MSI) status of the neoplastic cells remains controversial. In an attempt to clarify the situation in gastric carcinoma, we studied, by PCR/SSCP and sequencing, five mitochondrial genes and two D-loop regions in 32 gastric carcinomas that had been previously screened for MSI and mitochondrial common deletion. MtDNA alterations were detected in 26 carcinomas (81%). All the mtDNA mutations, which occurred mainly in the D-loop and ND1 and ND5 genes, were transitions. D-loop alterations (insertions and/or deletions) were not significantly associated with mutations in the coding regions. There was a trend towards an inverse relationship between the occurrence of mitochondrial common deletion and mtDNA mutations. No significant relationship was observed between MSI status and mtDNA mutations, whereas the mitochondrial common deletion appeared to be almost exclusively restricted to MSI-negative tumors. The latter finding--almost no gastric carcinoma with MSI-positive phenotype has large deletions of mtDNA--needs to be confirmed in a larger series and in tumors from other organs.

Extensive DNA fragmentation in oxyphilic cell lesions of the thyroid

The in situ end-labeling (ISEL) method demonstrates DNA fragmentation, commonly regarded as a marker of apoptosis. We investigated by the ISEL procedure a series of 52 thyroid lesions, including 24 lesions of mitochondrion-rich oxyphilic cells, both benign and malignant, and 28 non-oxyphilic control tumors. A high percentage of nuclear ISEL staining (approximating to 100% in most cases) was observed in the vast majority of oxyphilic cells from both adenomas and carcinomas, in the absence of morphological apoptotic changes and with no immunocytochemical evidence of caspase activation. This pattern of DNA fragmentation was not observed in non-oxyphilic lesions and was confirmed in total extracted DNA. Moreover, a peculiar cytoplasmic staining was also observed in oxyphilic cells from both benign and malignant lesions, probably related to abnormal fragmentation of mitochondrial DNA. Similar staining patterns were detected in oxyphilic cell tumors of other organs (parathyroids, salivary glands, and kidneys). These findings are consistent with an extensive DNA fragmentation peculiar to oxyphilic cells, which is not directly related to apoptosis and whose origin and biological significance are presently unknown.