Monoclonality of endocrine tumours: What does it mean?

Multiclonal tumor origin: Evidence and implications

An accurate understanding of the clonal origins of tumors is critical for designing effective strategies to treat or prevent cancer and for guiding the field of cancer risk assessment. The intent of this review is to summarize evidence of multiclonal tumor origin and, thereby, contest the commonly held assumption of monoclonal tumor origin. This review describes relevant studies of X chromosome inactivation, analyses of tumor heterogeneity using other markers, single cell sequencing, and lineage tracing studies in aggregation chimeras and engineered rodent models. Methods for investigating tumor clonality have an inherent bias against detecting multiclonality. Despite this, multiclonality has been observed within all tumor stages and within 53 different types of tumors. For myeloid tumors, monoclonal tumor origin may be the predominant path to cancer and a monoclonal tumor origin cannot be ruled out for a fraction of other cancer types. Nevertheless, a large body of evidence supports the conclusion that most cancers are multiclonal in origin. Cooperation between different cell types and between clones of cells carrying different genetic and/or epigenetic lesions is discussed, along with how polyclonal tumor origin can be integrated with current perspectives on the genesis of tumors. In order to develop biologically sound and useful approaches to cancer risk assessment and precision medicine, mathematical models of carcinogenesis are needed, which incorporate multiclonal tumor origin and the contributions of spontaneous mutations in conjunction with the selective advantages conferred by particular mutations and combinations of mutations. Adherence to the idea that a growth must develop from a single progenitor cell to be considered neoplastic has outlived its usefulness. Moving forward, explicit examination of tumor clonality, using advanced tools, like lineage tracing models, will provide a strong foundation for future advances in clinical oncology and better training for the next generation of oncologists and pathologists.

Identification of a novel RASD1 somatic mutation in a USP8-mutated corticotroph adenoma

Cushing's disease (CD) is caused by pituitary corticotroph adenomas that secrete excess adrenocorticotropic hormone (ACTH). In these tumors, somatic mutations in the gene USP8 have been identified as recurrent and pathogenic and are the sole known molecular driver for CD. Although other somatic mutations were reported in these studies, their contribution to the pathogenesis of CD remains unexplored. No molecular drivers have been established for a large proportion of CD cases and tumor heterogeneity has not yet been investigated using genomics methods. Also, even in USP8-mutant tumors, a possibility may exist of additional contributing mutations, following a paradigm from other neoplasm types where multiple somatic alterations contribute to neoplastic transformation. The current study utilizes whole-exome discovery sequencing on the Illumina platform, followed by targeted amplicon-validation sequencing on the Pacific Biosciences platform, to interrogate the somatic mutation landscape in a corticotroph adenoma resected from a CD patient. In this USP8-mutated tumor, we identified an interesting somatic mutation in the gene RASD1, which is a component of the corticotropin-releasing hormone receptor signaling system. This finding may provide insight into a novel mechanism involving loss of feedback control to the corticotropin-releasing hormone receptor and subsequent deregulation of ACTH production in corticotroph tumors.

Coexistence of malignant struma ovarii and cervical papillary thyroid carcinoma

CONTEXT

Struma ovarii is an uncommon monodermal teratoma in which thyroid tissue is the predominant element. Malignant transformation of struma ovarii is an even rarer occurrence.

CASE PRESENTATION

We describe a 42-year-old woman who underwent a total abdominal hysterectomy and bilateral salpingo-oophorectomy for a symptomatic left pelvic mass. Histology revealed malignant struma ovarii with classical papillary thyroid carcinoma expression. Ultrasonography of the cervical neck showed thyroid micronodules and a dominant 1-cm nodule in the left thyroid lobe. As the ovarian tumor was large, the patient underwent a total thyroidectomy with the intention of administering ¹³¹I therapy in an adjuvant setting. Histology of the cervical thyroid gland revealed bilateral multifocal papillary thyroid carcinoma with extrathyroidal extension and perithyroidal lymph node metastasis.

METHODS

Morphological (microscopy), immunohistochemical (Hector Battifora mesothelial cell 1, cytokeratin-19, galectin-3), and molecular (BRAF V600E, RAS, RET-PTC) characteristics and clonality analysis of the cervical thyroid and ovarian tumors were explored to distinguish them as separate malignancies.

RESULTS

The thyroid-type tumors from the cervical gland and ovary were discordant in terms of tissue histology and level of cytokeratin-19 expression. The clinical features and tumor profile results supported the independent existence of these two embryologically related, although topographically distinct, malignancies.

CONCLUSION

Our findings provided support for synchronous, albeit distinct, primary tumors in the ovary and cervical thyroid. "Field cancerization" and early genomic instability may explain multifocality in all thyroid-type tissue. In this regard, patients with malignant struma ovarii should undergo imaging of their thyroid gland for coexisting disease and thyroidectomy recommended for suspected malignancy or in preparation for radioiodine therapy.

Pituitary stem cell update and potential implications for treating hypopituitarism

Stem cells have been identified in organs with both low and high cell turnover rates. They are characterized by the expression of key marker genes for undifferentiated cells, the ability to self-renew, and the ability to regenerate tissue after cell loss. Several recent reports present evidence for the presence of pituitary stem cells. Here we offer a critical review of the field and suggest additional studies that could resolve points of debate. Recent reports have relied on different markers, including SOX2, nestin, GFRa2, and SCA1, to identify pituitary stem cells and progenitors. Future studies will be needed to resolve the relationships between cells expressing these markers. Members of the Sox family of transcription factors are likely involved in the earliest steps of pituitary stem cell proliferation and the earliest transitions to differentiation. The transcription factor PROP1 and the NOTCH signaling pathway may regulate the transition to differentiation. Identification of the stem cell niche is an important step in understanding organ development. The niche may be the marginal zone around the lumen of Rathke's pouch, between the anterior and intermediate lobes of mouse pituitary, because cells in this region apparently give birth to all six pituitary hormone cell lineages. Stem cells have been shown to play a role in recurrent malignancies in some tissues, and their role in pituitary hyperplasia, pituitary adenomas, and tumors is an important area for future investigation. From a therapeutic viewpoint, the ability to cultivate and grow stem cells in a pituitary predifferentiation state might also be helpful for the long-term treatment of pituitary deficiencies.

Molecular and trophic mechanisms of tumorigenesis

A significant proportion of pituitary macroadenomas, and by definition all microadenomas, regain trophic stability after an initial period of deregulated growth. Classical proto-oncogene activation and tumor suppressor mutation are rarely responsible, and no histologic or molecular markers reliably predict behavior. GNAS1 activation and the mutations associated with multiple endocrine neoplasia type 1 and Carney complex, aryl hydrocarbon receptor interacting protein gene mutations, and a narrowing region of chromosome 11q13 in familial isolated acromegaly together account for such a small proportion of pituitary adenomas that the pituitary adenoma pathogenic epiphany is surely yet to come.

Cell-to-cell fusion as a link between viruses and cancer

The ability to fuse cells is shared by many viruses, including common human pathogens and several endogenous viruses. Here we will discuss how cell fusion can link viruses to cancer, what types of cancers it can affect, how the existence of this link can be tested and how the hypotheses that we propose might affect the search for human oncogenic viruses. In particular, we will focus on the ability of cell fusion that is caused by viruses to induce chromosomal instability, a common affliction of cancer cells that has been thought to underlie the malignant properties of cancerous tumours.

Molecular Evidence for Independent Origin of Multifocal Neuroendocrine Tumors of the Enteropancreatic Axis

Neuroendocrine tumors of the enteropancreatic axis are often multifocal. We have investigated whether multifocal intestinal carcinoid tumors and multifocal pancreatic endocrine tumors arise independently or whether they originate from a single clone with subsequent intramural or intrapancreatic spread. Twenty-four cases, including 16 multifocal intestinal carcinoid tumors and eight multifocal pancreatic endocrine tumors, were studied. Genomic DNA samples were prepared from 72 distinct tumor nodules using laser capture microdissection. Loss of heterozygosity (LOH) assays were done using markers for putative tumor suppressor genes located on chromosomes 9p21 (p16), 11q13 (MEN1), 11q23 (SDHD), 16q21, 18q21, and 18q22-23. In addition, X chromosome inactivation analysis was done on the tumors from eight female patients. Twenty-two of 24 (92%) cases showed allelic loss in at least one tumor focus, including 15 of 16 (94%) cases of multifocal carcinoid tumors and 7 of 8 (88%) cases of multifocal pancreatic endocrine tumors. Eleven of 24 (46%) cases exhibited a different LOH pattern for each tumor. Additionally, 9 of 24 (38%) cases showed different LOH patterns among some of the coexisting tumors, whereas other coexisting tumors displayed the same allelic loss pattern. Two of 24 (8%) cases showed the same LOH pattern in every individual tumor. X chromosome inactivation analysis showed a discordant pattern of nonrandom X chromosome inactivation in two of six informative cases and concordant pattern of nonrandom X chromosome inactivation in the four remaining informative cases. Our data suggest that some multifocal neuroendocrine tumors of the enteropancreatic axis arise independently, whereas others originate as a single clone with subsequent local and discontinuous metastasis.

Molecular mechanisms and biological plausibility underlying the malignant transformation of endometriosis: a critical analysis

Although population-based studies have unequivocally reported an increased risk of ovarian cancer in women with endometriosis, the biological evidence supporting the idea of endometriosis as a preneoplastic condition is scanty and not well substantiated. The fundamental features of human neoplasms (monoclonal growth, genetic changes, mutations in tumour suppressor genes and replicative advantage) have been evaluated in endometriotic lesions but results obtained are discordant. It is plausible that ectopic glands may expand monoclonally but the entity of this phenomenon is debated. According to some allelotyping studies, from one-third to one-half of endometriosis lesions would harbour somatic genetic changes in chromosomal regions supposed to contain genes involved in ovarian tumourigenesis, especially for the endometrioid histotype. These findings would be consistent with the progression model for carcinogenesis from the benign precursor to ovarian cancer but they could not be unequivocally replicated. Gene mutational studies are rare in this context. A single group has found missense mutations and deletions of PTEN gene in about 20% of ovarian endometriotic cysts. Moreover, in a model of genetically engineered mice harbouring an oncogenic allele of K-ras resulting in benign lesions reminiscent of endometriosis, a conditional deletion of PTEN caused the progression towards the endometrioid tumour. Based on these data, the causal link between endometriosis and ovarian endometrioid/clear cell carcinomas remains to be defined both in terms of entity of association and of underlying molecular mechanisms.

Molecular pathogenesis of euthyroid and toxic multinodular goiter

The purpose of this review is to summarize current knowledge of the etiology of euthyroid and toxic multinodular goiter (MNG) with respect to the epidemiology, clinical characteristics, and molecular pathology. In reconstructing the line of events from early thyroid hyperplasia to MNG we will argue the predominant neoplastic character of nodular structures, the nature of known somatic mutations, and the importance of mutagenesis. Furthermore, we outline direct and indirect consequences of these somatic mutations for thyroid pathophysiology and summarize information concerning a possible genetic background of euthyroid goiter. Finally, we discuss uncertainties and open questions in differential diagnosis and therapy of euthyroid and toxic MNG.

p16 expression in Barrett's esophagus and esophageal adenocarcinoma: association with genetic and epigenetic alterations

Alteration of the p16 tumor suppressor gene has been implicated as a critical lesion in the molecular pathogenesis of esophageal adenocarcinoma. The aim of this study was to characterize the spectrum of p16 alterations in surgically resected esophageal tissues, comprising histologically normal esophageal squamous and gastric epithelia, premalignant Barrett's epithelia, and associated esophageal adenocarcinomas, and to explore associations between p16 mRNA expression and p16 mutations, deletions, promoter hypermethylation, p16 protein expression, and clinico-pathologic features for the same tissues. We have shown that while p16 mutations are uncommon (2%; 1/54), hypermethylation of the p16 promoter is detected in 43% (9/21) of histologically normal epithelia, in 77% (14/18) of associated Barrett's epithelia, and in 85% (18/21) of esophageal adenocarcinomas. However, p16 mRNA levels (relative to matched normal epithelia) were variable in Barrett's epithelia and adenocarcinomas, having no clear correlation with methylation status or other molecular and clinico-pathological parameters. These findings are consistent with a role for the p16 tumor suppressor gene early in the molecular progression of Barrett's epithelium to invasive esophageal adenocarcinoma, but do not support the notion that the detection of hypermethylation is systematically associated with low levels of expression.

The molecular pathogenesis of hereditary and sporadic adrenocortical and adrenomedullary tumors

Modern imaging modalities lead to frequent detection of adrenal masses, most of them incidental findings. Although the majority of adrenocortical and adrenomedullary tumors are benign, there are no reliable clinical and laboratory markers to distinguish most of them from malignant neoplasms. The molecular mechanisms underlying the pathogenesis of these tumors have recently begun to be unraveled. A fruitful avenue for the elucidation of tumorigenesis has been the study of adrenal tumors that are manifestations of hereditary or postzygotic genetic syndromes, because one knows the "first hit", i.e. the primary gene defect. In contrast, in sporadic adrenal tumors the first hit, possibly a somatic mutation of a tumor-related gene, is unknown, and therefore the sequence of genetic alterations is difficult to establish. In this article we review in addition to our own work the literature on molecular aspects of adrenocortical and adrenomedullary tumorigenesis.

Hyperplasia versus adenoma in endocrine tissues: are they different?

The traditional view holds that hyperplasia of endocrine glands is secondary to oversecretion of a trophic hormone. However, in most cases, the mechanism underlying this growth is the spontaneous proliferation of benign neoplasias. Pathologists still depend on subtle morphological criteria to delineate and further classify these tumours. Owing to their variable architecture, a bewildering nomenclature has emerged for these tumours, exemplified by the many names applied to the goitrous thyroid gland: hyperplasia, adenomatous goitre, adenomatoid nodules, benign nodular thyroid disease, adenoma, etc. This article reviews the evidence suggesting that: (1) the varied types of benign neogeneration of endocrine tissue, the spectrum of which ranges from 'simple hyperplasia' to 'true adenoma', involve the same process; (2) even clonality of a growing lesion cannot distinguish hyperplasia from neoplasia; and (3) the basic processes in both cases are not different from those that cause benign tumours in other organs.