Molecular Endocrine Pathology
Moderators: Dr. Ricardo Lloyd and Dr. George Kontogeorgos
Section 2 -
Molecular Pathology of Thyroid Tumors
Yuri E. Nikiforov
Department of Pathology and Laboratory Medicine
University of Cincinnati College of Medicine
During the last two decades we have witnessed a dramatic progress in our understanding of the
molecular biology of thyroid tumors. It has become clear that molecular pathogenesis of thyroid
papillary carcinomas involve mutations that activate mitogen-activated protein kinase (MAPK) pathway,
some of which represent promising targets for treatment with specific molecular inhibitors. The
identification of the gene responsible for the familial forms of medullary carcinomas has led to a
dramatic change in the management of patients with this disease, and is one of the first examples when
preventive surgery is performed solely on the basis of molecular genetic testing. The progress in
molecular biology is expected to affect virtually all aspects of thyroid pathology and provide
significant help in the diagnosis of thyroid tumors, determining tumor prognosis, and prediction of tumor
response to specific treatment modalities.
These tumors frequently have mutations of genes
coding for proteins which signal along the MAPK pathway. Activating mutations of the BRAF, RET, or RAS genes
are found in approximately 70% of all cases and rarely overlap in the same tumor .
Point mutations of the BRAF gene coding for serine/threonine kinase are
found in ~40% of papillary carcinomas
Virtually all mutations involve nucleotide 1799 and
result in a valine-to-glutamate substitution at residue 600 (V600E). Among thyroid tumors, the V600E
BRAF mutation is restricted to papillary carcinoma and poorly differentiated
and anaplastic carcinomas arising from papillary carcinoma. Therefore, the identification of this
mutation in cells from fine needle aspiration of the thyroid gland or in surgical material is virtually
diagnostic for papillary carcinoma
BRAF mutations are associated
with older patient age, classic papillary histopathology or the tall cell variant, a higher rate of
extrathyroidal extension, and more advance tumor stage at presentation .
RET/PTC rearrangement is another common genetic alteration in papillary
carcinomas . It results in fusion of the 3' portion of the RET receptor
tyrosine kinase gene to the 5' portion of various genes. Two most common rearrangement types, RET/PTC1 and RET/PTC3, are paracentric inversions
since both RET and its respective fusion
partner, H4 or ELE1 (NCOA4), reside on the long arm of chromosome 10. RET/PTC2
and several additional variants of the RET/PTC oncogene are rare. The
prevalence of RET/PTC in papillary carcinomas from the general population is
~20%, it has significant geographic variation and is higher in tumors from patients with history of
radiation exposure and in pediatric populations . In tumors arising after radiation exposure, RET/PTC1 was found to be associated with classic papillary histology, whereas
RET/PTC3 type was more common in the solid variant . Overall, tumors
with RET/PTC rearrangements usually presents at younger age, has classic
papillary histology, frequent psammoma bodies, and a high rate of lymph node metastases . Although
several studies reported the detection of RET/PTC in thyroid adenomas and
other benign thyroid lesions, it is generally accepted that strong RET/PTC
expression is restricted to papillary carcinomas.
Point mutations involving several specific sites (codons 12, 13, and 61) of the N-RAS, H-RAS, or K-RAS
genes are found in 10-15% of papillary carcinomas
Tumors with RAS
mutations are almost all the follicular variants of papillary carcinoma, this mutation also correlates
with significantly less prominent nuclear features, more frequent encapsulation, and low rate of lymph
node metastases . Mutations of the RAS gene are not restricted to
papillary carcinoma and also found in other benign and malignant thyroid tumors, as well as in neoplasms
from other tissues.
Most frequent genetic alterations in these tumors
include point mutations of the RAS genes and PAX8-PPARg rearrangement. RAS mutations, most commonly affecting N-RAS codon 61
or H-RAS codon 61, are found in 40-50% of follicular carcinomas, and may
correlate with tumor dedifferentiation and less favorable prognosis
PAX8-PPARg fusion results from the
. It occurs in ~25% of follicular carcinomas and in ~7% of
It has been suggested that follicular adenomas positive for this
rearrangement may be pre-invasive follicular carcinomas or tumors where invasion was overlooked .
Tumors with PAX8-PPARg tend to present at a
younger age, be smaller in size, and more frequently have vascular invasion than those with mutant RAS
The incidence of RAS and PAX8-PPARg mutations is significantly lower in
oncocytic follicular carcinomas.
RAS mutations can not be used as a diagnostic marker of follicular
carcinoma since they also occur with significant prevalence in follicular adenomas and the follicular
variant of papillary carcinoma. However, detection of PAX8-PPARg rearrangement may be of diagnostic value since it occurs almost exclusively in
follicular carcinomas. The rearrangement results in overexpression of PPARg protein that can be detected
by immunohistochemistry . However, only strong diffuse nuclear staining correlates with the presence
of rearrangement . The usage of this immunostaining may be challenging since not all commercially
available PPARg antibodies give a reliable result. The presence of strong diffuse PPARg staining in
the tumor nodule, especially if confirmed by RT-PCR or FISH, should justify the submission of additional
sections of the capsule and obtaining deeper levels of all suspicious areas in search for capsular or
Follicular carcinomas are characterized by a considerable rate of loss of heterozygosity (LOH) and
frequent losses of multiple chromosomal regions. The average rate of LOH per chromosome arm is ~20%
in follicular carcinomas, as compared to 6% in follicular adenomas, and only 3% in papillary carcinomas.
The most commonly deleted regions in follicular carcinomas are on chromosomes 2p, 3p, 9q, 10q, 11p, 15q,
and 17p. conventional follicular tumors. The presence of numerous mitochondria on oncocytic tumors may
be a consequence of somatic mutations and sequence variants in mitochondrial DNA .
Poorly Differentiated Carcinomas:
These tumors show a variable frequency
of BRAF, RAS, and RET/PTC mutations, which are characteristic of well differentiated papillary and
follicular carcinomas, as well as a substantial rate of p53 and β-catenin mutations, which are common in
anaplastic carcinomas . No genetic mutations unique for poorly differentiated carcinoma have been
identified to date. This suggests that poorly differentiated carcinoma, as a group, represents a
distinct step in the evolution from well differentiated to anaplastic thyroid carcinoma, rather than an
entirely separate type of thyroid malignancy.
Undifferentiated (Anaplastic) Carcinomas:
These tumors typically have a
highly unstable, complex karyotype with numerous gains and losses of whole chromosomes and smaller
chromosomal regions. Point mutations of RAS are found in approximately 60%
of undifferentiated carcinomas and mutations in exons 5-8 of p53 in 70-80% of cases. Mutations in exon 3
of the β-catenin (CTNNB1)
gene have been reported in 66% of undifferentiated carcinomas .
Molecular alterations include point mutations of the
RAS genes, most frequently involving N-RAS
codon 61, which are found in about 30% of adenomas. They are not specific for adenomas and also occur in
follicular and papillary carcinomas. PAX8-PPARg rearrangement was
reported in ~7% of adenomas, and the identification of PAX8-PPARg
in a follicular tumor should prompt an exhaustive search for vascular or capsular invasion.
Hyperfunctioning adenomas frequently show point mutations in the TSH receptor gene and occasionally in
the GSα (GSP) gene. Cytogenetic
alterations are found in less than half of adenomas and most frequently manifest as trisomy 7 or
translocations involving the long arm of chromosome 19. Oncocytic adenomas frequently show multiple
numerical chromosomal abnormalities and somatic mutations and sequence variants in mitochondrial DNA
Hyalinizing Trabecular Tumors:
This tumor, previously
known as hyalinizing trabecular adenoma, had been suspected for a long time to be related to papillary
carcinoma, since it shares with it several diagnostic nuclear features. Several recent reports have
identified RET/PTC rearrangement in a significant portion of these tumors,
suggesting their link to papillary carcinoma
However, the great majority of these tumors
behave as benign neoplasms, and their linkage to papillary carcinoma has not been fully proven.
RET proto-oncogene as a
key molecule in the development of medullary carcinoma, including both familial and sporadic forms of the
disease. In these tumors, RET is activated by point mutation, in contrast
to its activation by chromosomal rearrangement in papillary thyroid carcinomas. Germline mutations in
specific functional regions of RET are found in almost all patients with
familial forms of medullary carcinoma. In MEN 2A an familial medullary carcinoma, mutations are
typically located in the extracellular domain, within the cysteine rich region . Almost 90% of MEN
2A mutations affect a single codon 634, whereas in familial medullary carcinoma they are more evenly
distributed along the cysteine rich region . In MEN 2B, most of the mutations involve codon 918 in
the intracellular tyrosine kinase domain. In sporadic medullary carcinomas, somatic mutations of RET are found in 23-70% of cases. The vast majority of those affect codon 918,
although they have also been identified in few other regions.
- Kimura ET, et al. High prevalence of BRAF mutations in thyroid cancer: genetic evidence for constitutive activation of the RET/PTC-RAS-BRAF signaling pathway in papillary thyroid carcinoma. Cancer Res 2003 63: 1454-7.
- Cohen Y, et al. BRAF mutation in papillary thyroid carcinoma. J Natl Cancer Inst 2003 95: 625-7.
- Nikiforova MN, et al. BRAF mutations in thyroid tumors are restricted to papillary carcinomas and anaplastic or poorly differentiated carcinomas arising from papillary carcinomas. J Clin Endocrinol Metab 2003 88: 5399-404.
- Salvatore G, et al. Analysis of BRAF point mutation and RET/PTC rearrangement refines the fine-needle aspiration diagnosis of papillary thyroid carcinoma. J Clin Endocrinol Metab 2004 89: 5175-80.
- Cohen Y, et al. Mutational analysis of BRAF in fine needle aspiration biopsies of the thyroid: a potential application for the preoperative assessment of thyroid nodules. Clin Cancer Res 2004 10: 2761-5.
- Adeniran A, et al. Correlation between genetic alterations and microscopic features, clinical manifestations, and prognostic characteristics of thyroid papillary carcinomas. Am J Surg Pathol 2006 30:216-22.
- Santoro M, et al. Ret oncogene activation in human thyroid neoplasms is restricted to the papillary cancer subtype. J Clin Invest 1992 89: 1517-22.
- Nikiforov YE RET/PTC Rearrangement in Thyroid Tumors. Endocr Pathol 2002 13: 3-16.
- Nikiforov YE, et al. Distinct pattern of ret oncogene rearrangements in morphological variants of radiation-induced and sporadic thyroid papillary carcinomas in children. Cancer Res 1997 57: 1690-4.
- Namba H, SA Rubin, and JA Fagin Point mutations of ras oncogenes are an early event in thyroid tumorigenesis. Mol Endocrinol 1990 4: 1474-9.
- Ezzat S, et al. Prevalence of activating ras mutations in morphologically characterized thyroid nodules. Thyroid 1996 6: 409-16.
- Basolo F, et al., N-ras mutation in poorly differentiated thyroid carcinomas: correlation with bone metastases and inverse correlation to thyroglobulin expression, in Thyroid. 2000. p. 19-23.
- Garcia-Rostan G, et al., ras mutations are associated with aggressive tumor phenotypes and poor prognosis in thyroid cancer, in J Clin Oncol. 2003. p. 3226-35.
- Kroll TG, et al. PAX8-PPARgamma1 fusion oncogene in human thyroid carcinoma. Science 2000 289: 1357-60.
- French CA, et al., Genetic and biological subgroups of low-stage follicular thyroid cancer, in Am J Pathol. 2003. p. 1053-60.
- Nikiforova MN, et al. RAS point mutations and PAX8-PPAR gamma rearrangement in thyroid tumors: evidence for distinct molecular pathways in thyroid follicular carcinoma. J Clin Endocrinol Metab 2003 88: 2318-26.
- Nikiforova MN, et al. PAX8-PPARgamma rearrangement in thyroid tumors: RT-PCR and immunohistochemical analyses. Am J Surg Pathol 2002 26: 1016-23.
- Maximo V, et al. Mitochondrial DNA somatic mutations (point mutations and large deletions) and mitochondrial DNA variants in human thyroid pathology: a study with emphasis on Hurthle cell tumors. Am J Pathol 2002 160: 1857-65.
- Nikiforov YE Genetic alterations involved in the transition from well-differentiated to poorly differentiated and anaplastic thyroid carcinomas. Endocr Pathol 2004 15: 319-27.
- Garcia-Rostan G, et al. β-catenin dysregulation in thyroid neoplasms: down-regulation, aberrant nuclear expression, and CTNNB1 exon 3 mutations are markers for aggressive tumor phenotypes and poor prognosis. Am J Pathol 2001 158: 987-96.
- Papotti M, et al. RET/PTC activation in hyalinizing trabecular tumors of the thyroid. Am J Surg Pathol 2000 24: 1615-21.
- Cheung CC, et al. Hyalinizing trabecular tumor of the thyroid: a variant of papillary carcinoma proved by molecular genetics. Am J Surg Pathol 2000 24: 1622-6.
- Mulligan LM, et al. Genotype-phenotype correlation in multiple endocrine neoplasia type 2: report of the International RET Mutation Consortium. J Intern Med 1995 238: 343-6.
- Hansford JR and LM Mulligan Multiple endocrine neoplasia type 2 and RET: from neoplasia to neurogenesis. J Med Genet 2000 37: 817-27.