Mitochondrial DNA mutations in oxyphilic and chief cell parathyroid adenomas

Single parathyroid adenomas, which cause approximately 85% of the cases of primary hyperparathyroidism [1] are well-differentiated, benign, clonal tumors, which produce hypercalcemia through excessive secretion of parathyroid hormone (PTH). Activation of the cyclin D1 oncogene and inactivation of the MEN1 tumor suppressor gene are established pathogenetic contributors, but understanding of molecular pathogenesis in this disease remains incomplete. The mitochondrial genome has been identified as a possible target for somatic mutations that may promote tumorigenesis [2–10]. Mitochondrial abnormalities, including changes in structure, number, and respiratory enzyme components and transport systems, have been observed in many cancers. Homoplasmic somatic mutations of the mitochondrial genome have also been identified in many tumor types [2–10]. However, the pathogenetic significance of the mutations detected in those tissues is unclear.

The human parathyroid is an intrinsically low-turnover tissue [11], as such, benign tumors arising from this tissue have relatively low proliferative rates. The mitochondrial genome is an especially attractive potential target for mutations that could drive tumorigenesis in such a tissue. Mitochondrial DNA replicates frequently and independently of the nuclear genome [12], and over many years the potential for enhanced DNA damage from locally generated reactive oxygen species (ROS) may further enhance the accumulation of somatic mutations in the mitochondrial genome.

In many endocrine organs, especially the parathyroid glands, with age, increasing numbers of oxyphil cells appear [13, 14]. The oxyphil cells of normal parathyroid glands have a characteristic eosinophilic granular cytoplasm (Figure 1) which on electron microscopic examination is filled with abnormally shaped mitochondria [15, 16]. In contrast to the chief cells, the oxyphil cells in normal glands lack the organelles associated with protein synthesis and secretion, and lack immunohistochemically identifiable chromogranin and PTH [16]. The majority of parathyroid adenomas are composed predominately of chief cells (Figure 1), however there is a subset of hyperfunctioning parathyroid adenomas that contain greater than 90% oxyphil cells and are classified as oxyphil adenomas These tumors, in addition to having a cytoplasm filled with abnormal mitochondria, contain the organelles of protein synthesis and secretion and contain histochemically identifiable PTH. While the molecular basis for this phenotypic change is unknown, it has been hypothesized that mitochondria may accumulate in oxyphil cells due to abnormalities of the mitochondrial genome[17], supported by the finding of increased numbers of oxyphil cells in normal glands with increasing age, analogous to increased mtDNA damage that occurs with aging [16].

We therefore sought acquired somatic mutations of the mitochondrial genome in both typical chief cell parathyroid adenomas and oxyphil cell parathyroid adenomas using an especially rigorous approach, and compared these findings with the mitochondrial genomes of normal and hyperplastic parathyroid glands.

Eleven somatic mutations were identified in 6 of 18 chief cell adenomas (Table 2 and Figure 2). Two tumors contained mutations that were predicted to result in early stop codons in either the ND4 or ND6 genes, both subunits of mitochondrial Complex I. Three tumors contained mutations that were predicted to result in non-synonymous changes in subunits of either Complex I or Complex III. Two tumors contained mutations only in the highly polymorphic, but non-coding D-loop region.

Tumor ID	Tumor Type	Mutation	Gene affected	Expected Protein Change	Previously seen?
1	Oxyphil cell	12631T>C	ND5	99Ser>Pro	normal variant
2	Oxyphil cell	3173G>A	16S rRNA		novel
		6869C>T	CO1	no change	somatic mutation-cancer cell line
		12425delA	ND5	30Asn>Frameshift	novel
3	Oxyphil cell	559G>A*	D-loop		normal variant
		7028T	CO1	no change	normal variant
4	Oxyphil cell	3572insC	ND1	89Leu>Frameshift	novel
5	Oxyphil cell	15924A>G	Thr tRNA		LIMM
6	Oxyphil cell	304C>T	D-loop		normal variant
		11038delA	ND4	93Lys>Frameshift	novel
7	Oxyphil cell	310insC	D-loop		normal variant
8	Oxyphil cell	3566insC	ND1	87Thr>Frameshift	novel
9	Oxyphil Cell	4172T>C	ND1	289Leu>Pro	novel
		5026A>G	ND2	186His>Arg	somatic mutation-oral cancer
		10522G>A	ND4L	18Gly>Glu	novel
		12372G>A	ND5	no change	normal variant
10	Chief Cell	11038delA	ND4	93K>Frameshift	novel
		13577T>C	ND5	414Ile>Thr	novel
		14386T>C	ND6	no change	normal variant
11	Chief Cell	16311T>C	D-loop		normal variant
12	Chief Cell	12382A>G	ND5	16Ile>Val	novel
13	Chief Cell	15578T>C	CYTB	278Tyr>His	novel
14	Chief Cell	8281del8	non-coding		novel
		12753A>G	ND5	no change	novel
		14488delT	ND6	62Gly>Frameshift	novel
		16519T>C	D-loop		normal variant
15	Chief Cell	253C>A	D-loop		somatic mutation

* indicates that this mutation appeared to be heteroplasmic

LIMM-Lethal Infantile Mitochondrial Myopathy

Sixteen somatic mutations were identified in 9 of 12 oxyphil adenomas. Four tumors contained mutations that were predicted to result in early stop codons in either the ND1, ND4 or ND5 genes, all subunits of mitochondrial Complex I. Two tumors contained non-synonymous changes in Complex I subunits, one tumor contained a mutation in CO1 not predicted to change the amino acid sequence and one tumor contained a mutation affecting a tRNA gene. One tumor contained a D-loop mutation only. All mutations appeared to be homoplasmic, with the exception of one, which appeared to be heteroplasmic, a G to A transition at position 559, within the D-loop region. Homoplasmy refers to the presence of a particular sequence variant or mutation in virtually every copy of the mitochondrial genome within a single tumor cell sample (and within the limits of sensitivity of the sequencing technology), while heteroplasmy refers to the presence of more than one such detectable allele [20]. Heteroplasmy within a tumor would be indistinguishable from homoplasmy in a sub-population of the tumor's cells, since each tumor's cells are being analyzed collectively rather than on an individual cell-by-cell basis.

Mutations affecting Complex I genes, which represent 38.2% of the mitochondrial genome, accounted for 55.6% of all mutations and 22.2% of mutations occurred in the highly variable D-loop region.

To determine if the observed trend towards increased numbers of likely or potentially functionally significant mtDNA mutations in oxyphil adenomas as compared to chief cell adenomas was statistically significant, Fisher's Exact test was performed yielding a significant p-value of 0.02.

No somatic mutations occurred in any of the hyperplastic parathyroid glands. The one normal parathyroid for which paired thyroid tissue was available also contained no somatic mutations. No clustering of germline sequence variants occurred in any of the samples studied.

Mitochondrial abnormalities have been observed in many cancers and previous studies have demonstrated altered energy metabolism in cancer cells [2–10]. Homoplasmic somatic mutations of the mitochondrial genome have been identified in various tumor types. The mitochondrial genome is an especially attractive potential target for mutations that could drive tumorigenesis in a low turnover tissue such as the human parathyroid, given the frequent replication of mitochondrial DNA, independent of the nuclear genome and the enhanced potential for mitochondrial DNA damage by ROS. One might expect an increased potential for accumulation of somatic mutations in the mitochondrial genome in such low replicative tissues as the parathyroid over many years. Our data provide initial support for these hypotheses, and suggest that other slow-growing tumors arising from low-replication tissues should be similarly analyzed.

Mitochondrial DNA mutations in parathyroid adenomas tend to affect Complex I genes more often than other tumor types and more often then expected by chance alone. Additionally, D-loop mutations appear to be underrepresented in parathyroid adenomas compared to other tumor types. In contrast to many other studies, this study rigorously examines the entire mitochondrial genome in matched tumor/normal pairs from the same individual while many analogous studies of other types of tumors focused primarily on coding regions and/or the D-loop region or used population rather than control DNA from the same patient. Due to the wide range of mutation frequencies in the literature and discrepancies in study design, statistical analyses between our data and those from other related studies cannot generally be performed. The importance of observed differences between parathyroid adenomas and other tumor types remains unclear.

Our results raise the possibility that NADH dehydrogenase subunit genes might play a key role in tumorigenesis, perhaps especially in low-replication tissues. Few studies have directly examined this issue. The contribution of specific mtDNA mutations, affecting the ATP synthase subunit 6 [21, 22], to tumorigenesis has been examined, however little is known about the potential pathogenetic significance of the majority of mtDNA mutations identified to date. The contribution of specific germline sequence variants as predisposition alleles to specific tumor types has been suggested, however the majority of germline mtDNA sequence variants appear unlikely to directly contribute to tumorigenesis because of the frequency of such changes in individuals without tumors. Muller-Hocker, et al. examined respiratory chain function in normal parathyroid glands and hyperplastic parathyroids in uremic patients[23], finding defects in Complex III and IV in both normal and diseased glands. They also reported that chief cells in 20% of uremia-associated (secondary) hyperplastic parathyroid glands had defects of the respiratory chain, and that heart and muscle cells had more respiratory chain defects than parathyroid oxyphil cells. Together with reports of respiratory chain defects in normal, adenomatous, and hyperplastic parathyroid glands, the preferential clustering within NADH dehydrogenase subunit genes of the somatic mitochondrial mutations in our series of adenomas reinforces the intriguing suspicion that these mutations may be playing an integral role in parathyroid tumorigenesis. Given that mitochondria have recently been implicated in the regulation of intracellular calcium homeostasis [24, 25] and apoptosis [26], and that calcium sensing and signaling are crucial to parathyroid function and regulated PTH secretion, the possible relationship between calcium homeostasis and mitochondrial complex I enzyme function deserves further scrutiny.

Our data also provide additional support for the hypothesis that mitochondria accumulate in oxyphil cells due to abnormalities of the mitochondrial genome. Previous studies of Hurthle cell [27] and other oncocytic tumors [28] have demonstrated an association between a large deletion of mtDNA (Δ4977) and oncocytic/oxyphilic tumors in comparison to non-oncocytic tumors of the same tissue type. However, given its presence in both normal and tumor cells of various tissues, the potential pathogenetic significance of such a finding remains unclear. As such, the presence or absence of this deletion was not addressed by our experimental approach and was considered beyond the scope of this manuscript. Functioning oxyphil cell parathyroid adenomas are more likely to contain somatic mtDNA mutations than chief cell adenomas and the somatic mutations in oxyphil adenomas are more likely to be functionally significant. These differences between oxyphil and chief cell adenomas are statistically significant with a p-value of 0.02 (Fisher's Exact Test). Our results show marked similarity to the recent findings in thyroid tumors by Gasparre et al that disruptive mutations (nonsense and frameshift mutations) in Complex I genes are associated with an oncocytic phenotype [29]. It is therefore quite plausible that these molecular genetic lesions may make key contributions to determining the oxyphilic phenotype, as mitochondria may increase their replication rate as a compensatory mechanism for altered energy metabolism due to mtDNA mutations. Further studies of normal oxyphil cells that could shed additional light on this hypothesis should be encouraged.