Uncovering Mitochondrial Determinants of Racial Disparities in Ovarian Cancer

Abstract

Ovarian cancer (OC) incidence and mortality rates differ between racial groups. Mitochondrial genetic factors are now emerging as determinants of racial disparities in OC. A comprehensive understanding of the role of mitochondria in OC health disparities will help in developing novel therapeutic strategies targeting mitochondria to reduce or eliminate racial health disparities.

Introduction

OC is the most lethal gynecological cancer among women. However, African American (AA) women are disproportionately affected with OC when compared with Caucasian American (CA) women [1]. Increased mortality rate is also observed in patients of AA ancestry with a 5-year survival rate of 38% as compared to that of 45% in CA women with OC (based on data from the Surveillance Epidemiology and End Results (SEER) programⁱ). AA women are also substantially burdened more with late diagnosis of OC and often have more comorbidities (see Glossary) than CA women with OC [1]. Elevated levels of cancer antigen 125, a diagnostic marker of OC that indicates higher recurrence and reduced disease-free survival of OC, is often found in AA women post-treatment. Meanwhile, Hispanic and Asian women are diagnosed at a younger age, less burdened with advanced disease, and more prone to have endometrioid and clear cell tumor subtypes than CA and AA women [1].

The underlying mechanism of racial differences in OC is poorly understood. Although socioeconomic factors are highly attributed, aberrations in mitochondrial DNA (mtDNA) genes as well as mitochondrial proteins encoded by nuclear DNA (nDNA) in racial disparities in cancers have also recently been described [2]. Indeed, AA women respond better to the mitochondria inhibitor metformin than CA patients do, indicating the relevance of mitochondria in precision therapy based on race [3].

Mitochondria are dynamic organelles with multifunctional roles in cellular energy metabolism, cell signaling, and oxidative stress [2]. mtDNA encodes 37 mitochondrial proteins, while the remaining are encoded by nuclear DNA (nDNA) [2]. Mitochondria crosstalk with the nucleus through retrograde communication (signaling) [2]. Variants in mtDNA may genetically or epigenetically alter this communication, affecting mitochondrial bioenergetics, Ca²⁺ signaling, redox regulation, and metabolic rear-rangements in the cell [2]. As mitochondrial dysfunction is a hallmark of cancer, retrograde communication may provide a molecular basis and an underlying mechanism for cancer risk and aggressiveness [2] (Box 1). Conceivably, differences in mitochondrial function at the cellular level in various ethnic populations may alter organellar crosstalk and contribute to racial disparities [2]. In this review, we focus on mitochondrial genome variants and nuclear gene variants encoding mitochondrial proteins in OC to bring attention to the role of mitochondria in racial disparity.

Box 1. Mitochondrial Retrograde Signaling in OC.

Mitochondria play an important role in OC development through retrograde signaling in many different ways. Defects in mitochondria alter the expression of several oncogenes or tumor suppressor genes in breast and ovarian tumors [2]. Integrative analysis of the mitochondrial expressed proteins in OC patients and The Cancer Genome Atlas (TCGA) data also revealed changes in signaling pathways in the nucleus that were associated with mitophagy and energy metabolism including the tricarboxylic acid cycle (TCA) cycle pathway [4]. Various kinases such as PI3K/Akt kinase involved in OC initiation may also be activated by retrograde signaling [2]. miRNAs, such as miR-663, also control retrograde signaling by epigenetic regulation in breast cancer cells, and are upregulated in OC tissues where they facilitate the growth, migration, and invasion of OC cells [5].

Many mitochondrial metabolites including citrate, an intermediate of the TCA cycle, act as a trigger for retrograde signaling. When converted into acetyl-CoA, citrate can be exported to the cytoplasm and nucleus where it controls transcription and cytoplasmic processes such as autophagy [2]. Acetoacetate, derived from acetyl-CoA, promotes cancer proliferation through BRAF kinase and MAPK signaling pathways, which are crucial pathways in OC development [2]. OC cells have an active TCA cycle suggesting that mitochondrial retrograde signaling through TCA cycle metabolic intermediates may be a trigger for OC development. Furthermore, TCA cycle intermediates such as oxaloacetate and α-ketoglutarate are the precursors of amino acids, aspartate, and glutamate, respectively. Glutamate is also a product of glutamine metabolism. Abundance of certain metabolites such as glutamine, glutamate and aspartate were found to be elevated in AAs compared to CAs in bladder cancer, although metabolic differences in OC among racial groups still need to be studied.

mtDNA Encoded Genes in OC Racial Disparity

mtDNA variants behave functionally different among lineages. The same variant from ancient lineages may foster diet and environmental adaptation, while also contributing to bioenergetic disorders such as obesity, cardiovascular disease, and increased cancer risk [6]. Therefore, there is interest in determining whether mtDNA variants contribute to racial disparities in OC. A recent study examined mtDNA sequence variants with respect to differences in race/ethnicity and age across 118 OC tissues [7]. A total of 39 mtDNA variants were detected, with notable changes in tRNA, ATPase 6, and subunits of the oxidative phosphorylation OXPHOS complexes, COI and COIII, affecting races with different frequency (Table 1). Independent of age, many of the variants found in AA patients over CA patients appear to occur with the most frequency in advanced serous sub-type tissues. For example, the COI variant, C7028T was found with increasing frequency from borderline tumors to stage III/IV OC tumors [7]. However, age can be a factor among racial disparities. Variations in expression of somatic variant C9500T in COIII gene suggests ethnic differences for younger and older women with more advanced OC (Table 1). Despite these findings, it remains unclear how these changes affect OC initiation and progression as well as contribute to a higher incidence of OC in AA. Among mtDNA genes, the expression of ATPase 6 and COI are highly variable. This variation might be from natural selection to adapt to environmental conditions at that time. For example, mitochondrial OXPHOS drives ATP production, which is coupled to heat generation. As prehistoric humans moved northward from Africa, they encountered colder climate and dietary changes, from a low fat vegetarian diet to a high fat/high protein carnivorous diet that led to the accumulation of variations in mtDNA OXPHOS subunit complexes. This increase in mtDNA variants allowed humans to adapt by increasing the generation of heat through caloric consumption. However, these changes are also associated with increased risks for diseases including cancer [6]. mtDNA changes that are also influenced by dietary and lifestyle habits may occur differently in AA and CA. In contrast, individuals who remained in Africa do not have such variations in mtDNA [6].

Table 1.

Mitochondrial Proteins Involved in Racial Disparity in OC

Study/Refs	Type	Gene	Role of encoded protein	Variant/gene expression	Relevance in racial disparity
Study consisting of 118 tissues containing three subtypes (serous, endometroid, and mucinous) and 18 matched normal. Forty-nine tissues were derived from AA and 87 from CA women [7]	mtDNA-encoded genes	ATPase 6	ATP production, mtDNA maintenance, and cell apoptosis	T8548G	Identified in 95% of AA OC patients in contrast to only 22% of CA patients belonging to the same age group
		tRNA	tRNA metabolism, proper mitochondrial function	C7520T	Present in 74% of AA patients and only 26% of CA patients
		COI	Proper assembly of mitochondrial super complexes	C7028T	Found in 86% of AA OC women and 43% of CA OC women
		COIII	Proper assembly of mitochondrial super complexes	C9500T T9540C	Found in 47% of AA patients compared to 1% of CA patients aged >40 years Found in 23% of AA patients compared with 60% of CA patients aged >60 years Found at a high frequency in AA patients (88%) compared with CA patients (10%)
GSEA of OC using TCGA gene expression data of AA patients and CA OC patients [8]	Nuclear genes encoding mitochondrial proteins	OXPHOS complexes	ATP production and ROS generation	Upregulation of several expression of genes belonging to complexes I–V	Mitochondrial OXPHOS genes enriched in AA compared with OC tumors from CA women
Ovarian Cancer study of women of European ancestry from the OCAC with or without invasive OC and women with serous OC from AA and Asian ancestry [10]	Mitochondrial metabolite transport proteins	SLC25A45 MGST1	SLC25A45 is involved in transport of Long chain fatty acids. SLC25 family is involved in transport of ATP/ADP, amino acids, and metabolites such as malate, ornithine/citrulline. across mitochondrial membrane to drive OXPHOS. Protects against oxidative stress from wide range of ROS including lipid peroxides; maintains mitochondrial membrane integrity; plays a role in treatment response towards antiangiogenic drugs used in OC; perhaps by downregulating hypoxia-related genes.	SNP rs681309 SNP rs6488840	Associated with reduced risk for serous OC cases in the Asian population but not in thosewith CA or AA ancestry. Significantly associated with reduced serous OC risk in women of AA ancestry and CA but is not significantly associated with the Asian population
Study consisting of CA and AA women with OC and matched controls [14]	Dual nuclear and mitochondrial localized proteins	AR	Involved in androgen receptor signaling pathway	Short CAG and GGC repeat length	Short repeat length on either allele was associated with a twofold increase in OC risk in AAs (while the short CAG repeat lengths on both alleles increased the risk of OC fivefold. In contrast, no increase in risk for OC was found in CA women.

Nuclear DNA Encoded Mitochondrial Proteins in OC Racial Disparity

Mitochondrial OXPHOS, encoded mainly by the nucleus is a major source for ATP synthesis and plays roles in mitochondrial bioenergetics, biosynthesis, and signaling in OC [8]. In OC, an increased dependence on OXPHOS for survival corresponds with chemoresistant cell types, while reliance on glycolysis correlates with chemosensitivity. AAs have a higher incidence of aggressive forms of OC compared with other ethnic groups, and recent gene expression data across 23 different cancer types including OC revealed that mitochondrial OXPHOS is enriched in OC tumors from AA women compared with those derived from CA women [8] (Table 1). Subsequent analysis revealed increased expression of genes encoding mitochondrial proteins in tissues from AAs compared with CAs, such as nuclear transcription factors involved in OXPHOS regulation [8], ERR1, and PGC1α. However, these findings need to be validated in OC tissues. While these studies suggest that mitochondria contribute to the biological basis of OC health disparities, the mechanisms by which OXPHOS regulates OC in AA women remains unclear. One possibility is that increased mtDNA copy number, which confers an increased risk of OC and is found in advanced stages of OC [9], alters mitochondrial OXPHOS.

Mitochondrial Metabolite Transport Proteins in OC Disparities

Defects in cellular transport genes leads to accumulation of iron, hormones, and metabolites and may promote carcinogenesis. A study consisting of women of European ancestry from the Ovarian Cancer Association Consortium (OCAC) with or without invasive OC and women with serous OC from AA ancestry and Asian ancestry [10] identified SNP rs681309 in the intergenic region near SLC25A45 and SNP rs6488840 near gene, MGST1 (Table 1). SLC25A45 encodes a mitochondrial protein involved in the transport of solutes and metabolites to drive OXPHOS. SLC25A45 is downregulated during OC progression [11], and this SNP is associated with reduced risk for serous OC cases in Asian populations but not in those with CA or AA ancestry [10]. While the exact mechanism by which SLC25A45 regulates OC is unknown, the predicted substrate is long chain fatty acids. Moreover, the expression of these proteins are regulated at the epigenetic level and are dependent upon nutritional conditions [11]. Asian diet differs from western diet in the amount and type of long chain fatty acids consumed, with the Asian diet consuming a greater amount of omega-3 type. This consumption when accompanied by eastern style cooking methods might play a role in protecting against OC [12]. Therefore, we can speculate that the SNP rs681309 may be differentially regulated due to different dietary habits of Asians and therefore, have a reduced association with OC in the Asian population compared to AAs and CAs.

MGST1 is abundantly expressed in OC. It encodes the mitochondrial membrane protein that belongs to the MAPEG (membrane-associated proteins in eicosanoid and glutathione metabolism) superfamily and that localizes to the outer mitochondrial membrane and endoplasmic reticulum [13]. The SNP rs6488840 located near MGST1 is significantly associated with reduced OC risk in women of AA and CA ancestry [10] (Table 1), but is not significantly associated with the Asian population. More work is needed to understand how this SNP can contribute to racial disparities in OC.

Dual Nuclear and Mitochondrial Localized Proteins in OC Disparities

Some proteins that localize to both the nucleus and mitochondria have been implicated in the progression of OC. Specifically, activation of the androgen receptor signaling pathway is correlated with an increased risk of developing ovarian cancer. Known CAG and GGC repeat polymorphisms in exon 1 of the AR gene affect its transactivation activity. Moreover, CAG repeat polymorphisms in AR show racial differences in OC risk (Table 1). Analysis for shorter and longer CAG and GGC repeat length polymorphisms in the AR gene revealed that the short repeat length on either allele was associated with a twofold increase in OC risk in AAs [odds ratio (OR) = 2.2; 95% confidence interval (CI): 1.1–4.1], while the short CAG repeat lengths on both alleles increased the risk of OC fivefold (OR = 5.4; 95% CI: 1.4–1.7) [14]. In contrast, no increase in risk for OC was found in CA women. As CAG repeat length and androgen activity are inversely related, it is possible that short CAG repeat lengths contribute to the increased incidence of OC in AA [15].

Concluding Remarks

It is crucial to recognize differences in mitochondria as it relates to racial disparities when categorizing risk factors for OC incidence and mortality. Targeting mitochondrial determinants and identifying and understanding altered mitochondrial retrograde signaling in OC could pave the way for development of promising therapeutic strategies for the treatment of OC in racially diverse populations. In addition, future clinical trials selection criteria must include adequate numbers of underrepresented races such as AA, Hispanic, and Asian women to better identify biomarkers and to determine the benefits and risks associated with specific interventions in particular races of patients with OC.

Glossary

Comorbidities

two or more chronic diseases or conditions presented simultaneously in a patient.

Mitochondrial bioenergetics

refers to fundamental cellular processes related to energy production such as glycolysis, fatty acid metabolism, nucleotide biosynthesis, and iron metabolism etc.in the cell.

mtDNA copy number

measure of the number of mitochondrial genomes per cell. Altered mtDNA copy number is an indirect measurement of mitochondrial dysfunction (condition where mitochondria do not work efficiently due to any disease)

Retrograde communication

mitochondria to nuclear communication is called retrograde communication.

SNP

a single-nucleotide genetic variation at a specific position in the genome, and a commonly found genetic variation among people that may predispose them to any disease state.

The Cancer Genome Atlas (TCGA)

a cancer genomics program to identify molecular changes across various types of cancer and matched normal samples which may help to understand the causes of cancer and new ways to diagnose, prevent or treat cancer.