Mitochondrial DNA variant A4917G, smoking and spontaneous preterm birth

Fetal and obstetrics manifestations of mitochondrial diseases

During embryonic and neonatal development, mitochondria have essential effects on metabolic and energetic regulation, shaping cell fate decisions and leading to significant short- and long-term effects on embryonic and offspring health. Therefore, perturbation on mitochondrial function can have a pathological effect on pregnancy. Several shreds of evidence collected in preclinical models revealed that severe mitochondrial dysfunction is incompatible with life or leads to critical developmental defects, highlighting the importance of correct mitochondrial function during embryo-fetal development. The mechanism impairing the correct development is unknown and may include a dysfunctional metabolic switch in differentiating cells due to decreased ATP production or altered apoptotic signalling. Given the central role of mitochondria in embryonic and fetal development, the mitochondrial dysfunction typical of Mitochondrial Diseases (MDs) should, in principle, be detectable during pregnancy. However, little is known about the clinical manifestations of MDs in embryonic and fetal development. In this manuscript, we review preclinical and clinical evidence suggesting that MDs may affect fetal development and highlight the fetal and maternal outcomes that may provide a wake-up call for targeted genetic diagnosis.

Obstetric involvement in mitochondrial disorders: A review

This is the first review about obstetric involvement in mitochondrial disorders (MIDs). The purpose of the review was to discuss recent advances and knowledge about the type and frequency of obstetric complications in MIDs. A narrative review for preferred reporting items was performed in MEDLINE, Current Contents, EMBASE, Web of Science, Web of Knowledge, LILACS, SCOPUS, and Google Scholar. The author searched for studies examining obstetric complications in patients with a definite MID. Obstetric complications described in MIDs include eclampsia, preeclampsia, intra uterine growth retardation, polyhydramnion, oligoamnion, decreased fetal movements, premature delivery, stillbirth, blow weakness, dystocia, breech presentation, retained placenta, postnatal hemorrhage, low birth weight, and early postnatal death. The most common of these complications are polyhydramnion, stillbirth, premature delivery, and low birth weight. The data show that some obstetric complications are more common in MIDs than in healthy females. MIDs can be associated with various obstetric complications. Some of these complications are more common in pregnant females with MID compared with healthy pregnant females. Obstetricians should be aware of MIDs and should know that pregnant females with a MID have an increased risk of developing complications during pregnancy or delivery.

Circulating cell-free mitochondrial DNA in pregnancy

Circulating cell-free mitochondrial DNA (ccf-mtDNA) released upon cell injury or death stimulates diverse pattern recognition receptors to activate innate immune responses and initiate systemic inflammation. In this review, we discuss the temporal changes of ccf-mtDNA during pregnancy and its potential contribution to adverse pregnancy outcomes in pregnancy complications.

Validation of low-coverage whole-genome sequencing for mitochondrial DNA variants suggests mitochondrial DNA as a genetic cause of preterm birth

Preterm birth (PTB), or birth that occurs earlier than 37 weeks of gestational age, is a major contributor to infant mortality and neonatal hospitalization. Mutations in the mitochondrial genome (mtDNA) have been linked to various rare mitochondrial disorders and may be a contributing factor in PTB given that maternal genetic factors have been strongly linked to PTB. However, to date, no study has found a conclusive connection between a particular mtDNA variant and PTB. Given the high mtDNA copy number per cell, an automated pipeline was developed for detecting mtDNA variants using low‐coverage whole‐genome sequencing (lcWGS) data. The pipeline was first validated against samples of known heteroplasmy, and then applied to 929 samples from a PTB cohort from diverse ethnic backgrounds with an average gestational age of 27.18 weeks (range: 21–30). Our new pipeline successfully identified haplogroups and a large number of mtDNA variants in this large PTB cohort, including 8 samples carrying known pathogenic variants and 47 samples carrying rare mtDNA variants. These results confirm that lcWGS can be utilized to reliably identify mtDNA variants. These mtDNA variants may make a contribution toward preterm birth in a small proportion of live births.

Placental mitochondrial DNA mutational load and perinatal outcomes: Findings from a multi-ethnic pregnancy cohort

Mitochondria fuel placental activity, with mitochondrial dysfunction implicated in several perinatal complications. We investigated placental mtDNA mutational load using NextGen sequencing in relation to birthweight and gestational length among 358 mother-newborn pairs. We found that higher heteroplasmy, especially in the hypervariable displacement loop region, was associated with shorter gestational length. Results were similar among male and female pregnancies, but stronger in magnitude among females. With regard to growth, we observed that higher mutational load was associated with lower birthweight-for-gestational age (BWGA) among females, but higher BWGA among males. These findings support potential sex-differential fetal biological strategies for coping with increased heteroplasmies.

Spontaneous premature birth as a target of genomic research

Spontaneous preterm birth is a serious and common pregnancy complication associated with hormonal dysregulation, infection, inflammation, immunity, rupture of fetal membranes, stress, bleeding, and uterine distention. Heredity is 25-40% and mostly involves the maternal genome, with contribution of the fetal genome. Significant discoveries of candidate genes by genome-wide studies and confirmation in independent replicate populations serve as signposts for further research. The main task is to define the candidate genes, their roles, localization, regulation, and the associated pathways that influence the onset of human labor. Genomic research has identified some candidate genes that involve growth, differentiation, endocrine function, immunity, and other defense functions. For example, selenocysteine-specific elongation factor (EEFSEC) influences synthesis of selenoproteins. WNT4 regulates decidualization, while a heat-shock protein family A (HSP70) member 1 like, HSPAIL, influences expression of glucocorticoid receptor and WNT4. Programming of pregnancy duration starts before pregnancy and during placentation. Future goals are to understand the interactive regulation of the pathways in order to define the clocks that influence the risk of prematurity and the duration of pregnancy. Premature birth has a great impact on the duration and the quality of life. Intensification of focused research on causes, prediction and prevention of prematurity is justified.

Genetic studies of gestational duration and preterm birth

The fine control of birth timing is important to human survival and evolution. A key challenge in studying the mechanisms underlying the regulation of human birth timing is that human parturition is a unique to human event — animal models provide only limited information. The duration of gestation or the risk of preterm birth is a complex human trait under genetic control from both maternal and fetal genomes. Genomic discoveries through genome-wide association (GWA) studies would implicate relevant genes and pathways. Similar to other complex human traits, gestational duration is likely to be influenced by numerous genetic variants of small effect size. The detection of these small-effect genetic variants requires very large sample sizes. In addition, several practical and analytical challenges, in particular the involvement of both maternal and fetal genomes, further complicate the genetic studies of gestational duration and other pregnancy phenotypes. Despite these challenges, large-scale GWA studies have already identified several genomic loci associated with gestational duration or the risk of preterm birth. These genomic discoveries have revealed novel insights about the biology of human birth timing. Expanding genomic discoveries in larger datasets by more refined analytical approaches, together with the functional analysis of the identified genomic loci, will collectively elucidate the biological processes underlying the control of human birth timing.

Spontaneous preterm birth: advances toward the discovery of genetic predisposition

Evidence from family and twin-based studies provide strong support for a significant contribution of maternal and fetal genetics to the timing of parturition and spontaneous preterm birth. However, there has been only modest success in the discovery of genes predisposing to preterm birth, despite increasing sophistication of genetic and genomic technology. In contrast, DNA variants associated with other traits/diseases have been identified. For example, there is overwhelming evidence that suggests that the nature and intensity of an inflammatory response in adults and children are under genetic control. Because inflammation is often invoked as an etiologic factor in spontaneous preterm birth, the question of whether spontaneous preterm birth has a genetic predisposition in the case of pathologic inflammation has been of long-standing interest to investigators. Here, we review various genetic approaches used for the discovery of preterm birth genetic variants in the context of inflammation-associated spontaneous preterm birth. Candidate gene studies have sought genetic variants that regulate inflammation in the mother and fetus; however, the promising findings have often not been replicated. Genome-wide association studies, an approach to the identification of chromosomal loci responsible for complex traits, have also not yielded compelling evidence for DNA variants predisposing to preterm birth. A recent genome-wide association study that included a large number of White women (>40,000) revealed that maternal loci contribute to preterm birth. Although none of these loci harbored genes directly related to innate immunity, the results were replicated. Another approach to identify DNA variants predisposing to preterm birth is whole exome sequencing, which examines the DNA sequence of protein-coding regions of the genome. A recent whole exome sequencing study identified rare mutations in genes encoding for proteins involved in the negative regulation (dampening) of the innate immune response (eg, CARD6, CARD8, NLRP10, NLRP12, NOD2, TLR10) and antimicrobial peptide/proteins (eg, DEFB1, MBL2). These findings support the concept that preterm labor, at least in part, has an inflammatory etiology, which can be induced by pathogens (ie, intraamniotic infection) or "danger signals" (alarmins) released during cellular stress or necrosis (ie, sterile intraamniotic inflammation). These findings support the notion that preterm birth has a polygenic basis that involves rare mutations or damaging variants in multiple genes involved in innate immunity and host defense mechanisms against microbes and their noxious products. An overlap among the whole exome sequencing-identified genes and other inflammatory conditions associated with preterm birth, such as periodontal disease and inflammatory bowel disease, was observed, which suggests a shared genetic substrate for these conditions. We propose that whole exome sequencing, as well as whole genome sequencing, is the most promising approach for the identification of functionally significant genetic variants responsible for spontaneous preterm birth, at least in the context of pathologic inflammation. The identification of genes that contribute to preterm birth by whole exome sequencing, or whole genome sequencing, promises to yield valuable population-specific biomarkers to identify the risk for spontaneous preterm birth and potential strategies to mitigate such a risk.

Immunostimulatory role of mitochondrial DAMPs: alarming for pre-eclampsia?

Mitochondria are critical signaling organelles that play an integral cellular role in the activation of diverse physiological responses to perturbation. Mitochondrial damage-associated molecular patterns (DAMPs) act as redox signaling nodes synchronizing mitochondrial metabolism with triggering of inflammation. Oxidative stress and inflammation are implicated in the pathogenesis of pre-eclampsia; however, the mechanisms involved in the novel crosstalk between these two pathogenic pathways are less well elucidated. In this review, we show that mitochondrial redox signals are paramount for regulating and maintaining the inflammatory response to danger signals. Mitochondrial DNA (mtDNA) represents a mitochondrial DAMP and is often liberated as signal of mitochondrial dysfunction. This review will explore the mechanistic role of mitochondrial DNA in directly coordinating adaptive changes in the maternal inflammatory status in pre-eclampsia through recruitment of innate immune cells and subsequent cytokine production. Finally, we provide emerging evidence of elevated circulating mitochondrial DAMPs in pre-eclampsia.

The genetics of preterm birth: Progress and promise

Preterm birth is the single leading cause of mortality for neonates and children less than 5 years of age. Compared to other childhood diseases, such as infections, less progress in prevention of prematurity has been made. In large part, the continued high burden of prematurity results from the limited understanding of the mechanisms controlling normal birth timing in humans, and how individual genetic variation and environmental exposures disrupt these mechanisms to cause preterm birth. In this review, we summarize the outcomes and limitations from studies in model organisms for birth timing in humans, the evidence that genetic factors contribute to birth timing and risk for preterm birth, and recent genetic and genomic studies in women and infants that implicate specific genes and pathways. We conclude with discussing areas of potential high impact in understanding human parturition and preterm birth in the future.

Mitochondrial DNA sequence variation is largely conserved at birth with rare de novo mutations in neonates

OBJECTIVE

Mitochondrial DNA (mtDNA) encodes the proteins of the electron transfer chain to produce adenosine triphosphate through oxidative phosphorylation, and is essential to sustain life. mtDNA is unique from the nuclear genome in so much as it is solely maternally inherited (non-mendelian patterning), and shows a relatively high rate of mutation due to the absence of error checking capacity. While it is generally assumed that most new mutations accumulate through the process of heteroplasmy, it is unknown whether mutations initiated in the mother are inherited, occur in utero, or occur and accumulate early in life. The purpose of this study is to examine the maternally heritable and de novo mutation rate in the fetal mtDNA through high-fidelity sequencing from a large population-based cohort.

STUDY DESIGN

Samples were obtained from 90 matched maternal (blood) and fetal (placental) pairs. In addition, a smaller cohort (n = 5) of maternal (blood), fetal (placental), and neonatal (cord blood) trios were subjected to DNA extraction and shotgun sequencing. The whole genome was sequenced on the Illumina HiSeq platform (Illumina Inc., San Diego, CA), and haplogroups and mtDNA variants were identified through mapping to reference mitochondrial genomes (NC_012920).

RESULTS

We observed 665 single nucleotide polymorphisms and 82 insertions-deletions variants identified in the cohort at large. We achieved high sequencing depth of the mtDNA to an average depth of 65X (range, 20-171X) coverage. The proportions of haplogroups identified in the cohort are consistent with the patient's self-identified ethnicity (>90% Hispanic), and all maternal-fetal pairs mapped to the identical haplogroup. Only variants from samples with average depth >20X and allele frequency >1% were included for further analysis. While the majority of the maternal-fetal pairs (>90%) demonstrated identical variants at the single nucleotide level, we observed rare mitochondrial single nucleotide polymorphism discordance between maternal and fetal mitochondrial genomes.

CONCLUSION

In this first in-depth sequencing analysis of mtDNA from maternal-fetal pairs at the time of birth, a low rate of de novo mutations appears in the fetal mitochondrial genome. This implies that these mutations likely arise from the maternal heteroplasmic pool (eg, in the oocyte), and accumulate later in the offspring's life. These findings have key implications for both the occurrence and screening for mitochondrial disorders.

The case for the continuing use of the revised Cambridge Reference Sequence (rCRS) and the standardization of notation in human mitochondrial DNA studies

Since the determination in 1981 of the sequence of the human mitochondrial DNA (mtDNA) genome, the Cambridge Reference Sequence (CRS), has been used as the reference sequence to annotate mtDNA in molecular anthropology, forensic science and medical genetics. The CRS was eventually upgraded to the revised version (rCRS) in 1999. This reference sequence is a convenient device for recording mtDNA variation, although it has often been misunderstood as a wild-type (WT) or consensus sequence by medical geneticists. Recently, there has been a proposal to replace the rCRS with the so-called Reconstructed Sapiens Reference Sequence (RSRS). Even if it had been estimated accurately, the RSRS would be a cumbersome substitute for the rCRS, as the new proposal fuses--and thus confuses--the two distinct concepts of ancestral lineage and reference point for human mtDNA. Instead, we prefer to maintain the rCRS and to report mtDNA profiles by employing the hitherto predominant circumfix style. Tree diagrams could display mutations by using either the profile notation (in conventional short forms where appropriate) or in a root-upwards way with two suffixes indicating ancestral and derived nucleotides. This would guard against misunderstandings about reporting mtDNA variation. It is therefore neither necessary nor sensible to change the present reference sequence, the rCRS, in any way. The proposed switch to RSRS would inevitably lead to notational chaos, mistakes and misinterpretations.

No observed association for mitochondrial SNPs with preterm delivery and related outcomes

BACKGROUND

Preterm delivery (PTD) is the leading cause of neonatal morbidity and mortality. Epidemiologic studies indicate recurrence of PTD is maternally inherited, creating a strong possibility that mitochondrial variants contribute to its etiology. This study examines the association between mitochondrial genotypes and PTD and related outcomes.

METHODS

This study combined, through meta-analysis, two case-control, genome-wide association studies: one from the Danish National Birth Cohort Study and one from the Norwegian Mother and Child Cohort Study (MoBa) conducted by the Norwegian Institute of Public Health. The outcomes of PTD (≤36 wk), very PTD (≤32 wk), and preterm prelabor rupture of membranes (PPROM) were examined. A total of 135 individual single-nucleotide polymorphism (SNP) associations were tested using the combined genome from mothers and neonates (case vs. control) in each population and then pooled via meta-analysis.

RESULTS

After meta-analysis, there were four SNPs for the outcome of PTD below P ≤ 0.10 and two below P ≤ 0.05. For the additional outcomes of very PTD and PPROM, there were three and four SNPs, respectively, below P ≤ 0.10.

CONCLUSION

Given the number of tests, no single SNP reached study-wide significance (P = 0.0006). Our study does not support the hypothesis that mitochondrial genetics contributes to the maternal transmission of PTD and related outcomes.

Identification of fetal and maternal single nucleotide polymorphisms in candidate genes that predispose to spontaneous preterm labor with intact membranes

OBJECTIVE

The purpose of this study was to determine whether maternal/fetal single nucleotide polymorphisms (SNPs) in candidate genes are associated with spontaneous preterm labor/delivery.

STUDY DESIGN

A genetic association study was conducted in 223 mothers and 179 fetuses (preterm labor with intact membranes who delivered <37 weeks of gestation [preterm birth (PTB)]), and 599 mothers and 628 fetuses (normal pregnancy); 190 candidate genes and 775 SNPs were studied. Single locus/haplotype association analyses were performed; the false discovery rate was used to correct for multiple testing.

RESULTS

The strongest single locus associations with PTB were interleukin-6 receptor 1 (fetus; P=.000148) and tissue inhibitor of metalloproteinase 2 (mother; P=.000197), which remained significant after correction for multiple comparisons. Global haplotype analysis indicated an association between a fetal DNA variant in insulin-like growth factor F2 and maternal alpha 3 type IV collagen isoform 1 (global, P=.004 and .007, respectively).

CONCLUSION

An SNP involved in controlling fetal inflammation (interleukin-6 receptor 1) and DNA variants in maternal genes encoding for proteins involved in extracellular matrix metabolism approximately doubled the risk of PTB.

Association of mitochondrial allele 4216C with increased risk for sepsis-related organ dysfunction and shock after burn injury

Impaired mitochondrial activity has been linked to increased risk for clinical complications after injury. Furthermore, variant mitochondrial alleles have been identified and are thought to result in decreased mitochondrial activity. These include a nonsynonymous mitochondrial polymorphism (T4216C) in the nicotinamide adenine dinucleotide dehydrogenase 1 gene (ND1), encoding a key member of complex I within the electron transport chain, which is found almost exclusively among Caucasians. We hypothesized that burn patients carrying ND1 4216C are less able to generate the cellular energy necessary for an effective immune response and are at increased risk for infectious complications. The association between 4216C and outcome after burn injury was evaluated in a cohort of 175 Caucasian patients admitted to the Parkland Hospital with burns covering greater than or equal to 15% of their total body surface area or greater than or equal to 5% full-thickness burns under an institutional review board-approved protocol. To remove confounding unrelated to burn injury, individuals were excluded if they presented with significant non-burn-related trauma (Injury Severity Score > or =16), traumatic or anoxic brain injury, spinal cord injury, were HIV/AIDS positive, had active malignancy, or survived less than 48 h postadmission. Within this cohort of patients, carriage of the 4216C allele was significantly associated by unadjusted analysis with increased risk for sepsis-related organ dysfunction or septic shock (P = 0.011). After adjustment for full-thickness burn size, inhalation injury, age, and sex, carriage of the 4216C allele was associated with complicated sepsis (adjusted odds ratio = 3.7; 95% confidence interval, 1.5-9.1; P = 0.005), relative to carriers of the T allele.

What causes racial disparities in very preterm birth? A biosocial perspective

Very preterm birth (<32 weeks' gestation) occurs in approximately 2% of livebirths but is a leading cause of infant mortality and morbidity in the United States. African-American women have a 2-fold to 3-fold elevated risk compared with non-Hispanic white women for reasons that are incompletely understood. This paper reviews the evidence for the biologic and social patterning of very preterm birth, with attention to leading hypotheses regarding the etiology of the racial disparity. A systematic review of the literature in the MEDLINE, CINAHL, PsycInfo, and EMBASE indices was conducted. The literature to date suggests a complex, multifactorial causal framework for understanding racial disparities in very preterm birth, with maternal inflammatory, vascular, or neuroendocrine dysfunction as proximal pathways and maternal exposure to stress, racial differences in preconceptional health, and genetic, epigenetic, and gene-environment interactions as more distal mediators. Interpersonal and institutionalized racism are mechanisms that may drive racially patterned differences. Current literature is limited in that research on social determinants and biologic processes of prematurity has been generally disconnected. Improved etiologic understanding and the potential for effective intervention may come with better integration of these research approaches.

Association of mitochondrial allele 4216C with increased risk for complicated sepsis and death after traumatic injury

OBJECTIVES

Numerous studies have linked impaired mitochondrial activity with increased risk for clinical complications after injury. Furthermore, a number of nonsynonymous polymorphisms have been identified within the mitochondrial genome that are believed to impair cellular respiration. These DNA variants include a nonsynonymous polymorphism (T4216C) in the NADH dehydrogenase 1 gene (ND1), which encodes a key member of Complex I of the electron transport chain. We hypothesized that trauma patients who carry the ND1 4216C allele may be less able to generate the cellular energy necessary to mount an effective immune response and are at increased risk for death as well as sepsis complicated by organ dysfunction or shock.

METHODS

We enrolled a cohort of 136 patients admitted to the Parkland Hospital Surgical intensive care unit (ICU) with significant trauma (Injury Severity Score > or = 16), > or =16 years of age, and with a minimum intensive care unit stay of > or =24 hours under a protocol approved by the UTSW and Parkland IRBs. Patients with brain death, spinal cord injury, active malignancy, HIV/AIDS or who survived <48 hours after admission were excluded. Clinical data were collected prospectively and T4216C was genotyped by polymerase chain reaction-restriction fragment length polymorphism.

RESULTS

After multivariate adjustment for mechanism, severity of injury, units of packed red blood cells given in the first 24 hours, age, gender, and race/ethnicity, carriage of the 4216 C-allele was significantly associated with increased risk for sepsis complicated by organ dysfunction or septic shock (adjusted odds ratio [aOR] = 3.68; 95%CI: 1.17-11.52; p = 0.02) as well as death (aOR = 4.56; 95% CI: 1.05-19.79; p = 0.04), relative to carriers of the T-allele.

CONCLUSION

Carriage of the mitochondrial 4216C-allele increases the risk for infectious complications and death after severe trauma.