Preterm labour and delivery: a genetic predisposition

Preterm delivery (PTD) complicates as many as 10% of pregnancies in the United States. Moreover, prematurity accounts for more than 70% of the consequent neonatal and infantile morbidity and mortality. Serious long-term complications include cerebral palsy, respiratory disease, blindness and deafness. Despite substantial basic scientific, translational and clinical investigation in recent years, the PTD rate (10%) and the low birthweight rate (7%) remain largely unchanged. Indeed, the very aetiology and pathophysiology of PTD remain unknown in most cases. In short, PTD continues to constitute a major clinical and public health challenge of the highest order, a circumstance further compounded by the controversy surrounding the efficacy of current therapeutic regimens. In an effort to address the relevant knowledge gap, we put forth the hypothesis that PTD results, at least in part, from a genetic predisposition. Evidence supporting the hypothesis that certain women have a genetic predisposition to deliver preterm is growing. Moreover, the discovery of a gene mutation predisposing to PTD would constitute a major breakthrough for future research into the biology, prediction, and therapy of preterm labour. Presented here is a discussion of the evidence to support a genetic predisposition to PTD, molecular techniques proposed to study the genetics of preterm labour, and plausible candidate genes that warrant further investigation.

Genetic expression by fetal chorionic villi during the first trimester of human gestation

OBJECTIVE

The growth and differentiation of the embryo and the contiguous placental structures are fundamental to human reproduction and survival. Little is known, however, about the genetic control of these processes during early human development. Normal placentation is the result of a well-orchestrated sequence of events that consists of cellular adhesion and limited invasion controlled by relatively unknown genetic processes. We hypothesized that genes expressed by first-trimester chorionic villi constitute critical regulators of placentation and hence of early human development. Our objective was therefore to isolate and characterize genes, both known and unknown, expressed by the human placenta during the first trimester.

STUDY DESIGN

Tissues collected consisted of placental material collected during first-trimester elective pregnancy terminations. Fetal chorionic villi were separated grossly from maternal decidual and quickly frozen in liquid nitrogen for ribonucleic acid preservation. Tissues from different gestational ages were kept separate. Total ribonucleic acid was extracted, messenger ribonucleic acid was isolated, and complementary deoxyribonucleic acid was synthesized. Complementary deoxyribonucleic acid was cloned into the ZAP Express lambda vector (Stratagene, La Jolla, Calif). Automated sequencing of random plaques was done. Sequence homology was searched for with the Basic Local Assignment Search Tool to search the Genbank database (National Center for Biotechnology Institute, Bethesda, Md). In the event that a known gene sequence was derived, no further workup was undertaken. If no homology was identified, the complete complementary deoxyribonucleic acid insert was sequenced in its entirety. Novel genes were further characterized by tissue-specific patterns, cellular localization, and chromosomal location. Expression by fetal villi was confirmed by reverse transcriptase polymerase chain reaction.

RESULTS

We isolated a number of genes known to be expressed at the maternal-fetal interface. Seventeen of 186 random clones were >1 kilobase in length and did not display homology with known genes, and these may therefore constitute novel genes critical for placentation. One of the clones from a human chorionic villi complementary deoxyribonucleic acid library at 12 weeks' gestation is a 7-kilobase gene that is also abundantly expressed in human fetal brain, lung, liver, and kidney. In situ hybridization localized the transcript to the fetal renal glomerulus.

CONCLUSIONS

Our findings indicate that the placenta serves as a rich source for potential novel gene expression. Seventeen clones were >1 kilobase in length and are potential novel genes involved in early first-trimester placentation. One of these 17 potential novel genes is expressed in abundance in several fetal tissues, which suggests a role in early human fetal development. Other potential novel genes are currently being characterized. The powerful molecular techniques that we used to isolate genes expressed by early fetal chorionic villi will lead us to a better understanding of the genetic control of normal human reproduction. They also may be used to study obstetric and other human disease.

A promoter mutation in the tumor necrosis factor alpha gene is not associated with preeclampsia

Preeclampsia is associated with increased plasma concentrations of tumor necrosis factor alpha (TNF alpha) and TNF receptors. A mutation in the promoter region of the TNF alpha gene (TNF T2) has been described which is associated with increased transcription of the gene. Due to the familial predisposition of preeclampsia, we hypothesized that this promoter mutation in the TNF alpha gene may contribute to the genetic etiology of preeclampsia. Our objective was to determine the allele frequency of this mutation in a population with well-characterized preeclampsia and with hemolysis, elevated liver enzymes, low platelets (HELLP) syndrome as compared with normotensive controls. DNA was extracted from blood of 131 women with severe preeclampsia, 75 women with HELLP syndrome, and 41 normotensive gravid controls. Genotypes were determined using the polymerase chain reaction (PCR), allele-specific restriction with Nco1, and agarose gel electrophoresis. Results were analyzed with a chi 2 contingency table. No significant differences were found between patients with severe preeclampsia, HELLP syndrome, normotensive gravid controls, and previously published allele frequencies. The frequency of the TNF T2 allele is not increased in patients with preeclampsia or HELLP syndrome. Therefore, this promoter mutation is probably not a major genetic cause of preeclampsia. As more genes are cloned, sequenced and localized, this will enable investigators to take this 'candidate gene approach' to investigate potential genetic causes of preeclampsia.

A prospective evaluation of fetal pericardial fluid in 506 second-trimester low-risk pregnancies

OBJECTIVE

To measure fetal pericardial fluid in low-risk second-trimester pregnancies and to evaluate outcome for those with measurements greater than 2 mm.

METHODS

Five hundred and six women were referred for sonography between 16 and 25 weeks' gestation for common obstetric indications (dating, fetal survey, and placental location) unrelated to an increased risk of anomalies. All cases were evaluated with two-dimensional and M-mode real-time ultrasonography with the use of a mechanical sector transducer. The maximum distance of the fetal hypoechoic cardiac rim was recorded. We reviewed maternal and infant charts for those with measurements greater than 2 mm.

RESULTS

Median (range) maternal age was 25 (15-42) years. Median gravidity and parity were two (1-14) and one (0-11), respectively. Median estimated gestational age was 20.4 (16.3-24.9) weeks. Fetal pericardial fluid was seen in 360 of 506 (71%) fetuses. Of these 360 fetuses, the mean distance (+/- 2 standard deviation) of the fetal hypoechoic cardiac rim was 1.20 mm +/- 0.91 mm (95% confidence interval 1.15, 1.25). Among the 506 cases, the maximum measurement was 3 mm. Ten of the 506 (2%) cases had measurements greater than 2 mm. None of these ten fetuses had a cardiac structural abnormality or arrhythmia, and perinatal outcome was unremarkable.

CONCLUSION

During second-trimester fetal ultrasonographic examination, visualization of pericardial fluid up to 2 mm in the fetus with current high-resolution technology is common and should not be regarded as pathologic.

A promoter mutation that increases transcription of the tumor necrosis factor-alpha gene is not associated with preterm delivery

OBJECTIVE

Increased amniotic fluid concentrations of tumor necrosis factor-alpha are observed in women with preterm labor and subsequent preterm birth. We tested whether a mutation in the promoter region of tumor necrosis factor-alpha gene, TNF T2, which increases transcription of the gene, is more frequent in a preterm delivery cohort.

STUDY DESIGN

Deoxyribonucleic acid was extracted from whole blood of 203 women and 44 fetuses delivered at < 37 weeks of estimated gestational age. The polymerase chain reaction was used to amplify the promoter region of the tumor necrosis factor-alpha gene. The resulting polymerase chain product was subjected to allele-specific enzymatic digestion with Nco I. Fragments were size fractionated on a 3% Metaphor agarose gel stained with ethidium bromide. Results were analyzed with use of a chi 2 contingency table.

RESULTS

No statistically significant differences for either the TNF T1 or TNF T2 allele frequencies were found between women or fetuses delivered preterm compared with a control group or previously published allele frequencies.

CONCLUSIONS

The frequency of this tumor necrosis factor-alpha promoter mutation, TNF T2, is not increased in either women or fetuses delivered at < 37 weeks' gestation. Basal levels of tumor necrosis factor-alpha are unlikely to affect a woman's risk of preterm delivery. Tumor necrosis factor-alpha variants should not be used as a predictive test for preterm delivery.

The factor V Leiden mutation is not a common cause of recurrent miscarriage

Some investigators suggest that placental thrombosis and infarction can cause recurrent miscarriage. We have shown that the common missense mutation in the factor V gene, the Leiden mutation, which renders factor Va resistant to cleavage inactivation by activated protein C, predisposes to placental thrombosis and spontaneous miscarriage. Our objective was to determine the frequency of the Leiden mutation in a population with well-characterized idiopathic recurrent miscarriage. DNA was extracted from whole blood of 40 couples with a history of idiopathic recurrent miscarriage and 25 couples with a history of proven fertility (seven or more live births). The polymerase chain reaction was used to amplify exon 10 of the factor V gene followed by allele-specific restriction with Mnl1 for mutation detection. Results were analyzed with a chi 2 contingency table. None of the 40 women with idiopathic recurrent miscarriage carried the mutation and only one of their reproductive partners was heterozygous for the mutation. Similarly, none of the control women carried the mutation, and only one of the 25 control male partners was heterozygous for the mutation. In our referral population, the factor V Leiden mutation which predisposes to thrombosis is not a common cause of recurrent miscarriage.

Fetal carriers of the factor V Leiden mutation are prone to miscarriage and placental infarction

OBJECTIVES

The factor V Leiden mutation is the most common genetic predisposition to thrombosis. However, little is known concerning the reproductive outcome of mutation carriers or prenatal expressivity of this thrombogenic mutation. Our purpose was to examine whether this mutation presents phenotypically as miscarriage or idiopathic placental thrombosis.

STUDY DESIGN

We performed two studies. First, a case-control comparison to determine whether fetal or maternal carriers of the factor V Leiden mutation are at risk for spontaneous miscarriage was performed, and, second, a cohort study evaluating placental infarction in fetuses carrying this mutation was performed.

RESULTS

We found a twofold increase in the factor V Leiden carrier frequency in 12 of 139 (8.6%) abortuses compared with 17 of 403 (4.2%) unselected pregnant women seen in the labor and delivery suite and, even more remarkable, a tenfold increase in the fetal carrier frequency in 10 of 24 (42%) placentas with > 10% placental infarction compared with 7 of 372 (1.9%) placentas with < 10% placental infarction.

CONCLUSIONS

These findings suggest a prenatal phenotype and effects of this mutation at the fetoplacental interface. If large prospective studies confirm these findings, then testing for this thrombogenic mutation should be considered in women and placental tissue from spontaneous abortuses and placentas with evidence of placental infarction. In addition to identifying individuals and families at risk for thrombosis, this information may help to improve our understanding of hemostasis and circulatory disturbances at the fetoplacental interface.

The incidence of the factor V Leiden mutation in an obstetric population and its relationship to deep vein thrombosis

OBJECTIVE

A common missense mutation in the factor V gene, the Leiden mutation, renders factor Va resistant to cleavage inactivation by activated protein C and predisposes patients to thrombotic events. We sought to evaluate the prevalence of the Leiden mutation and the associated thromboembolic events in a community hospital's low-risk obstetric population.

STUDY DESIGN

Deoxyribonucleic acid was extracted from whole blood of 407 women. The polymerase chain reaction was used to amplify exon 10 of the factor V gene, followed by enzymatic digestion with MnI 1 for mutation detection. Medical charts were reviewed and patient characteristics, including age, gravidity, parity, obstetric complications, medical complications, and mode of delivery, were recorded.

RESULTS

Fourteen of the 407 women carried the factor V Leiden mutation (13 heterozygotes and 1 homozygous mutant) for an allele frequency of 3%, consistent with the published carrier rate. Four of the 14 carriers (28%) had deep venous thrombosis, whereas the frequency of deep venous thrombosis in this obstetric population was <1%. Another patient carrying the mutation had a consumptive coagulopathy of unknown etiology at 20 weeks' gestation, necessitating delivery.

CONCLUSIONS

The Leiden mutation is relatively common in the general obstetric population. The high rate of deep venous thrombosis noted in our series suggests the need for genetic testing for this mutation in women with a thrombotic event during pregnancy.

The factor V Leiden mutation may predispose women to severe preeclampsia

OBJECTIVE

A recent study showed that resistance to activated protein C may underlie some cases of severe preeclampsia. A common missense mutation in the factor V gene, the Leiden mutation, is the most frequent genetic cause of resistance to activated protein C. Our objective was to determine whether this mutation is more prevalent in patients with severe preeclampsia than in normotensive controls.

STUDY DESIGN

Deoxyribonucleic acid was extracted from whole blood of 158 gravid women meeting criteria of The American College of Obstetricians and Gynecologists for severe preeclampsia and 403 normotensive gravid women. The polymerase chain reaction was used to amplify exon 10 of the factor V gene, followed by allele-specific restriction with Mnl 1 for mutation detection. Results were analyzed with a chi(2) contingency table.

RESULTS

No patients were homozygous for the Leiden mutation. Fourteen of 158 women with severe preeclampsia (8.9%) were heterozygous for the Leiden mutation compared with 17 of 403 normotensive gravid controls (4.2%). The difference in frequency between women with severe preeclampsia and normotensive controls was statistically significant, chi(2) 4.686, p = 0.03.

CONCLUSIONS

Our data suggest that carriers of the factor V Leiden mutation are at increased risk for severe preeclampsia. Deoxyribonucleic acid analysis for the factor V Leiden mutation could serve as one component of a genetic screening profile for preeclampsia and other adverse pregnancy outcomes. Women who carry this mutation are at increased risk for deep venous thrombosis. Carriers of this common thrombophilic mutation may be identified so that adequate counseling regarding future contraceptive usage and effective thromboembolic prophylaxis during pregnancy and surgical procedures may be offered.