Invasive Trophoblast Antigen (Hyperglycosylated Human Chorionic Gonadotropin) in Second-Trimester Maternal Urine as a Marker for Down Syndrome: Preliminary Results of an Observational Study on Fresh Samples

Urine tests for Down's syndrome screening

BACKGROUND

Down's syndrome occurs when a person has three copies of chromosome 21, or the specific area of chromosome 21 implicated in causing Down's syndrome, rather than two. It is the commonest congenital cause of mental disability and also leads to numerous metabolic and structural problems. It can be life-threatening, or lead to considerable ill health, although some individuals have only mild problems and can lead relatively normal lives. Having a baby with Down's syndrome is likely to have a significant impact on family life. The risk of a Down's syndrome affected pregnancy increases with advancing maternal age.Noninvasive screening based on biochemical analysis of maternal serum or urine, or fetal ultrasound measurements, allows estimates of the risk of a pregnancy being affected and provides information to guide decisions about definitive testing. Before agreeing to screening tests, parents need to be fully informed about the risks, benefits and possible consequences of such a test. This includes subsequent choices for further tests they may face, and the implications of both false positive and false negative screening tests (i.e. invasive diagnostic testing, and the possibility that a miscarried fetus may be chromosomally normal). The decisions that may be faced by expectant parents inevitably engender a high level of anxiety at all stages of the screening process, and the outcomes of screening can be associated with considerable physical and psychological morbidity. No screening test can predict the severity of problems a person with Down's syndrome will have.

OBJECTIVES

To estimate and compare the accuracy of first and second trimester urine markers for the detection of Down's syndrome.

SEARCH METHODS

We carried out a sensitive and comprehensive literature search of MEDLINE (1980 to 25 August 2011), EMBASE (1980 to 25 August 2011), BIOSIS via EDINA (1985 to 25 August 2011), CINAHL via OVID (1982 to 25 August 2011), The Database of Abstracts of Reviews of Effectiveness (The Cochrane Library 2011, Issue 7), MEDION (25 August 2011), The Database of Systematic Reviews and Meta-Analyses in Laboratory Medicine (25 August 2011), The National Research Register (archived 2007), Health Services Research Projects in Progress database (25 August 2011). We studied reference lists and published review articles.

SELECTION CRITERIA

Studies evaluating tests of maternal urine in women up to 24 weeks of gestation for Down's syndrome, compared with a reference standard, either chromosomal verification or macroscopic postnatal inspection.

DATA COLLECTION AND ANALYSIS

We extracted data as test positive or test negative results for Down's and non-Down's pregnancies allowing estimation of detection rates (sensitivity) and false positive rates (1-specificity). We performed quality assessment according to QUADAS (Quality Assessment of Diagnostic Accuracy Studies) criteria. We used hierarchical summary ROC (receiver operating characteristic) meta-analytical methods to analyse test performance and compare test accuracy. We performed analysis of studies allowing direct comparison between tests. We investigated the impact of maternal age on test performance in subgroup analyses.

MAIN RESULTS

We included 19 studies involving 18,013 pregnancies (including 527 with Down's syndrome). Studies were generally of high quality, although differential verification was common with invasive testing of only high-risk pregnancies. Twenty-four test combinations were evaluated formed from combinations of the following seven different markers with and without maternal age: AFP (alpha-fetoprotein), ITA (invasive trophoblast antigen), ß-core fragment, free ßhCG (beta human chorionic gonadotrophin), total hCG, oestriol, gonadotropin peptide and various marker ratios. The strategies evaluated included three double tests and seven single tests in combination with maternal age, and one triple test, two double tests and 11 single tests without maternal age. Twelve of the 19 studies only evaluated the performance of a single test strategy while the remaining seven evaluated at least two test strategies. Two marker combinations were evaluated in more than four studies; second trimester ß-core fragment (six studies), and second trimester ß-core fragment with maternal age (five studies).In direct test comparisons, for a 5% false positive rate (FPR), the diagnostic accuracy of the double marker second trimester ß-core fragment and oestriol with maternal age test combination was significantly better (ratio of diagnostic odds ratio (RDOR): 2.2 (95% confidence interval (CI) 1.1 to 4.5), P = 0.02) (summary sensitivity of 73% (CI 57 to 85) at a cut-point of 5% FPR) than that of the single marker test strategy of second trimester ß-core fragment and maternal age (summary sensitivity of 56% (CI 45 to 66) at a cut-point of 5% FPR), but was not significantly better (RDOR: 1.5 (0.8 to 2.8), P = 0.21) than that of the second trimester ß-core fragment to oestriol ratio and maternal age test strategy (summary sensitivity of 71% (CI 51 to 86) at a cut-point of 5% FPR).

AUTHORS' CONCLUSIONS

Tests involving second trimester ß-core fragment and oestriol with maternal age are significantly more sensitive than the single marker second trimester ß-core fragment and maternal age, however, there were few studies. There is a paucity of evidence available to support the use of urine testing for Down's syndrome screening in clinical practice where alternatives are available.

A model for calculating the within-subject biological variation and likelihood ratios for analytes with a time-dependent change in concentrations; exemplified with the use of D-dimer in suspected venous thromboembolism in healthy pregnant women

BACKGROUND

Within-subject biological variation and reference change value (RCV) are difficult to calculate for an analyte with a changing concentration. The aim of this study was to develop a model to examine if it was possible to transform an analyte with a time-dependent change in concentration into a 'steady-state' situation by the use of 'multiples of the median' (MoM) and its natural logarithm (lnMoM). In addition, we wanted to extend the RCV concept, using likelihood and odds ratios, to calculate the post-test probabilities for disease. D-dimer in pregnancy is used as an example.

METHODS

Blood samples from 18 healthy pregnant and 18 healthy non-pregnant women were collected every fourth week. MoM of the D-dimer concentrations was calculated for each four-week interval to obtain a 'steady-state' situation for the D-dimer concentrations. The 'normalized' values were then transformed to the lnMoM to obtain a Gaussian distribution, used for the estimation of biological variation.

RESULTS

Median D-dimer concentrations increased six-fold during pregnancy. Within-subject variation (SD) of lnMoM D-dimer was 0.27 during pregnancy and 0.23 in non-pregnant women, with RCVs of 0.72 and 0.90, respectively.

CONCLUSIONS

By using the lnMoM model, an increasing concentration of an analyte can be transformed to a steady-state situation and the within-subject biological variation and its derived parameters can be calculated.

hCG, five independent molecules

INTRODUCTION

The hCG amino acid sequence supports 5 glycoproteins. All are called hCG forms. This review examines all 5 molecules, the hormone as produced by the placental syncytiotrophoblast cells, the sulfated hormone produced by the pituitary gonadotrope cells, the hyperglycosylated hCG autocrine made by placental cytotrophoblast cells, and the autocrine cancer promoters hyperglycosylated hCG, hCGß and hyperglycosylated hCGß as made by all malignancies. This review examines all the molecules and multiple proven functions, ranging from evolution to cancer promotion to hormone action.

RESULTS AND DISCUSSION

hCG forms are critical super-growth factors in humans, with an exceptional wide range of functions.

Human chorionic gonadotropin and associated molecules

When considering human chorionic gonadotropin (hCG) and hCG tests, it is important to realize that it is not a single biological molecule. The regular form of hCG produced by differentiated syncytotrophoblast cells (regular hCG) is a hormone made with the primary function of maintaining the myometrial and decidual spiral arteries and the vascular supply of the placenta during the full course of pregnancy. Hyperglycosylated hCG (hCG with double-size O-linked oligosaccharides) is made by undifferentiated cytotrophoblast cells. This is an autocrine hormone with separate functions, it maintains invasion as in implantation of pregnancy and malignancy in gestational trophoblastic diseases. A hyperglycosylated free beta-subunit is produced by a high proportion of all malignancies. This functions as an autocrine hormone to promote the growth and invasion of the malignancy. It is important to realize when ordering an hCG test what you are measuring and whether the test ordered will detect appropriately these three variant of hCG as well as their degradation products. Most automated commercial laboratory tests, point-of-care test and over-the-counter tests are limited in what is detected, focusing only on regular hCG. This is in part due to the US FDA, who only consider hCG as a pregnancy test, and to whom only detection of regular hCG is critical. This may be a cause of test errors since primarily hyperglycosylated hCG is produced in early pregnancy, choriocarcinoma and germ cell testicular malignancies, and only free beta-subunit may be produced in other germ cell malignancies (all applications for hCG test). The exceptions are the older style hCGbeta radioimmunoassay and the Siemen's Immulite platform hCG test which detect all variant and their degradation product appropriately. Regardless of test specificity limitations, assays for hCG variants are widely used clinically in pregnancy detection, early pregnancy detection, prediction of spontaneously aborting and ectopic pregnancies and prediction of trisomy pregnancies. hCG tests are essential in managing gestational trophoblastic diseases, whether hydatidiform mole, invasive mole or choriocarcinoma, and are very useful in management of testicular malignancies and other germ cell malignancies.

New discoveries on the biology and detection of human chorionic gonadotropin

Human chorionic gonadotropin (hCG) is a glycoprotein hormone comprising 2 subunits, alpha and beta joined non covalently. While similar in structure to luteinizing hormone (LH), hCG exists in multiple hormonal and non-endocrine agents, rather than as a single molecule like LH and the other glycoprotein hormones. These are regular hCG, hyperglycosylated hCG and the free beta-subunit of hyperglycosylated hCG. For 88 years regular hCG has been known as a promoter of corpus luteal progesterone production, even though this function only explains 3 weeks of a full gestations production of regular hCG. Research in recent years has explained the full gestational production by demonstration of critical functions in trophoblast differentiation and in fetal nutrition through myometrial spiral artery angiogenesis. While regular hCG is made by fused villous syncytiotrophoblast cells, extravillous invasive cytotrophoblast cells make the variant hyperglycosylated hCG. This variant is an autocrine factor, acting on extravillous invasive cytotrophoblast cells to initiate and control invasion as occurs at implantation of pregnancy and the establishment of hemochorial placentation, and malignancy as occurs in invasive hydatidiform mole and choriocarcinoma. Hyperglycosylated hCG inhibits apoptosis in extravillous invasive cytotrophoblast cells promoting cell invasion, growth and malignancy. Other non-trophoblastic malignancies retro-differentiate and produce a hyperglycosylated free beta-subunit of hCG (hCG free beta). This has been shown to be an autocrine factor antagonizing apoptosis furthering cancer cell growth and malignancy. New applications have been demonstrated for total hCG measurements and detection of the 3 hCG variants in pregnancy detection, monitoring pregnancy outcome, determining risk for Down syndrome fetus, predicting preeclampsia, detecting pituitary hCG, detecting and managing gestational trophoblastic diseases, diagnosing quiescent gestational trophoblastic disease, diagnosing placental site trophoblastic tumor, managing testicular germ cell malignancies, and monitoring other human malignancies. There are very few molecules with such wide and varying functions as regular hCG and its variants, and very few tests with such a wide spectrum of clinical applications as total hCG.

Human trophoblast function during the implantation process

The implantation process involves complex and synchronized molecular and cellular events between the uterus and the implanting embryo. These events are regulated by paracrine and autocrine factors. Trophoblast invasion and migration through the uterine wall is mediated by molecular and cellular interactions, controlled by the trophoblast and the maternal microenvironment. This review is focused on the molecular constituents of the human trophoblast, their actions and interactions, including interrelations with the uterine endometrium.

Maternal Serum Invasive Trophoblast Antigen and First-Trimester Down Syndrome Screening

Background: In the United States, Down syndrome screening is still performed mainly in the second trimester, using 3 or 4 markers. Moving screening into the first trimester has the advantage of earlier diagnosis. Currently, first-trimester screening typically includes maternal serum pregnancy-associated plasma protein-A (PAPP-A), the free β-subunit of human chorionic gonadotropin (free β), and ultrasound measurement of nuchal translucency thickness (NT). The current report describes a case–control study of serum invasive trophoblast antigen (ITA) and its possible inclusion in first-trimester screening for Down syndrome.

Methods: As part of an earlier observational study, serum samples from 54 Down syndrome and 276 matched unaffected pregnancies were collected between 9 and 15 weeks of gestation. Samples had been aliquoted and stored at −20 °C for 8 years. ITA was measured and converted to weight-adjusted multiples of the median (MoM). The distributions of other first-trimester markers are from a single published study.

Results: Median ITA MoM in Down syndrome pregnancies increase as gestational age increases (2.02 MoM at 11 and 2.44 MoM at 13 completed weeks). At 75% detection, maternal age in combination with ITA and PAPP-A measurements have an 8.0% false-positive rate, slightly lower than the 8.8% found for the free β and PAPP-A combination; adding NT measurements reduces false positives for the 2 combinations to 2.0% and 1.8%, respectively.

Conclusion: Serum ITA appears to be a useful first-trimester Down syndrome marker that could replace free β measurements while maintaining performance.

Second trimester serum markers

Prenatal screening for Down syndrome in the early second trimester with multiple maternal serum markers has been available for more than 15 years. The multiple marker combination with the highest screening performance currently available is alpha-fetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and inhibin A, together with maternal age (so-called quad marker test). With this combination, a detection rate of 80% at a 5% false positive rate is achieved. Inhibin A, the newest addition to second trimester serum screening, is an alpha-beta subunit hormone of placental origin, and is measured using a monoclonal two-site ELISA validated for use in prenatal screening. Quality control parameters for inhibin A measurement are acceptable and are monitored through the proficiency testing program administered by the College of American Pathologists. Research into other possible second trimester screening markers has included studies on the maternal urine and serum levels of an hCG variant, hyperglosylated hCG (h-hCG; invasive trophoblast antigen). Recent data indicate that h-hCG is similar to hCG itself, although its measurement in maternal urine may improve the performance of the established serum marker combinations. With the introduction of first trimester screening markers and their use in an integrated first and second trimester marker approach to screening, and with the fact that many women do not seek prenatal care until the early second trimester, prenatal screening for Down syndrome using second trimester serum markers remains a major resource in obstetrical care.

Specific Measurement of O-Linked Core 2 Sugar-Containing Isoforms of Hyperglycosylated Human Chorionic Gonadotropin by Antibody B152

There have been a significant number of reports on the clinical utility of measurement of 'hyperglycosylated' isoforms of the pregnancy hormone, human chorionic gonadotropin (hCG). Although there are a variety of hCG isoforms which can be termed 'hyperglycosylated', the measurements were all made using a unique antibody designated B152. This antibody was raised using a choriocarcinoma-derived form of hCG, which was hyperglycosylated with N- and O-glycans and was also 100% 'nicked' hCG. Antibody B152 was recently mapped to a linear epitope around a single O-glycan on the beta-subunit of hCG at residue number 132. Thus, the antibody can only measure isoforms of hCG that possess a core 2 type of branched O-glycan on this portion of the hCG beta-subunit. Isoforms that are hyperglycosylated in the hCG alpha-subunit or only on the N-glycans of hCGbeta will not be recognized by antibody B152. Apparently, measurements of these core 2 hCG isoforms have important clinical application in early pregnancy during which they are the predominant isoform of hCG until the 6th week of gestation. The secretory pattern of these isoforms can be used to predict the health status of the pregnancy in fertility clinics. Moreover, the measurements of these core 2 hCG isoforms are more useful than standard hCG for the prediction of Down syndrome pregnancies. The core 2 isoforms are also of important use in cancer diagnosis and monitoring since their concentration appears to correlate with malignancy.

Maternal Serum Invasive Trophoblast Antigen (Hyperglycosylated hCG) as a Screening Marker for Down Syndrome during the Second Trimester

Background: Approximately two million pregnancies in the United States are screened for Down syndrome annually by use of second-trimester maternal serum markers. At present, a combination of four markers can identify 75% of affected pregnancies when 5% of screened women are classified as candidates for amniocentesis. Although not currently included in screening panels, invasive trophoblast antigen (ITA) is a promising screening marker in serum or urine in both the second and first trimesters. This study aims at better defining the screening performance of serum ITA in the second trimester.

Methods: In an earlier study, serum samples from an unbiased sampling of 45 Down syndrome (cases) and 238 unaffected (control) pregnancies between 14 and 20 weeks of gestation were collected from various centers in the United States. Samples were aliquoted and stored at −20 °C for 8 years. We measured ITA in these samples and determined the screening performance both univariately and in combination with other screening markers.

Results: The median ITA in Down syndrome pregnancies was >3.00 multiples of the median, higher than that found for human chorionic gonadotropin (hCG) or free β-hCG. At a 5% false-positive rate, ITA univariately detected 38% and 40% of Down syndrome pregnancies, respectively, when assigned by date of last menstrual period or ultrasound date. Modeling yielded rates of 45% and 48%. ITA correlated strongly with hCG and free β-hCG. When substituted for either of these in a multiple marker panel, ITA performed comparably.

Conclusions: This study indicates that serum ITA is an effective marker for Down syndrome. It is highly correlated with both hCG and free β-hCG and could replace either of them in a multiple marker panel.