First trimester screening: results, risk calculation. Pregnancy-associated plasma protein A (PAPP-A) Papp a decreased causes

Description

Determination method Immunoassay.

Material under study Blood serum

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Pregnancy-associated plasma protein-A. In prenatal screening in the first trimester of pregnancy, it is a risk marker for Down syndrome and other chromosomal abnormalities of the fetus.

PAPP-A is a high molecular weight glycoprotein (m.v. about 800 kDa). During pregnancy, it is produced in large quantities by the trophoblast and enters the maternal circulation system; its concentration in the mother's blood serum increases with increasing gestational age. Based on its biochemical properties, PAPP-A is classified as a metalloprotease. It has the ability to break down one of the proteins that binds insulin-like growth factor. This causes an increase in the bioavailability of insulin-like growth factor, which is an important factor in fetal development during pregnancy. It is assumed that PAPP-A is also involved in the modulation of the maternal immune response during pregnancy. A similar protein is also present in low concentrations in the blood of men and non-pregnant women. The physiological role of PAPP-A continues to be investigated.

A number of serious clinical studies indicate the diagnostic significance of PAPP-A as a screening marker for the risk of fetal chromosomal abnormalities in the early stages of pregnancy (in the first trimester), which is fundamentally important in the diagnosis of chromosomal abnormalities. The level of PAPP-A is significantly reduced in the presence of trisomy 21 (Down syndrome) or trisomy 18 (Edwards syndrome) in the fetus. In addition, this test is also informative in assessing the threat of miscarriage and termination of pregnancy in the short term.

An isolated study of PAPP-A level as a risk marker for Down syndrome has diagnostic value starting from 8 - 9 weeks of pregnancy. In combination with the determination of beta-hCG (human chorionic gonadotropin), the determination of PAPP-A is optimally carried out at about 12 weeks of pregnancy (11 - 14 weeks). After 14 weeks of pregnancy, the diagnostic value of PAPP-A as a risk marker for Down syndrome is lost.

It has been established that the combination of this test with the determination of the free beta subunit of hCG (or total beta hCG), ultrasound data (nuchal translucency thickness), and assessment of age-related risk factors significantly increases the effectiveness of prenatal screening for Down syndrome in the first trimester of pregnancy, bringing it to 85 - 90% detection rate for Down syndrome with 5% false positive results. The study of PAPP-A as a biochemical marker of congenital and hereditary pathology in the fetus in combination with the determination of hCG at 11 - 13 weeks of pregnancy is currently included in the scheme of screening examinations of pregnant women by Order of the Moscow Department of Health No. 144 of April 4, 2005. first trimester.

Detection of deviations in the levels of biochemical markers in the mother’s blood is not an unconditional confirmation of fetal pathology, but, in combination with the assessment of other risk factors, is the basis for the use of more complex special methods for diagnosing fetal abnormalities.

Limits of detection: 0.03 mU/ml-100 mU/ml

Preparation

It is preferable to take blood in the morning on an empty stomach, after 8-14 hours of overnight fasting (you can drink water), or 4 hours after a light meal during the day.

On the eve of the study, it is necessary to exclude increased psycho-emotional and physical stress (sports training), alcohol intake, and smoking an hour before the study.

Indications for use

• Screening examination of pregnant women to assess the risk of chromosomal abnormalities of the fetus in the 1st and early 2nd trimesters of pregnancy (11 - 13 weeks).
• History of severe complications of pregnancy (in order to assess the threat of miscarriage and stop the development of pregnancy in the short term).
• The woman is over 35 years old.
• The presence of two or more spontaneous abortions in the early stages of pregnancy.
• Bacterial and viral infections (hepatitis, rubella, herpes, cytomegalovirus) suffered during the period preceding pregnancy.
• The presence in the family of a child (or a history of a fetus of an terminated pregnancy) with Down's disease, other chromosomal diseases, or congenital malformations.
• Hereditary diseases in close relatives.
• Radiation exposure or other harmful effects on one of the spouses before conception.

General information about the study

Pregnancy-associated plasma protein A (PAPP-A) is a zinc-containing enzyme (metalloproteinase). During pregnancy, it is produced in large quantities by fibroblasts in the outer layer of the placenta and decidua and is found in the maternal bloodstream as a high molecular weight protein fraction.

The PAPP-A enzyme cleaves protein fragments from the insulin-like growth factor and increases its biological activity, thereby ensuring the full growth and development of the placenta. In addition, it is able to inactivate some enzymes in the blood (trypsin, elastase, plasmin) and modulate the immune response of the maternal body. Its content in the blood increases with the progression of pregnancy. It does not significantly depend on parameters such as the sex and weight of the child. Only during the period of intensive formation of the placenta (7-14th week of pregnancy) is there a strong relationship between the level of PAPP-A and the concentration of estradiol. After childbirth, PAPP-A declines rapidly over several days.

With a chromosomal abnormality with fetal malformations, the concentration of PAPP-A in the blood decreases significantly from the 8th to the 14th week of pregnancy. The most dramatic decrease is observed with trisomies on the 21st, 18th and 13th chromosomes. In Down syndrome, the PAPP-A indicator is an order of magnitude lower than in the norm. The level of PAPP-A in the mother's blood serum drops even more sharply if the fetus has a genetic pathology with multiple malformations - Cornelia de Lange syndrome.

The test is prescribed in combination with the determination of the beta subunit of human chorionic gonadotropin and an ultrasound examination of the thickness of the nuchal translucency. This comprehensive examination is recommended for screening for Down syndrome and other chromosomal abnormalities of the fetus in the first trimester of pregnancy (10-13 weeks). A separate determination of PAPP-A is most informative at 8-9 weeks. After 14 weeks of pregnancy, the value of this indicator as a marker of the risk of chromosomal abnormalities is lost, since the level corresponds to the norm even with pathology.

Based on the results of this test, a decision is made on the advisability of prescribing additional methods of examining the fetus. However, the PAPP-A level cannot serve as a criterion for diagnosis. In normal pregnancies, the test result can be false positive in 5%, and fetal chromosomal abnormalities are detected in only 2-3% of pregnant women with reduced PAPP-A levels. In the United States, using this test in the first trimester of pregnancy, about 85% of cases of Down syndrome and 95% of Edwards syndrome are detected. If the result is positive, additional examinations are necessary, including chorionic villus puncture, amniocentesis with genetic testing of the material obtained.

PAPP-A protein can be found in minimal amounts in men and non-pregnant women. An increase in PAPP-A is recorded after damage to atherosclerotic plaques in acute coronary syndrome and unstable angina. This protein is being actively studied as a marker for the prognosis of coronary heart disease, but has not yet received widespread use in cardiac laboratory tests.

What is the research used for?

• To screen for possible chromosomal abnormalities in the fetus.
• To assess the threat of premature termination of pregnancy or miscarriage, to predict the course of pregnancy.

When is the study scheduled?

When examining pregnant women in the first trimester (analysis is recommended at 10-13 weeks of pregnancy), especially in the presence of risk factors for the development of pathology:

• age over 35 years;
• miscarriage and severe complications of pregnancy in the past;
• chromosomal pathologies, Down's disease or congenital malformations in previous pregnancies;
• hereditary diseases in the family;
• past infections, radiation exposure, taking medications in the early stages of pregnancy or shortly before it that have a teratogenic effect (can cause congenital defects and fetal anomalies).

Historical background and description of RARR-A

The human placenta is the source of a wide variety of specific proteins that are not found at all or in small quantities in normal serum. During pregnancy, they can be found in the maternal circulatory system. Such proteins include both hormones (human chorionic gonadotropin, human placental lactogen) and other proteins of placental origin. One of them is (pregnancy-associated plasma protein A, PAPP-A).

In 1974, Lin et al. isolated a group of proteins from retroplacental blood serum called pregnancy-associated proteins A, B, C, and D. PAPP-A is produced by the placenta, and its secretion increases with increasing gestational age. PAPP-A is only detected in the maternal circulatory system.

Recently, PAPP-A has attracted interest as a promising marker for a number of pathological conditions that occur during pregnancy, such as threatened premature termination of pregnancy and ectopic pregnancy. PAPP-A has been found to be the most specific of the earliest biochemical markers for trisomy 21, Down syndrome. In addition, recent data indicate the possibility of using PAPP-A in cardiology for the diagnosis of pathological conditions such as unstable angina.

Structure of RAPP-A

The active form of PAPP-A, which has proteolytic activity, is a homodimer weighing ~400 kDa. In blood plasma, only less than 1% of the total amount of PAPP-A is a homodimer and exhibits activity. The rest, most of PAPP-A in the bloodstream is found in the form of an inactive heterotetrameric complex with the proform of eosinophil major basic protein (proMBP). The complex consists of two PAPP-A molecules and two proMBP molecules and has a mass of ~500 kDa (Fig. 1). At the same time, PAPP-A as part of the complex does not exhibit proteolytic activity. The PAPP-A molecule consists of two subunits with a molecular weight of about 200 kDa each and is secreted into the blood by trophoblast cells in the form of a dimer. Dimerization of PAPP-A subunits occurs through the formation of a disulfide bond at Cys-1130, and the proMBP subunit occurs through two disulfide bonds. There are also two disulfide bonds between each PAPP-A and proMBP subunits.

Rice. 1. Schematic representation of the heterotetrameric PAPP-A/proMBP complex and the homodimeric form of PAPP-A.

The PAPP-A and proMBP subunits are highly glycosylated, and the total carbohydrate content is 13.4% and 38.6% of the total weight, respectively, and the total carbohydrate content is 17.4%. The carbohydrate components of both proteins are very different. PAPP-A contains carbohydrate components linked to the peptide by an N-glycosidic bond. ProMBP contains components linked to the peptide by both O- and N-glycosidic bonds.

Each subunit contains 1547 amino acid residues and is formed from a larger precursor. The PAPP-A amino acid sequence contains several repeats. Firstly, these are the so-called lin-notch repeats 1-3 (lin-notch repeats, LNR1-3), which regulate early tissue differentiation, 26-27 aa long, two of which are located near the active center, and the third is near the C-terminus of the polypeptide. Secondly, these are short consensus repeats 1-5 (SCR1-5) with a length of 57-77 aa. each following each other in the C-terminal region of the amino acid sequence PAPP-A. The active site includes the Glu483 residue and the nearby extended zinc-binding motif HEXXHXXGXXH (residues 482 - 492), as well as the highly conserved Met556 residue. The active site lies in the gap located between the two halves of the catalytic domain. The structure diagram of PAPP-A is shown in Figure 2.

Rice. 2. Scheme of the structure of the PAPP-A polypeptide chain.

The PAPP-A dimer found in blood plasma actively binds to the cell surface. PAPP-A adhesion occurs due to the non-covalent interaction of amino acid residues located in the SCR-3 and SCR-4 repeats with heparin and heparan sulfate exposed on the cell surface. When the PAPP-A dimer binds to the cell surface, the enzyme does not lose its proteolytic activity.

At the same time, the complex of PAPP-A and proMBP does not exhibit the ability to cell adhesion. It is believed that the heparan sulfate of the proMBP molecule competes with cell surface polysaccharides for binding to the SCR-3 and SCR-4 sites on the PAPP-A molecule. As a result, PAPP-A, which is in complex with proMBP, does not have free SCR-3 and −4 sites and cannot be retained at the cell surface.

Clinical use

The level of PAPP-A in the blood is normal and pathological

Low persistent levels of PAPP-A proteinase expression have been detected using mRNA hybridization techniques in many tissue types (both reproductive and non-reproductive), including kidney, colon and bone marrow cells, with PAPP-A levels in plasma blood does not depend on gender and age. A significant increase in the level of PAPP-A in the blood plasma is observed in women during pregnancy: by the end of the sixth month it reaches 50 mg/l.

Currently, a screening system - a short but quite informative examination - is being quite successfully introduced into medical practice. It usually includes a small list of diagnostic procedures, after which one can suspect or deny a person’s pathology. Moreover, such studies immediately form groups of people where activities for additional diagnostic search are required.

This method has proven itself most widely in obstetrics - there now exists a whole system called prenatal screening (that is, carried out before the birth of the child). The initial examination is performed in the first trimester of pregnancy - at about 12 weeks. Ultrasound examination of the fetus at this time provides little information, revealing only gross malformations. Therefore, biochemical tests with good sensitivity come to the rescue.

Now for diagnosis, two indicators are determined in the mother’s blood - PAPP-A and hCG (human chorionic gonadotropin). The first of them also has a rather complex name - pregnancy-associated plasma protein A. Their level directly depends on the correct development of the membranes, as well as the normal formation of the embryo itself. Therefore, a change in these indicators above or below normal may be a sign of serious pathology.

Early prenatal screening

Under this concept, women usually imagine performing only an ultrasound examination, which is remembered due to the visualization effect. The fact that blood was taken from a vein seems to be an insignificant event. Namely, a biochemical analysis at about 12 weeks provides basic information about the normal course of pregnancy. Modern early screening is comprehensive and includes three components:

1. Ultrasound only “superficially” concerns the examination of the embryo itself - only the coccygeal-parietal size is assessed, as well as the ratio of the head and torso. More attention is paid to its location in the uterine cavity - it is important to exclude a frozen or regressing pregnancy, or its ectopic nature.
2. HCG assessment already reveals the normal processes of growth and development from the inside. Human chorionic gonadotropin has a hormone-like effect, ensuring the proper formation of the membranes and placenta. Therefore, changes in its level are characteristic of many pathological conditions and diseases that affect the maternal or child’s body.
3. The PAPP-A analysis is already more specific - normally, the blood content of this biologically active substance at 12 weeks is within normal limits. If there is a decrease in it, then there is a high probability of chromosomal abnormalities in the fetus (most often Down syndrome).

The results of prenatal screening are assessed only by a doctor, who, based on them, makes a decision on the need for additional diagnostics.

RARR-A

Determining the level of this substance in the blood began to be carried out relatively recently - the test itself has become an excellent addition to the analysis for hCG. This was due to the high sensitivity of the latter method - too many diseases and conditions cause changes in its indicators. Therefore, a simple and quick test was developed, albeit with a strange and complex name:

• The method is based on determining the level of pregnancy-associated plasma protein A in the blood. In a woman’s body, this substance is responsible for regulating the work of insulin-like growth factors, which affects growth processes in the body.


• As a result of research, a connection has been proven between a decrease in PAPP-A levels in pregnant women and chromosomal abnormalities in the fetus. Moreover, the greatest sensitivity and specificity was observed in the case of changes in the number of chromosomes. Therefore, the test began to be used primarily for the early detection of Down syndrome.
• But the indicators have diagnostic value only in the first trimester - from 8 to 13 weeks. Therefore, this biochemical analysis needs to be combined with early prenatal screening.
• Carrying out the technique in the second trimester no longer gives informative and accurate results.

The standard units of measurement for this analysis are mU/ml, although the modern integral index (MoI) is more often used to evaluate the results.

Grade

A quantitative change in the level of PAPP-A only in a certain case is a cause for serious concern. Moreover, a pronounced decrease in the indicator, combined with increased hCG numbers, has diagnostic significance:

• During pregnancy, PAPP-A normally reaches its maximum values ​​by the 12th week. At this time, its content in venous blood can vary from 0.7 to 6 mIU/ml. Moreover, the large scatter of values ​​attracts attention - this is why simultaneous assessment of the test for hCG levels is important.


• An increase in the level of protein A is quite nonspecific and is often observed during ongoing regeneration processes. Therefore, increased analysis numbers may occur after a variety of soft tissue injuries.
• If the indicators are reduced, it is recommended to interpret them using MoM - a special coefficient. It allows you to calculate the relative value of PAPP-A taking into account various risk factors. If a deviation is noted there, then the woman requires additional examination.

An established low level of protein serves as an indication for invasive diagnostics - taking a fragment of fetal membranes, amniotic fluid or umbilical cord blood for analysis.

hCG

Human chorionic gonadotropin is a fairly sensitive, but almost nonspecific indicator. This means that changes in its level are observed in a number of diseases, making it impossible to accurately indicate one of them. Therefore, its normal level accurately indicates only one thing - the physiological course of pregnancy:

1. This biologically active substance begins to be released into the blood shortly after fertilization of the egg. Already after approximately 6 days it is possible to determine trace amounts using specific tests.


2. The substance has an effect similar to sex hormones, additionally stimulating the growth of membranes. Therefore, an increase in its quantity is normally observed throughout the entire first trimester of pregnancy, when the most intense processes of growth and development of the fetus occur.
3. The hCG level gradually increases until week 12, after which it freezes within certain limits for some time. Then there is a gradual decrease to minimum values ​​by the time the child is born.
4. Pregnancy tests are based on the biochemical reaction of the strips to human chorionic gonadotropin excreted in the urine. Such an analysis can be called qualitative - its result does not give exact figures.
5. More informative is the assessment of the level of hCG in the blood serum, which allows you to correlate the obtained values ​​with temporary norms that change depending on the duration of pregnancy.

The wide range of fluctuations of this analysis also implies the use of an integral rating scale (MoM), which makes it possible to clarify the influence of risk factors on the outcome.

Grade

The absolute values ​​of the analysis, like those of the study on PAPP-A, are measured using a quantitative indicator - in mU/ml. But the biologically active substance is determined throughout pregnancy, which requires standard values ​​for almost every week:

• Normally, the level of hCG in the blood by week 12 already decreases slightly, ranging from 6,000 to 103,000 mIU/ml. It is noteworthy that a little earlier these figures could be three times higher, without being a sign of pathology.


• An isolated increase (without connection with PAPP-A) may be associated with an incorrect assessment of the gestational age, multiple births, and endocrine disorders in the mother. Also, a similar picture is often observed when using synthetic gestagens (Duphaston).
• A combined increase in hCG and protein A almost always indicates a chromosomal abnormality in the fetus. Most often, screening allows one to suspect Down, Edwards or Patau syndrome, which is confirmed by an in-depth examination.
• A decrease in hCG levels is a sign of pathology that makes further pregnancy impossible or risky. The cause may be ectopic attachment of the embryo, a frozen or non-developing pregnancy, as well as the threat of spontaneous abortion.

Human chorionic gonadotropin is also assessed in late prenatal screening, where a decrease in its values ​​indicates delayed growth and development of the fetus.

Concept of MoM

Since all people are quite different, and laboratory standards are standard, a high error arises when assessing biochemical parameters. To simplify the interpretation of analyzes for PAPP-A and hCG in each woman, a scale of average values ​​was created. Its work is based on the following principles:

1. Initially, absolute indicators of these biological substances are required - in mIU/ml. They can either be within the limits of normal values ​​for analysis, or be higher or lower than them.
2. Particular attention is paid to borderline numbers, which, with further assessment, may go beyond the normal limits.
3. The primary MoM is calculated - an indicator obtained based on the ratio of a woman’s test result to the average indicator for a specific period of pregnancy.
4. Then the woman undergoes a special questionnaire (often on a computer), which identifies or excludes the main risk factors. The main indicators are age, race, body weight, bad habits, IVF, concomitant diseases and complicated pregnancy.
5. Taking into account the risks, the final MoM is derived, which can also shift above or below normal fluctuations. In the absence of pathology, the indicator usually ranges from 0.5 to 2.5.

Such an assessment of the results allows for an individual approach to the examination of each patient, allowing one to suspect a particular disease already at the screening stage. This simplifies the further diagnostic search for the disease, allowing you to select the most adequate methods for its detection.