D Pandey, P Kumar, G Chaturvedi, Maniarasi
human chorionic gonadotropin, placenta, pregnancy, ß-subunit
D Pandey, P Kumar, G Chaturvedi, Maniarasi. Human Chorionic Gonadotropin: "The Magic Molecule". The Internet Journal of Endocrinology. 2008 Volume 5 Number 1.
hCG is a glycoprotein molecule composed of two dissimilar subunits, alpha(α) and beta(ß) subunits, linked together with hydrogen and disulphide bonds. All cells produce hCG. However hCG molecule secreted by the placenta has a longer half life, owing to the glycosylation of its ß-subunit. The only definitely known function of hCG, is to support the corpus luteum of pregnancy. Uses of hCG include diagnosis of normal as well as nonviable, ectopic and molar pregnancy, as a tumor marker and for aneuploidy screening, while for therapy it is used in various steps of assisted conception, recurrent or threatened abortion, preterm labor and as antifertility vaccine. Many more areas are yet to be explored, where this enigmatic molecule has the capacity to do wonders.
Human chorionic gonadotropin (hCG) derives its name from the fact that it is biologically similar to gonadotropins secreted by the pituitary gland (Follicular Stimulating Hormone or FSH and Luetinizing Hormone or LH), is produced by chorionic tissue and is the first hormone demonstrated in human species prior to its recognition in animals. 1
Story of hCG dates back to 1530 BC, as described in Berlin Papyrus. 2 In Egypt pregnancy test consisted of moistening two cloth bags one filled with barley and the other with wheat grains with women's urine. Sprouting meant pregnancy test is positive. When wheat sprouted it was considered a baby girl while only barley meant a male conceptus. A study done in 1963 to determine the accuracy of this ancient pregnancy test surprisingly revealed a 70% rate of it being right.
It took more than 3000 years for the first pregnancy test based on hCG to be developed, initiating a new era in pregnancy testing. In 1927 Selmar Aschheim and Bernhard Zondek described A-Z test, which identified the presence of hCG in urine. 3 To test for pregnancy, a woman's urine was injected into an immature rat. In case of pregnancy, the rat would have estrous reaction and “blut punkte” (blood points) in the ovary, determined by a laparotomy approximately 100 h after the injection. Ascheim and Zondak had named this hormone – “prolan”, derived from the Latin word proles, meaning offspring. During early studies of the A-Z test, the scientists discovered that testicular tumors could also produce hCG. The 1930s saw a frenzied increase in shooting up all sorts of poor little creatures with hCG may it be a rabbit, frog, or a toad. These tests were expensive, required sacrifice of animals, and were slow, often taking days to get results.
Three decades later, in 1960, hemagglutination inhibition test, for pregnancy was developed by Wide and Gemzell. 4 This test was an immunoassay rather than a bioassay, was much faster and cheaper, but still relatively insensitive, especially for early diagnosis of pregnancy.
During 1970-1972 scientists at National Institute of Health (NIH) learned more about the properties of hCG. Using various methods, they identified two subunits of hCG. They found that the beta-subunit is where the immunologic and biologic specificity of hCG molecule resides.
FDA approval was sought in the year 1976 by Warner Chilcott for “Early Pregnancy Test (EPT)” later known as the “Error Proof Test” the first home pregnancy test kit in the United States.
After this first successful utilization, the magic molecule of hCG is trying to spread its pseudopodia in each and every territory of medical sciences, be it the sphinx of infertility, aging, obesity, or cancer.
Biochemistry of hCG molecule
Though the isolation and identification of hCG was performed as early as 1960s, the crystallographic structure was first determined by Lapthorn et al. in 1994. 5 hCG is a 40,000 dalton glycoprotein molecule composed of two dissimilar subunits, alpha(α) and beta(β) subunits, linked together with hydrogen and disulphide bonds.
The α subunit of hCG is similar to other glycoprotein hormones FSH, LH and TSH (thyroid stimulating hormone), but the β subunit is different in the sense that the terminal 28 to 30 amino acids of the carboxyl terminus are not found in any other glycoprotein hormone.
All cells produce hCG. However hCG molecule secreted by the placenta has a longer half life, owing to the glycosylation of β subunit, a unique function of chorionic tissue, that prevents its degradation. 4
hCG and its isoforms
hCG in the biological fluids, is not a single molecule, rather a mixture of different isoforms. Isoforms mean intact hCG, free subunits and degraded forms of these. International Federation of Clinical Chemistry (IFCC) established a working group in 1995 with the aim of improving standardization of hCG determinations. The nomenclature adopted by IFCC includes hCG, its subunits hCGα and hCGβ, partially degraded or nicked forms of hCG (hCGn) and hCGβ (hCGβn), and beta-core fragment (hCGβcf). 6 In addition to these variants, recently described hyperglycosyled isoform is of potential clinical utility. Produced by invasive cytotrophoblasts, it is the predominant form in second and the third week of pregnancy, slowly replaced by regular hCG. Specific measurement of this hyperglycosylated isoform is useful in suspected invasive gestational trophoblastic disease.
Point to be appreciated is that the clinical significance of these isoforms is not for routine diagnosis and follow up of normal pregnancy but for the follow up of gestational trophoblastic tumors, and germ cell tumors. Moreover various commercial kits manufactured to measure hCG have different specificity to different isoforms.
Functions of hCG
The only definitely known function of hCG to date, is to support the corpus luteum of pregnancy, 7 allowing continued progesterone production and maintenance of the gestational endometrium.
It might stimulate steroidogenesis in early fetal testis, ensuing androgen production and masculine differentiation. 8 This hypothesis is supported by the fact that in anencephalic fetus where the pituitary is absent, external genitalia are well developed in a male fetus, possibly owing to the presence of hCG. hCG gene is expressed in fetal kidney and adrenal glands, suggesting that hCG may affect the development and function of these organs. 9 In addition hCG may regulate placental development by influencing cytotrophoblast differentiation. 10
Laboratory test for hCG
Commercially available pregnancy test kit consists of a disposable plastic device which has three main parts – a well to pour drops of urine, another opening or window to see test result, and in between a column shielded by plastic this contains tracer in liquid phase antibody. If hCG is present in the urine poured into the well it will move towards tracer antibody in liquid phase and will bind to it, will continue to flow towards solid phase capture antibody present in a nitrocellulose membrane in the window, immobilized in a line shape, which glows with color if the poured urine contains hCG.
Diagnostic uses of hCG
Neither single nor serial measurements can confirm the location of gestational sac. Ectopic pregnancy may present with rising, falling or plateau hCG levels. Serial measurement is useful to confirm viability rather to identify ectopic pregnancy. Demonstration of normal doubling over 48 h supports diagnosis of fetal viability, but does not rule out ectopic pregnancy. Similarly, falling level confirms non-viability, but does not rule out ectopic pregnancy. 21
Still in ectopic pregnancy hCG is helpful in conjunction with ultrasound, as well as to guide methotrexate treatment and its follow up.
Therapeutic uses of hCG
Recently, hCG produced by recombinant techniques in Chinese hamster ovary cells has become commercially available. In ART, a dose of 250 µg of rhCG has been found to be equivalent 27 or at least as effective as 10,000 IU of urinary hCG in inducing final stages of oocyte maturation. Furthermore, the use of rhCG was associated with significantly better patient tolerance. 28
A recent study, published in 2008, found no difference in mean gestational age at delivery, birth weight, and preterm birth, concluding that even if there is not much evidence to support the use of hCG in threatened or recurrent abortions, there is no harm too, in giving the benefit of doubt as hCG treatment in early gestation does not increase chances of adverse effect in neonates. 30
hCG in special situations
Thoughts to ponder
In early pregnancy, vertical transmission of HIV is extremely rare due to high concentration of hCG, as the β-subunit is active against HIV. By the second trimester women carrying female fetus have higher hCG levels than those carrying male fetus. hCG acts as an endogenous tocolytic in normal pregnancy. It may provide a promising pharmacological approach to the pervasive problem of preterm labor. Qualitative hCG testing of cervico-vaginal washings appears to be useful predictor of preterm rupture of membranes.
A double-blind, placebo-controlled, randomized clinical trial of rhCG done by Liu PY et al. showed beneficial effects on muscle strength, physical function as well as activity in older men with partial age-related androgen deficiency.
In 1954, Dr. Simeons, a British-physician claimed hCG injections could suppress appetite, burn stored fat and redistribute fat. Unfortunately, he had no clinical evidence to validate his claims. Since than a number of studies have been done quite extensively. However, not a single study indicates hCG has any benefit for weight loss. Though this use of hCG has not been approved by FDA, weight reduction clinics are using it with immense patient satisfaction.
hCG, is found, curiously enough at the surface of all cancer cells. Connection between cancer and pregnancy was first muted by Dr Cohnheim. Many scientific studies have correlated increased β-hCG levels with increased risk of malignancy and metastasis. Studies have shown hCG inhibits Kaposi sarcoma associated angiogenesis and tumor growth. It is also being postulated to be helpful in the prevention of breast cancer.
hCG is a glycoprotein hormone with unique β-subunit. Its main function is to supports corpus luteum. It has many diagnostic and therapeutic benefits. Many more areas are yet to be explored, where this enigmatic molecule has the capacity to do wonders.
Dr Deeksha Pandey Assistant Professor Department of obstetrics & Gynecology Kasturba Medical College, Manipal, Karnataka, INDIA Pincode: 576104 Phone: 91-820-2922211 (O) 91-820-2571758 (R) Mobile: 91-9241216016 Email: email@example.com firstname.lastname@example.org email@example.com