DEFINITION
Anti-Müllerian hormone also known as AMH is a protein that, in humans, is encoded by the AMH gene. It inhibits the development of the Müllerian ducts (paramesonephric ducts) in the male embryo. It has also been called Müllerian inhibiting factor (MIF), Müllerian-inhibiting hormone (MIH), and Müllerian-inhibiting substance (MIS). It is named after Johannes Peter Müller.
Wikipedia
At the early stages of development in mammals, fetuses of both sexes have two pairs of ducts: the Wollfian and the Mullerian ducts. In the 1940s, Alfred Jost showed that a testicular product different from testosterone was responsible for the regression of Mullerian ducts in the male fetus. This product was called Mullerian Inibiting Substance (MIS). More than twenty years ago the human gene for anti-Müllerian hormone (AMH) was isolated and sequenced.
THE GENE
CHEMICAL STRUCTURE AND IMAGES
The AMH and its receptors
The AMH is a member of the transforming growth factor (TGFβ) superfamily. AMH is an homodimer disulfide-linked glycoprotein with a molecular weight of 140 kDa. The gene is located on the short arm of chromosome 19 in humans, band 19p 13.3 (Fig. 1).
Fig. 1: chromosome 19
The AMH gene is 275 bp long, it is divided into five exons. The 3' part of the fifth exon codes for the bioactive part of the molecule and is extremely GC rich.
The expression of the AMH is sexually dimorphic.
In males, MIS expression begins at 9 weeks gestation in the fetal testes and continues at high levels until puberty, when expression levels fall dramatically.
In females, MIS is produced only postnatally in granulosa cells from puberty through menopause at levels similar to adult males, after which expression ceases.
The most striking effect of AMH is its capacity to induce regression of the Müllerian ducts, the anlage of the female internal reproductive organs. In the absence of AMH, Müllerian ducts of both sexes develop into uterus, cervix, Fallopian tubes and the upper part of the vagina.
Human MIS is cleaved into its N- and C- terminal domains most likely by means of furin or arelated prohormone convertase PC5, expressed in the gonads. The purified C-terminal domain is the biologically active moiety and cleavage is required for biological activity. A secondary cleavage site, whose significance is unknown, is observed less frequently at residues 229–230.
The role of the amino-terminal domain in vivo may be to assist in protein folding and to facilitate delivery of the C-terminal peptide to its receptor.
A recent study showed that cleaved and C-terminal AMH can bind to AMHRII and stimulate phosphorylation of Smads, whereas noncleavable full-length AMH and also the N-terminal pro-region alone were completely inactive.
Fig. 2: Upon ligand binding to the type II receptor, the type I receptor is recruited into the receptor complex and becomes phosphorylated by the type II receptor. Activation of the type I receptor results in phosphorylation of the downstream Smad proteins. Phosphorylated receptor-specific Smads translocate to the nucleus where it regulates gene expression.
When relevant for the function
SYNTHESIS AND TURNOVER
Human AMH is synthesised as a 560 amino acid precursor with a 24-25 amino acid leader containing a 16-18 amino acid signal sequence and a putative 7-8 residue pro-sequence (Fig.3). The carboxyl-terminal region of AMH shares homology with that of members of the transforming growth factor-b. Most members of this family require proteolytic cleavage at a site 110 amino acids from the carboxyl terminus to be active. While the full-length AMH molecule is active in organ culture, a cleavage site 109 amino acids from the C-terminus releases a more active fragment. However, the cleaved N-terminal domain interacts and enhances the activity of the C-terminus.
AMH is encoded by a 2.75kb gene divided into 5 exons, characterised by a high GC content. The human AMH gene maps on chromosome 19 p13.3. While the mouse and rat promoters contain an almost perfect TATA box, the human AMH promoter lacks consensus TATA or CCAAT box elements. AMH transcription of the human gene has been shown to contain a functional initiator (Inr) element that is specifically recognised by transcription factor TFII-I. Cloning of 3.6kb of the 5'-flanking sequences of the human AMH gene allowed to identify a major transcription initiation site and three minor sites, a putative oestrogen response element at –1772 and an Sp1 binding site at –303.
Anti-Müllerian hormone in disorders of sex determination and differentiation. 2005
Following AMH gene activation and protein expression, AMH binds to the serine/threonine kinase AMH-receptor 2 (AMHR2), a homologue to TGFβ-related proteins. AMHR2 phosphorylates the AMH receptor 1 (AMHR1), serine/threonine kinases, which are also called activin-like receptor kinase 2 (ALK2) and ALK3. ALK2 and 3 most likely form a heteromeric complex, but a third factor ALK6 has also been implicated in the AMH signalling. The AMHR1 phosphorylation induces a TGF-β-like signal cascade of gene regulation involving protein–protein interactions and phosphorylation events including the Smad proteins.
DNA sequence variations of the entire anti-Müllerian hormone (AMH) gene promoter and AMH protein expression in patients with the Mayer–Rokitanski–Küster–Hauser syndrome. 2005
AMH/MIS: what we know already about the gene, the protein and its regulation. 2003
CELLULAR FUNCTIONS
In the foetus
Müllerian ducts are present and the specific AMH receptor is expressed in the foetuses of both sexes at the time when sexual differentiation is triggered. It is, therefore, the production of AMH by the testis which are responsible for the initiation of Müllerian duct regression in the male foetus. AMH is, in fact, one of the earliest Sertoli cell-specific protein expressed by the gonad. AMH is secreted by the human testis from the sixth week of gestational age (eighth week of amenorrhea) and provokes irreversible Müllerian duct regression, which is completed by the end of Week 9. Yet, AMH expression by Sertoli cells remains at high levels through foetal and prepubertal life, which indicates that the end of the critical window of Müllerian duct regression is dependent on the expression pattern of AMH type II receptor.
Reviewed by Josso et al. 2001.
An immunoassay to detect human müllerian inhibiting substance in males and females during normal development. 1990
AMH in the testis
Except for a transient decline in the peri-natal period, testicular AMH secretion is maintained at high levels until puberty, when Sertoli cell maturation is characterised by a decreasing AMH activity.
The decline of AMH production by Sertoli cells during puberty in the boy is related to the stage of pubertal development rather than age. The most significant decrease in serum AMH is observed between stages II and III of pubertal development, in coincidence with the increase of intratesticular testosterone concentration—an event occurring earlier than the elevation of serum testosteron (Fig. 4).
Fig.4: Serum AMH is high until puberty, when it declines as the intratesticular concentration of testosterone increases. Note the delay existing between the increase of testosterone concentration within testicular tissue and in serum. In the foetus and new-born, testosterone does not inhibit AMH expression owing to the lack of expression of the androgen receptor (AR) in Sertoli cells, which increases progressively after birth.
AMH in the ovary
Ovarian granulosa cells, the homologous to testicular Sertoli cells, also produce AMH but with several differences: AMH expression only begins at the peri-natal period, remains low throughout reproductive life and becomes undetectable after menopause. Granulosa cells of primary and small cavitary follicles show homogeneous AMH expression, in larger follicles, AMH is mainly produced in cells near the oocyte and in few cells surrounding the antrum, and no AMH production is observed in atretic follicles and theca cells.
Clinical utility of AMH measurement in the male
Serum AMH determination is useful in the evaluation of children with non-palpable gonads, with or without ambiguous genitalia. It signals the existence of functional testicular tissue and allows a distinction to be made between gonadal dysgenesis and dissociated tubular-interstitial dysfunction. Serum AMH is a useful marker in the follow-up of male patients with precocious puberty or hypogonadotrophic hypogonadism, as well as of patients with sex cord stromal tumours of the gonads. Furthermore, AMH determination on the seminal plasma of men with non-obstructive azoospermia may be used as a marker of the existence of testicular spermatozoa when intracytoplasmic sperm injection is considered.
Clinical utility of AMH measurement in the female
In women at the onset of puberty, AMH, like inhibin B, is formed by the granulosa cells of the maturing ovarian follicle, but not by the primordial follicles and also not by the antral follicles under direct FSH regulation in the final stage of follicular growth. AMH is the biological regulator of folliculogenesis and of primordial follicular rupture. It reduces the rate of follicle conversion from the primordial to the growing stage and regulates follicle growth by inhibiting FSH-induced conversion from the early to the late stage (Fig. 5)
AMH is an ideal marker for ovarian functional reserve because it is formed only by the primary follicles, which are potentially capable of maturation, and the secondary follicles. There is thus a very good correlation between the serum AMH level and the number of follicles potentially capable of maturation and thus also the ovarian functional reserve. The AMH level falls continuously with increasing age, corresponding to the loss of ovarian functional reserve.
Reduced levels point to restricted ovarian functional reserve and a poor response to ovarian
stimulation. Over 80% of women show reduced ovarian functional reserve, with a level of < 1 μg/l, and an inadequate ovarian response to stimulation is seen in 90%.
In the table below we can see the relationship between fertility and AMH level in women.
BIOLOGICAL FUNCTION
Cell signaling and Ligand transport
AMH employs a heteromeric receptor system consisting of single membrane spanning serine threonine kinase receptors called types I and II, respectively. The type II receptor imparts ligand binding specificity and the type I receptor mediates downstream signaling when activated by the type II receptor (as seen in Fig. 2).
The activated type I receptor then phosphorylates the cytoplasmic Sma- and Mad-related proteins (Smads) 1, 5, or 8, which migrate into the nucleus and, in concert with other transcription factors, regulate responsive genes.
The type II receptor for AMH (AMHRII) is one of five type II receptors in the TGF-β family. AMH and AMHRII are mutually specific. In contrast, the other four type II receptors for activin (ActRIIA, ActRIIB), BMP (BMPRII), and TGF-β (TβRII), each interact with multiple ligands, and all other TGF-β family ligands interact with multiple type II receptors.
In the fetus, mesoepithelial cells expressing AMHRII in the coelomic epithelium covering the urogenital ridge migrate into and become part of the mesenchymal cells surrounding the Mullerian duct epithelium. It is interesting to note that loss of function mutations in the type II receptor as well as the AMH ligand itself, are causes of Persistent Müllerian Duct Syndrome in humans.
Development of an efficiently cleaved, bioactive, highly pure FLAG-tagged recombinant human Mullerian Inhibiting Substance. 2013
Processing of Anti-Müllerian Hormone Regulates Receptor Activation by a Mechanism Distinct from TGF-β. 2013
REGULATION
In males
The regulation of AMH after birth, in males, is complex: basal levels of AMH are independent of gonadotropin regulation, for example, during childhood. Throughout pubertal development, AMH correlates negatively with serum testosterone. This correlation persist if androgen levels are abnormally high but gonadotropins are low.
These findings suggest that AMH is down-regulated by androgens and not directly by gonadotropins. In patients with androgen insensitivity or deficient androgen production, serum AMH levels are extremely elevated indicating that, without the inibitory androgen effect, FSH stimulates AMH production.
Anti-Müllerian hormone in men with normal and reduced sperm concentration and men with maldescended testes. 2009
In females
A recent study shows that AMH expression can be differentially regulated by estradiol depending on the estrogen receptors (ER -α or -β) and suggest that its decrease in granulosa cells of growing follicles, which mainly express ERβ, and during controlled ovarian hyperstimulation is due to the effect of estradiol.
Differential regulation of ovarian anti-müllerian hormone (AMH) by estradiol through α- and β-estrogen receptors. 2012
DIAGNOSTIC USE
AMH levels in women are lower than in men throughout life. In women AMH serum levels can be almost undetectable at birth, with a subtle increase noted after puberty. Serum AMH levels have been measured at different times during the menstrual cycle, suggesting minimal fluctuation. The peak value seems to be reached in the late follicular phase.
Anti-Mullerian hormone plasma levels in spontaneous menstrual cycle and during treatment with FSH to induce ovulation. 2004
Hence AMH is relatively convenient to determine, especially as it seems to exhibit a relatively stable expression during the menstrual cycle, making it an attractive determinant of ovarian activity.
The use of AMH in gynaecology is rapidly expanding, since it has been shown to be a useful marker, in follicular fluid or in serum, of polycystic ovary syndrome
The influence of circulating anti-Müllerian hormone on ovarian responsiveness to ovulation induction with gonadotrophins in women with polycystic ovarian syndrome: a pilot study. 2013
and granulosa cell tumours.
Anti-Müllerian hormone inhibits growth of AMH type II receptor-positive human ovarian granulosa cell tumor cells by activating apoptosis. 2011
and also to predict ovarian response in women undergoing ovulation induction in preparation for assisted reproductive technology Anti-mullerian hormone and antral follicle count as predictors of ovarian response in assisted reproduction. 2013
By Francesca Bastai
