Category Archives: Genetics

Genetics

Anti-Mullerian Hormone: The Blood-Based Biological Clock?


Many women choose to delay starting a family for various reasons, but how long is too long to wait? Is there some way to determine the time remaining on a woman’s “biological clock” to help guide family planning? A new biomarker measured in blood, anti-Müllerian hormone (AMH), has been proposed to do exactly that but there are some important limitations that must be considered before rushing out to the closest doctor’s office to request an AMH measurement.

First, some background. Women are born with approximately one million primordial ovarian follicles and only about one thousand of these remain when a woman reaches menopause. Over the course of a woman’s reproductive years, these primordial follicles come out of hibernation and develop into immature follicles by accumulating theca cells that produce testosterone and granulosa cells that convert testosterone to estradiol. Each cycle, in response to follicle-stimulating hormone (FSH), one of these immature follicles becomes the dominant, mature follicle that ultimately releases an egg through the process of ovulation. Some immature follicles exit the development pathway and become nonviable while others continue to develop for possible selection as the dominant follicle in a subsequent cycle. The key point is that the granulosa cells of these immature follicles produce AMH, which can be measured in serum or plasma as a direct reflection of the number of immature follicles. If more immature follicles are present, the serum/plasma AMH concentration will be higher. If fewer immature follicles are present, the AMH concentration will be lower. At first glance, measuring AMH would seem to be the ideal way to determine a woman’s reproductive lifespan – if AMH is high, many immature follicles remain and menopause is years away.

Unfortunately, it’s not quite that simple. While elevated AMH concentrations do reflect a large number of immature follicles, this doesn’t necessarily guarantee fertility. Polycystic ovary syndrome (PCOS) is a condition marked by the presence of many immature AMH-secreting follicles and women with PCOS typically have elevated serum/plasma AMH concentrations. AMH has been shown to inhibit the effects of FSH and AMH excess prevents immature follicles from reaching the final stages of development, resulting in impaired fertility for many women with PCOS. While an AMH concentration within the age-appropriate reference interval is a favorable indicator of fertility, higher is not necessarily better as very high AMH concentrations may indicate an underlying anovulatory condition.

At the other extreme, low age-specific serum/plasma AMH concentrations have been associated with impaired fertility in women in their 30s and may predict earlier menopause but low AMH concentrations are substantially harder to interpret in girls and younger women – precisely the population for whom an early estimate of reproductive lifespan would be most valuable. Low AMH concentrations in healthy women in their teens and 20s have not been associated with impaired fertility and survivors of childhood cancers with low AMH concentrations have achieved pregnancy. Furthermore, circulating AMH concentrations are reduced by lifestyle factors like oral contraceptive use and smoking, complicating the connection between AMH concentration and reproductive lifespan.

While studies of large numbers of women show that a low age-specific AMH concentration is associated with earlier menopause, it’s difficult to predict the age at menopause for an individual woman using a serum/plasma AMH concentration. The rate of decline in serum/plasma AMH concentrations varies from woman to woman, meaning that two women with identical AMH concentrations one year may have very different AMH concentrations the following year. Furthermore, the onset of menopause is a complex trait determined by genetic factors, environmental exposures and other influences like smoking, alcohol consumption and previous pregnancies. Ultimately, while AMH does reflect the number of immature follicles, its ability to predict onset of menopause and guide family planning decisions is questionable at the present time.

Currently, the most appropriate clinical use of AMH measurement is to predict response to ovarian stimulation in women undergoing in vitro fertilization (IVF). Women with a high AMH concentration (and a large number of immature follicles) who undergo IVF are at increased risk of ovarian hyperstimulation syndrome (OHSS), a potentially fatal condition marked by abdominal fluid retention, blood clots, altered electrolyte concentrations and kidney failure. Using a moderate ovarian stimulation protocol in women with a high AMH concentration has been shown to reduce the risk of OHSS while increasing the number of pregnancies and live births per IVF cycle started. At the other end of the spectrum, women with a low AMH concentration are enrolled in a more intensive stimulation protocol to maximize egg retrieval while those with undetectable AMH are offered alternate treatment options as the chance of IVF success is low.

It’s possible that one day AMH may be routinely measured to predict the onset of menopause but for now, its most promising uses are limited to PCOS diagnosis (still some kinks to be worked out there too) and customization of ovarian stimulation protocols to improve IVF outcomes while minimizing the occurrence of OHSS.

Can a personalized approach improve IVF success rates?


Test Tube Baby

This post was written by Robert D. Nerenz, PhD, an assistant professor of pathology and laboratory medicine at the University of Kentucky, in Lexington.

In the United States, an estimated one in seven couples experience infertility and for many of these couples, in vitro fertilization (IVF) represents their best chance of achieving pregnancy. However, IVF cycles constitute a significant expense (approximately $12,500 per cycle), disrupt patients’ daily lives and only result in a healthy, live birth 30% of the time! Furthermore, the majority of IVF cycles performed in the United States involve the transfer of multiple embryos. This is of particular concern because multiple embryo transfer carries a finite risk of a multiple gestation pregnancy. Bringing multiple infants to term is associated with an increased risk of poor fetal and maternal outcomes including decreased birth weight, increased rate of fetal death, preeclampsia, gestational diabetes and preterm labor. Clearly, there is a significant need to improve IVF success rates while also minimizing the likelihood of multiple gestation pregnancies.

One strategy that may accomplish both of these goals is to perform “single embryo transfer” by implanting one embryo that has a high likelihood of producing pregnancy and, ultimately, a live birth. This is the focus of an upcoming symposium at the AACC meeting to be held July 29th at 10:30 am in Atlanta, Georgia. Fertility clinics around the world currently attempt to do this by observing embryos under a microscope and choosing the best embryo on the basis of its physical appearance. Unfortunately, this approach does not provide any information about the embryo’s genetic status. This is an important limitation because aneuploidy (the gain or loss of a chromosome) is the most common cause of pregnancy loss. It is also estimated to occur in ≥10% of clinical pregnancies and becomes more frequent with increasing maternal age.

To ensure that aneuploid embryos are not selected for transfer, several research groups have developed methods collectively known as comprehensive chromosome screening (CCS). CCS involves culturing embryos for 5-6 days, removing a few cells from the trophectoderm (the outer cell layer that develops into the placenta), isolating the DNA from those cells and assessing the copy number of each chromosome using techniques such as quantitative PCR, comparative genomic hybridization, or single nucleotide polymorphism arrays. Following determination of the embryos’ genetic status, only embryos with the normal number of chromosomes are chosen for transfer. In multiple prospective, randomized controlled trials described here and here, CCS has been shown to increase the pregnancy rate and decrease the frequency of multiple gestation pregnancies. As a result, CCS is beginning to make the transition from the research setting to use with patients.

The ability to transfer only euploid embryos represents the most promising application of novel technologies to IVF but ongoing research is focused on other ways to improve the IVF success rate. Many different groups are analyzing the culture medium that embryos are grown in prior to implantation. It is hoped that this will provide information about the embryos’ metabolic health and might help identify which embryos are most likely to result in pregnancy and live birth. Other groups are evaluating endometrial gene expression profiles to assess endometrial receptivity and ultimately determine the best time to perform embryo transfer. While both of these approaches have technical limitations and are not quite ready for primetime, they have the potential to greatly improve our current standard of care and may be ready for clinical use in the near future.

Conventional aneuploidy screening remains “most appropriate” choice for general population


OpinionThe American Congress of Obstetricians and Gynecologists (ACOG) have updated their guidance on cell-free DNA (cfDNA) screening tests for fetal aneuploidy. In it, they state that any patient (i.e. women at high-risk OR low-risk for having an affected pregnancy) may choose cfDNA testing but they caution that conventional screening tests are more appropriate. This document replaces an earlier opinion, published in 2012, which clearly stated that cfDNA screening tests should not be offered to the general obstetrical population because they are considered to be at low-risk.

So ACOG went from recommending that cfDNA testing not be performed on low-risk women to say that they may choose cfDNA testing. Why the subtle change? Well, as ACOG correctly notes, the landscape of cfDNA is changing rapidly. New studies are published frequently and those that have examined the performance of cfDNA tests in  low-risk women have reported that the test performs just as well in them as it does in high-risk women.

However, they make an important point about a metric that doesn't get the attention it deserves. The positive predictive value (PPV). See here for background. Because the prevalence of fetal aneuploidy in low-risk women is lower than it is in high-risk women, a "positive" or "abnormal" test result in low-risk women is more likely to be a false-positive result. For example, a positive result in a 25-year-old woman gives a 33% chance that the fetus is affected but that chance increases to 87% in a high-risk woman.

The report also calls out the "no result" problem. cfDNA tests fail to produce a result in 1-8% of samples tested, usually due to a low amount of fetal DNA in the blood sample. It's becoming clear that women with samples that fail to produce a result are at increased risk of having an affected fetus. According to ACOG, these women she be offered diagnostic testing such as fetal karyotyping using amniotic fluid obtained by amniocentesis.

Other notable points contained within the updated guidance include:

  • Caution about not routinely performing microdeletion screening (offered by some labs) because it has not been fully validated in clinical studies.
  • Clearly indicating that a negative or normal result does not rule out the possibility of an affected fetus.
  • Providing genetic counseling to patients about test limitations and that decisions such as pregnancy termination should not be based on these screening tests.
  • A reminder that cfDNA tests do not screen for neural tube or ventral wall defects

This certainly won't be the final say that ACOG has on cfDNA aneuploidy screening tests. Indeed, they state that "It will be critical to remain abreast of this rapidly changing technology to provide patients with the most effective, accurate, and cost-conscious methods for aneuploidy screening."

Preimplantation Genetic Diagnosis and Screening


BlastocystPreimplantation genetic testing is a way of examining the genetic features of a developing embryo during the process of in vitro fertilization, before pregnancy. After the egg is fertilized with sperm, the embryos develop to the cleavage-stage. On day 3 after fertilization, a single blastomere is removed from the embryo for genetic evaluation using techniques such as PCR, FISH, or comparative genomic hybridization.

Preimplantation genetic diagnosis (PGD) is used to select embryos without certain genetic disorders. This testing including three major groups of disease: sex-linked disorders, single gene defects, and chromosomal disorders.

Preimplantation genetic screening (PGS), is not used to detect disease, but as a screen to select embryos for such things as: matching HLA type in order to be a tissue donor for an affected sibling, selecting gender, selecting embryos with the least predisposition for developing certain cancers, and selecting embryos with a higher chance of implantation and therefore increase the likelihood of achieving pregnancy. Medscape has an excellent overview of PGD and PGS.

For women of advanced maternal age or couples with known genetic mutations, the ability to screen for embryos free of certain genetic mutations is reassuring. However, as with many medical interventions associated with human reproduction, PGS has raised ethical questions. For instance, as stated earlier, PGS can be used to select for a preferred gender. In some cases this is to avoid a sex-specific disease. Other times this is done for so-called "family planning" or "gender balance." In other words, selecting a gender because of personal preference. Some feel this is discriminatory and should not be allowed. In other cases, embryos have been tested so that the resulting child would be compatible to serve as a stem cell donor for a sick sibling (much like the popular fiction book "My Sister's Keeper").   There have also been cases where parents have requested the selection of affected embryos so that the child has the same minor disability, such as deafness or dwarfism, as the parents. Some preimplantation genetics laboratories agree to do this type of testing and some do not.

The New York Times recently ran an article discussing this issue. The article states that:

"In the United States, there are no regulations that limit the method’s use. The Society for Assisted Reproductive Technology, whose members provide preimplantation diagnosis, says it is 'ethically justified' to prevent serious adult diseases for which 'no safe, effective interventions are available.' The method is 'ethically allowed' for conditions 'of lesser severity' or for which the gene increases risk but does not guarantee a disease."

The January issue of Clinical Chemistry published a Question and Answer piece entitled "The Ethical Implications of Preimplantation Genetic Diagnosis." A podcast interview with two of the authors is also available.

The paper summarized the opinions of an ethicist, an attorney, and the director of a preimplantation genetics laboratory. The ethicist indicated that in the past, PGD has focused mainly on reducing the risk of transmitting serious diseases. In the future, he sees a shift away from lifesaving interventions to more ‘eugenically’ inspired interventions. That is, looking for traits that parents do not want in their children and selecting for traits that they do want in an attempt to pass them on. The morality of eugenics is a key moral as this technology moves forward.

Indeed it will be interesting to see where the future of this technology lies. Although it is practiced routinely, the indications, utility, and outcomes of PGD and PGS are still being defined.