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Genetic polymorphism and recurrent miscarriage

Reproductive BioMedicine Online, 6, 29, pages 657 - 658

Editorial Comment

Despite increased efforts recurrent miscarriages still account for at least 1-2% of women in the reproductive age. One third can not be explained following elaborate investigations. Detecting further reasons is highly welcome, even if it seems hard to distinguish against the population variations. Therefore it is necessary that statistical power and clinical consequences are respected. Both articles, Haroun et al. 2014 and Andraweera et al 2014 seem to take care of this fact, therefore pointing to additional pathways. To accelerate the process of finding relevant influencing factors, genome wide studies seem to be appropriate. This needs to be accompanied by basic research for example in understanding the genetic background of meiotic processes to comprehend recurrent aneuploidies.

PD Dr. Tina Buchholz Scientific director for IFFS

It is well accepted that recurrent miscarriage is not due to a single underlying cause, but is a heterogeneous condition associated with several different maternal factors such as antiphospholipid syndrome and other prothrombotic conditions, acquired and congenital uterine pathology, immunological, endocrine and subtle endometrial factors. Genetic factors also contribute to recurrent miscarriage. Parental chromosomal anomalies, such as balanced translocations, are found in 4% of couples with recurrent miscarriage. A larger proportion of cases of recurrent miscarriage are associated with recurrent aneuploidy of the implanting embryo. In sporadic miscarriage, embryo aneuploidy is found in over 50% of cases ( Ljunger et al., 2011 ). It is quite possible that some cases of recurrent miscarriage are a consequence of a consecutive run of embryo aneuploidy, which may or may not recur again when conception happens again.

In this issue of the journal, two groups of investigators examined the association between recurrent miscarriage and genetic polymorphisms. In the first study, Haroun et al. (2014) found that the allele frequency of the trefoil factor 3 gene, which is an inflammatory mediator expressed in the endometrium in the peri-implantation phase, differed significantly between women with or without a history of recurrent miscarriage. In the second study, Andraweera et al. (2014) reported that the prevalence of polymorphisms in the fibrinolytic system genes was increased in Sinhalese women with recurrent miscarriage when compared with controls.

Two recent reviews on genetic studies (Daher et al, 2012 and Rull et al, 2012) revealed that over 80 studies have been carried out to date to examine the association between genetic polymorphisms and recurrent miscarriage. A wide variety of genes have been studied, ranging from genes relating to hereditary thromphophilia, such as factor V Leiden mutation, prothrombin G20210A mutation andMTHFRC667T encoding the methylenetetrahydrofolate reductase system, various cytokine genes particularlyIL-6, IL-10andTNF-a, angiogenesis mediator genes and steroid receptor genes. The large number of genetic polymorphisms reported to show an association with recurrent miscarriage is not at all surprising, but entirely in keeping with our current understanding of the physiology of implantation, which is a rather lengthy and complex process, involving multiple factors such as steroid hormones, cytokines, angiogenic mediators and immune competent cells.

What is the clinical relevance of the findings of the two studies reported in this issue? The two studies were conducted with reliable, validated genotyping methods; their findings are therefore not in doubt. The two studies add further support to the notion that there is a genetic basis for recurrent miscarriage. However, the identification of a putative genetic factor based on a genetic polymorphism study is only a starting point. The clinical relevance of these two studies has yet to be established. It may be unfair to make the criticism that a genetic polymorphism study is a fishing exercise, partly because multiple comparisons are often made in this type of study, which increases the likelihood of a false positive result when the significance of the observation is determined by a P value of less than 5%. It is often necessary to apply correction in studies of this kind to reduce the likelihood of a false positive finding. Nevertheless, it is reassuring that both Andraweera et al. (2014) and Haroun et al. (2014) did make allowance for the multiple comparisons and made appropriate correction to reduce the false discovery rate (FDR) ( Benjamini et al., 2001 ).

Genetic polymorphism studies should be followed by carefully planned clinical studies to establish the functional relevance of the observation. For example, does the observed genetic polymorphism lead to a change in demonstrable physiological function? It is of interest that the authors of both papers published in this issue of the journal share the same view. In the study conducted by Haroun et al. (2014) on trefoil factor 3 gene variant, the authors acknowledged that the effect of the observed polymorphism on gene expression level, especially in the endometrium, is unknown and warrants investigation. Andraweera et al. (2014) similarly concluded that functional studies are required to elucidate the cellular and molecular effects of the observation, one example of which is to correlate the various fibrinolytic pathway gene polymorphisms with a validated functional measurement such as plasma plasminogen activator inhibitor 2 (PAI-2).

Alternatively, the functional significance of the polymorphism may be explored by examining the prognostic value of the observation. Is it associated with a reduced chance of a successful live birth in a subsequent pregnancy? The sample size required depends on how large the impact of the observed genetic polymorphism is on the outcome. The smaller the impact, the larger is the sample size required to establish the functional significance. Ideally, the likely impact on outcome should be estimated from the available initial genetic study and retrospective cohort study, so that the sample size could be more accurately calculated. Andraweera et al. (2014) were correct to point out that most individual polymorphism have small effects on disease risk so that the likely sample size required prospectively is going to be large; the number of women with recurrent miscarriage to be recruited is almost certainly over a thousand, especially as only around 50% of women with recurrent miscarriage belong to the unexplained category and not everyone will conceive again. The follow-up study should also collect information on any intervention that could have altered the outcome, as it may well be necessary to analyse this category of subjects separately. Moreover, the follow up study should, if possible, collect detailed clinical information on any subsequent loss, for example, does it always occur at a specific gestational age? How often does it occur at a very early stage, before any ultrasound evidence of a pregnancy is established (biochemical loss)? How often does it occur after a foetal heartbeat has been detected? It is of interest to note that Andraweera et al (2014) did observe that some of the genetic polymorphisms differed significantly between the miscarriage and control populations only if first trimester losses were included, but that there was no significance difference if later losses were included. The detailed follow-up information prospectively collected should help us understand how the observed genetic condition leads to the miscarriage.

Finally, it is worthy to note that the two studies reported in this issue have used different criteria to define recurrent miscarriage. Andraweera et al. (2014) defined it as two consecutive first trimester losses (without further qualification), whereas Haroun et al. (2014) defined it as three or more consecutive miscarriages in the first or second trimesters of pregnancy. This is perhaps not surprising given that different international professional bodies such as the American Society for Reproductive Medicine (ASRM) and the Royal College of Obstetricians & Gynaecologists (RCOG ) do not follow a unified definition, with the former recommending two consecutive clinical losses (excluding biochemical losses) ( American Society for Reproductive Medicine, 2008 ) and the latter recommending three losses (with no specific guidance on whether or not biochemical losses should be excluded) ( Royal College of Obstetricians and Gynaecologists, 2011 ). For now, researchers should decide for themselves which definition is the more appropriate, bearing in mind that if stricter criteria are used, recruitment of subjects is more difficult and takes longer, but the subjects successfully recruited are likely to be less heterogeneous and so the power of the study is likely to be greater.

To conclude, geneticists and clinicians should work together to carefully plan follow-up studies to establish the clinical relevance of various genetic polymorphism studies on recurrent miscarriage.