Unexplained recurrent miscarriages are associated with an aberrant sperm protamine mRNA content

STUDY QUESTION: Are unexplained recurrent miscarriages associated with abnormal protamine-1 and protamine-2 mRNA levels in spermatozoa? SUMMARY ANSWER: Both protamine-1 and protamine-2 mRNA levels as well as the protamine-1 to protamine-2 mRNA ratio in spermatozoa from men whose female partners experienced two or more consecutive miscarriages were significantly different compared to those from both healthy control men and subfertile couples undergoing IVF/ICSI. WHAT IS KNOWN ALREADY: Aberrant sperm protamine ratios are known to be associated with male-factor infertility. Data from this study suggest that the protamine mRNA ratio may additionally affect early embryo development. STUDY DESIGN, SIZE, DURATION: The study population was recruited from men whose female partners presented with two or more consecutive unexplained miscarriages in a consultation for recurrent pregnancy loss between 2014 and 2016. At the research laboratory of the Urological Clinic of the University Giessen, spermatozoa from cases and controls were subjected to reverse transcription quantitative PCR (RTqPCR) using specific primer pairs for protamine-1 and protamine-2. PARTICIPANTS/MATERIALS, SETTING, METHODS: Protamine-1 and protamine-2 mRNA levels were analysed in semen samples from 25 men whose female partners experienced at least two consecutive idiopathic miscarriages before the 20th week of gestation. The couples were recruited during consultation at the Fertility Center of the LMU Munich, Germany, and at the Clinical Division of Gynecologic Endocrinology and Reproductive Medicine of the Medical University of Vienna, Austria. Results were compared with those from 32 healthy donors (WHO, 2010) recruited at the Department of Urology, Pediatric Urology and Andrology, Giessen, Germany, and 107 men whose partners participated in an IVF/ICSI program at the Fertility Center of the LMU Munich, Germany. MAIN RESULTS AND THE ROLE OF CHANCE: Protamine-1 and protamine-2 mRNA levels as well as the protamine mRNA ratio and all routine semen parameters revealed significant differences between recurrent miscarriage couples and healthy volunteers (P < 0.01). When comparing recurrent miscarriage couples with couples undergoing IVF/ICSI, Ct-values of protamine-1 and protamine-2 mRNAs were significantly higher and the protamine mRNA ratio was significantly lower in RM couples (P < 0.01). When comparing protamine mRNA levels and the protamine mRNA ratio with routine semen parameters, a significant negative correlation was evident between progressive motility and the protamine-2 mRNA level (P = 0.015), as well as between non-progressive motility and the protamine mRNA ratio (P = 0.023). LIMITATIONS REASONS FOR CAUTION: Although our data demonstrate significant abnormalities in RM, larger sample sizes will be needed to confirm our results. Larger sample sizes should also balance the fact that we had to focus mainly on median protamine mRNA levels. Finally, men in the healthy control group were younger in age than those in the case group, which might have introduced some bias, at least concerning the classic semen parameters. Moreover, only protamine mRNA instead of protein levels could be measured. WIDER IMPLICATIONS OF THE FINDINGS: Although the exact mechanism remains to be elucidated, our data suggest that protamine mRNA levels in spermatozoa are not only important for successful fertilization, but also for proper development of the early embryo.