The sperm protamine mRNA ratio as a clinical parameter to estimate the fertilizing potential of men taking part in an ART programme

STUDY QUESTION: Could the protamine-1 to protamine-2 mRNA ratio serve as a biomarker to estimate the fertilizing capacity of sperm from men taking part in an IVF/ICSI programme?

SUMMARY ANSWER: The protamine mRNA ratio clearly discriminates between fertile and subfertile men and sperm with a normal protamine mRNA ratio exhibit a higher fertilizing capacity in IVF/ICSI.

WHAT IS KNOWN ALREADY: Aberrant sperm protamine ratios are associated with male factor infertility and mRNA ratio is comparable with protein ratio (due to transcriptional stop in elongating spermatids).

STUDY DESIGN, SIZE, DURATION: The study population was drawn from subfertile men, whose female partners participated in IVF or ICSI programmes between September 2010 and February 2012. Normozoospermic healthy volunteers served as controls. Sperm cells were lysed, mRNA extracted, reverse transcribed and subjected to real-time quantitative PCR using specific primer pairs for protamine-1 and protamine-2. Relative protamine-1 and protamine-2 mRNA levels were analysed with the Mann-Whitney U-test (two-tailed).

PARTICIPANTS/MATERIALS, SETTING, METHODS: Quantitative RT-PCR for protamines 1 and 2 has been performed in ejaculates from 32 normozoospermic volunteers (control, University Clinic Giessen, Germany) and 306 patients, whose female partners took part in an IVF (n = 76; University Clinic Hamburg, Germany and Shanghai Jiaotong University, China) or an ICSI (n = 230; University Clinic Munich, Germany and Kinderwunschzentrum Wiesbaden, Germany) programme.

MAIN RESULTS AND THE ROLE OF CHANCE: The sperm protamine mRNA ratio in normozoospermic men (0.98 ± 0.3) differed significantly from that of ICSI patients (Munich 0.81 ± 0.1; Wiesbaden 0.78 ± 0.2; P < 0.001), while processed samples obtained from IVF patients revealed a normal protamine mRNA ratio (Hamburg 1.0 ± 0.07; Shanghai 1.0 ± 0.54). Normal protamine mRNA ratios were associated with a significantly higher total motile sperm count and a significantly higher percentage of progressively motile sperm. Sperm with a normal protamine mRNA ratio revealed a higher fertilization capacity (fc) in both IVF (53.6% of patients with fc > 80%; P = 0.017) and ICSI (65.1% of patients with fc > 70%; P = 0.028).

LIMITATIONS, REASONS FOR CAUTION: The protamine mRNA ratio in an individual sperm cell used for ICSI may be different from the overall value obtained from a semen aliquot.

WIDER IMPLICATIONS OF THE FINDINGS: Data are in line with current literature and suggest the protamine mRNA ratio as a diagnostic marker to estimate the fertilizing capacity of sperm.

STUDY FUNDING: The German Research Foundation (DFG) to K.S., W.W. and A.P. (STE 892/9-2), as well as to A.S. and H.C.O. (SP721/1-3).

COMPETING INTEREST(S): None.