Sci Transl Med-2011-Rozen-92ps31

B ETA DEFENSIN 126An estimated 10 to 15% of couples at some point su er from infertility, which is de ned as the inability to conceive a er 1 year of unprotected intercourse (1). O en, multiple factors contribute to infertility, and many treatment decisions rely on trial and error (2–4). In many couples, genetically deter-mined characteristics of the male’s semen probably play at least some role, and this may be compounded by reduced fertility in the female partner. However, little is known about the genetic determinants of male fer-tility beyond e ects mediated by reduced sperm count (5–9), as well as a few addition-al rare variants that account for a small frac-tion of male infertility (10–12). In this issue of Science Translational Medicine , Tollner et al . (13) o er a welcome advance showing that a common variant in the gene encoding beta defensin 126, DEFB126, probably con-tributes to infertility.DEFB126 is a member of a diverse yet underinvestigated family of genes, many of which encode antimicrobial proteins pres-ent in the skin or mucous membranes (14). Some beta defensins are expressed primarily in the epididymis (15), a small organ on the side of each testis that serves to store sperm and contributes to their maturation (Fig. 1A). It is hypothesized that some of the beta defensins expressed in the epididymis help to protect the organ from infection-causing microbes. However, among the epididymal beta defensins, the DEFB126 protein has a di erent function. In a model organism, itcoats the plasma membrane of sperm (16)(Fig. 1B). Functional studies in macaques show that the DEFB126 protein coat helps sperm to penetrate cervical mucus (17), shields them from recognition by the female immune system (18), and promotes their at-tachment to the epithelium of the oviduct (fallopian tube) (Fig. 1A) (19).A COMMON FUNCTIONAL GENETICVARIANT IN DEFB126In examining the nucleotide sequence of hu-man DEFB126, Tollner and colleagues no-ticed an extremely common sequence variant (13). is variant, denoted “del,” involves the deletion of two cytosine nucleotides starting at position 317 of the reference coding se-quence (CDS) of the gene (Concensus CDS no. 12990.1). us, although the reference variant encodes a predicted 111–amino acid protein, the del variant results in a frame shi that removes the stop codon. is results in a “nonstop” mRNA in which the last six amino acids of the reference protein product (PVSPTG) are predicted to be replaced by a 26–amino acid sequence: RFSHWLNIP AS-VSCSRIP DSLKQRGL. e authors further reported that in the epididymis, a del/del homozygote had DEFB126 mRNA levels that were only 10% of those in a reference-allele homozygote (13). ey hypothesize that this might be the result of suppression of the non-stop mRNA by cellular surveillance mecha-nisms. In any case, the surface of sperm from del/del homozygotes showed dramati-cally less glycosylation of the type associated with DEFB126 (20)—speci cally, O-linked galactose-GalNAc (N -acetylgalactosamine) glycans (13). is lack of sperm glycosylation is presumably due to reduced levels of the DEFB126 protein itself, although this was notmeasured directly. Importantly, the amount of this coating correlates with the ability of sperm to penetrate hyaluronic acid (HA) (13). Because HA is an established surrogate for cervical mucus (21, 22), the near inability of sperm from del/del men to penetrate HA indicates that these sperm would have great di culty penetrating cervical mucus.DEL/DEL AND FERTILITYNaturally, some questions then followed: Does homozygosity for the del variant im-pede sperm penetration of cervical mucus or other functions mediated by DEFB126 in vivo? Furthermore, does this then a ect fer-tility? To address the latter question, Tollner et al . studied DEFB126 genotypes and fertil-ity in approximately 500 newlywed couples (13). ey found that a er 2 years, couples in which the man’s genotype was del/del were signi cantly less likely to have had a pregnancy than were the other couples. A second, con rmatory analysis found that in any given month, couples in which the man’s genotype was del/del were 30% less likely than were the other couples to have a birth. e con uence of the experimental and human genetic studies provides strong evidence that men with the del/del genotype have reduced fertility. Nevertheless, the ge-netic ndings will need to be replicated in a larger cohort of couples or over a longer period of time before we can be con dent in the e ects of the del/del DEFB126 genotype on male fertility.It is also important to recognize that re-duced penetration of cervical mucus might not be the only reason that del/del homo-zygotes have lower fertility. In macaques, DEFB126 also helps sperm avoid recogni-tion by the female immune system (18). In addition, DEFB126 promotes sperm attach-ment to the epithelium of the oviduct (19), the organ in which fertilization normally takes place. us, beyond reducing sperm penetration of cervical mucus, the del/del genotype could conceivably contribute to infertility by abrogating these or other, still-unknown, functions of DEFB126. e geographically widespread, high prevalence of the del variant—close to 50% of alleles in populations from Asia, Europe, and Africa—is surprising in light of the ob-served reduced fertility of del/del homozy-gotes. Fundamentally, we can only specu-late about why the del variant is extremely common in so many populations. Tollner and colleagues show possible evidence ofI N F E R T I L I T YDefending Male FertilitySteve Rozen**E-mail: steve.rozen@.sgDuke–NUS Graduate Medical School Singapore, 8 Col-lege Road, 169857 Singapore. A n estimated 10 to 15% of couples su er from infertility, and many treatment decisions rely on trial and error. In this issue of Science Translational Medicine , Tollner and colleagues provide strong evidence from a human genetics study that a common variant in the beta defensin 126 gene, the “del” variant, can reduce male fertility substantially. In addition, they show a plausible mechanism for reduced fertility: Sperm from del/del homozygotes lack an important component of their glycoprotein coat and have di culty penetrating a surrogate for cervical mucus. If replicated in future studies, these ndings promise to guide choices about the timing and type of assisted reproduction interventions—and further hint at the possibility of treating sperm from del/del homozygotes to promote fertility.o n J u l y 21, 2011s t m .s c i e n c e m a g .o r g D o w n l o a d e d f r o mC R ED I T : P . H UE Y /S C I E N C E T R A N S L A T I O N A L M E D I C I N Ebalancing selection, in which heterozygotes mig ht have a selective advantag e over ho-mozygotes for either allele. But the authors’ genetic studies (13) show no evidence that heterozy otes have any advanta e over reference-allele homozyg otes in terms of time to pregnancy or birth. us, any het-erozygote advantage would have to be medi-ated by other e ects, such as, speculatively, resistance to infection.CLINICAL IMPLICATIONS e g enetic nding s by Tollner et al . o er two clinical opportunities. A future oppor-tunity is the possibility of treating sperm from del/del men to restore g lycosylated DEFB126. In previous work, the authors showed that it is possible to remove and then restore the DEFB126 coat from ma-caque sperm (Fig. 1B) (23). Restoring g ly-cosylated DEFB126 would depend on iden-tifying an abundant source of the protein with the necessary pattern of glycosylation. e more immediate clinical opportunity is that of guiding choices about the timing andtype of assisted reproduction intervention (Fig. 1A). In g eneral terms, if it is known that time to preg nancy is likely to be longbecause the man has the del/del genotype, then resorting immediately to assisted re-production may be attractive. is would beespecially true if the woman has dwindlingovarian reserves and is also contributing to the couple’s infertility. is situation is be-coming more common as women increas-ing ly delay childbearing , which results indecreased female fertility (24–28).If it is con rmed that the del/del geno-type depresses fertility by impeding sperm penetration of the cervical mucus, then apossible intervention is intrauterine insemi-nation (Fig. 1A). In this technique, washed sperm are deposited directly into the uterusvia a catheter inserted throug h the cervi-cal canal, thus mechanically bypassing the cervical mucus. is can be combined withovarian hyperstimulation to increase thechances of successful fertilization. If, how-ever, it turns out that the del/del genotypedepresses fertility through additional mech-anisms [such as poor evasion of the femaleimmune system (18) or suboptimal interac-tions with the oviductal epithelium (19)],possible interventions would include in vi-tro fertilization (IVF) or intracytoplasmicsperm injection (ICSI) (Fig. 1A). For IVF,the sperm and an egg are combined in a mi-crodroplet, and the sperm can fertilize the egg naturally, albeit outside of the body. For ICSI, the sperm is injected directly into the cytoplasm of the egg. For both assisted re-production techniques, embryos are placed in the uterus a few days a er fertilization; if an embryo implants, a pregnancy results.With either technique, any requirement for the DEFB126 protein coat would be by-passed because DEFB126 must be removedfrom the sperm as a nal step before fertil-ization (23).It is too soon to predict how this new knowledg e about the DEFB126 g enotype(13) may aect the multifaceted decisions reg arding the use of assisted reproduc-tion technolog ies for human preg nancy. Itis clear that this genetic information could lead to more informed assisted reproduc-tion, in addition to possible new methods of treating sperm. In the meantime, we eagerly await a more complete understanding of the epidemiolog y of the DEFB126 del variant and of the mechanisms by which it increases time to pregnancy.REFERENCES AND NOTES 1. M. G. Hull, C. M. Glazener, N. J. Kelly, D. I. Conway, P. A.Foster, R. A. Hinton, C. Coulson, P. A. Lambert, E. M. Watt,K. M. 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DEFB126 helps the sperm to penetrate the cervical mucus, probably helps them to evade the female immune system, and promotes their attachment to the epithelial lining of the oviducts. (B ) Macaque sperm stained with an antibody for DEFB126. Reproduced with permission from (16). CervixFertile sperm with DEFB126 coating Less-fertile sperm, which lack the DEFB126 coatingClinical implication: Treat sperm to provide a coating of glycosylated DEFB126Location of cervical mucusUterusOvary Oviduct AB Oviduct OvaryTestisEpididymisClinical implication: Intrauterine insemination Clinical implications:In vitro fertilizationIntracytoplasmic sperm injectiono n J u l y 21, 2011s t m .s c i e n c e m a g .o r g D o w n l o a d e d f r o mloaded from /content/media/op-timalevaluation2010.pdf.4. P. J. Rowe, F. H. Comhaire, T. B. Hargreave, A. M. A. Mah-moud, WHO Manual for th e Standardized Investigation and Diagnosis of the Infertile Male (Cambridge Univ. Press, Cambridge, 2000).5. C. Krausz, S. 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Showstack, The economic impactof the assisted reproductive technologies. Nat. Cell Biol.4,(Suppl. 1), S29–S32 (2002).29. Funding: The author is supported by the Singapore Min-istry of Health and the Agency for Science, Technology,and Research. Competing Interests: The author declaresno competing interests.Citation: S. Rozen, Defending male fertility. Sci. Transl. Med.3, 92ps31 (2011).10.1126/scitranslmed.3002743onJuly21,211stm.sciencemag.orgDownloadedfrom。