fertility supplement review Genetic dissection of mammalian fertility pathways Martin M. Matzuk*†‡# and Dolores J. Lamb†§ Departments of *Pathology, †Molecular and Cellular Biology and ‡Molecular and Human Genetics, and §Scott Department of Urology, Baylor College of Medicine, Houston, TX 77030, USA #e-mail: [email protected] The world’s population is increasing at an alarming rate and is projected to reach nine billion by 2050. Despite this, 15% of couples world-wide remain childless because of infertility. Few genetic causes of infertility have been identified in humans; nevertheless, genetic aetiologies are thought to underlie many cases of idiopathic infertility. Mouse models with reproductive defects as a major phenotype are being rapidly created and discovered and now total over 200. These models are helping to define mechanisms of reproductive function, as well as identify potential new contracep- tive targets and genes involved in the pathophysiology of reproductive disorders. With this new information, men and women will continue to be confronted with difficult decisions on whether or not to use state-of-the-art technology and hormonal treatments to propagate their germline, despite the risks of transmitting mutant genes to their offspring. espite advances in assisted reproductive have been produced by spontaneous muta- Where it all begins technologies, infertility is a major health tions, fortuitous transgene integration, Reproductive development and physiology problem worldwide. Approximately 15% of retroviral infection of embryonic stem are evolutionarily conserved processes couples are unable to conceive within one cells, ethylnitrosurea (ENU) mutagenesis across eutherian mammalian species and year of unprotected intercourse. The fertil- and gene targeting technologies3,7,8. These many other vertebrates, including marsupi- ity potential of a couple is dependent on mutations affect all aspects of reproduc- als15, amphibians, reptiles, birds and the coordinated and combined functions of tion, including ovarian development and fish16–19. Several genes required for verte- Dboth male and female reproductive sys- function, testis determination, spermatoge- brate fertility are also highly conserved in tems. Anatomic defects, gametogenesis dys- nesis, sperm function, genital tract devel- evolution, with orthologues in Drosophila function, endocrinopathies, immunologic opment and function, sexual behaviour, melanogaster (for example, vasa (DDX4), problems, ejaculatory failure and environ- fertilization and early embryonic develop- fat facets (DFFRY) and boule (DAZ)20–22). mental exposures are significant causes of ment, and therefore have contributed Propagation of the vertebrate germline infertility. Although several infertility dis- much to our understanding of infertility. requires development of the gonads, the orders are associated with defined genetic For example, male infertility is observed in site of future gamete production. The indif- syndromes (for example, cystic fibrosis and the spontaneous mutant models hypogo- ferent gonad forms during foetal develop- Turner’s Syndrome1,2), almost a quarter of nadal (hpg)9 and testicular feminization ment, primordial germ cells enter the clinical infertility cases of either sex are (tfm)10 and in models created by transgene gonad primordium and the tissue eventu- idiopathic in nature, in part as a result of a integration, such as the kisimo mouse ally differentiates along a female (ovarian) poor understanding of the basic mecha- model (which arose by transgene disrup- or male (testicular) pathway; this differen- nisms regulating fertility. It is thought that tion of the Theg gene11) and retroviral dis- tiation dictates the formation of the sec- genetic defects underlie many of these ruption of the Bclw12, Mtap13 and Spnr14 ondary sex organs19. Although there may be unrecognized pathologies. On the basis of genes. These models are improving our spatiotemporal variations of these process- over 200 infertile or subfertile genetic knowledge of the genetic basis of mam- es in different species (for example, in mice mouse models (see Supplementary malian infertility and suggest that in the and humans, gonadal sex determination Information Table; also see ref. 3), it is not future, clinical technologies must advance occurs in utero, whereas in marsupial surprising that the diagnosis of idiopathic to enable analysis of many more genes mammals, it occurs after birth), they even- infertility is common in the clinic4,5. when an infertile couple enters the clinic. tually yield ovaries that produce eggs or In this review, we discuss causes of mam- Currently, karyotype analysis, sequence testes that generate spermatozoa. malian infertility with an emphasis on the analysis of the cystic fibrosis transmem- Defects in sexual differentiation path- genetic basis of fertility defects in humans brane conductance regulator gene and Y ways can cause infertility in mice and and mice. Animal models have defined key chromosome deletion analysis (for males) humans of both sexes (Fig. 1)23–25. In 1959, signalling pathways and proteins involved are the only genetic tests commonly offered through the analysis of human XO in reproductive physiology6. Mouse models to infertile patients4,5. (Turner syndrome) females and XXY Nature Cell Biology & Nature Medicine www.nature.com/fertility s41 fertility supplement From a distance, they will Sex determination/development Endocrinopathies come In both sexes, the primordial germ cells Hypogonadotropic Pseudohermaphroditism (PGCs) are defined histologically as alka- Hypogonadism (GNRH, KAL, (NR5A1) PC1, GNRHR, LEP, PCSK1) line-phosphatase-positive embryonic Sex reversal (SOX9, SRY, Pituitary/gonadotropin defects cells39,40. In the mouse, these cells divide NR0B1) (LHB, CGLHR, HESX1, LHX3, Denys-Drash (WT1) rapidly under the influence of transform- PROP1) Pseudovaginal perineoscrotal Steroid biosynthesis (StAR, ing growth factor-β (TGF-β) superfamily hypospadias (SRD5A, SRD5A2) CYP21, TDD, CYP17) Cryptorchidism signals; knockout models lacking bone Steroid metabolism (SRD5, (HOXA10, INSL3, GREAT) SRD5A) morphogenetic protein-4 (BMP-4) or CBAVD (CFTR) Steroid action (AR, ESR1) PMDS (AMH, AMHR) BMP-8b, or the downstream cytoplasmic- to-nuclear relay proteins, SMAD1 and Genetic aetiologies of SMAD5, have defects in PGC develop- human male infertility ment41–44. At mid-gestation, the PGCs begin one of the longest journeys of any mam- Myotonic dystrophy malian cell, migrating from their origin at Klinefelter syndrome (DMPK) (XXY-XXXY) the base of the yolk sac, along the hind-gut, Noonan syndrome (PTPN11) Mixed gonadal dysgenesis Sickle cell anaemia (HBB) to eventually enter the genital ridge. Factors (45X,46XY) β-thalassemia (HBB) Translocations (including required for this migration in humans are Kartagener syndrome Robertsonian) (DNAI1,DNAH5) unknown, although chemoattractants and Inversions Primary ciliary dyskinesia Deletions cell adhesion factors have been implicat- (DNAI1, DNAH5) Y Chromosome microdeletions 45 Fanconi anaemia (FANCA) ed . In the mouse, mutations in Kit recep- XX male Ataxia telangiectasia XY female tor (KITR) and Kit ligand (KITL) genes (ATM) block PGC migration, causing infertility, but not altering sexual differentiation46. Sperm production and function Chromosomal (numerical/structural) Few known human mutations result in a reduction of the PGC or follicle pool, Figure 1 Genetic aetiologies of human male infertility. Developmental disorders causing human male although girls with Turner’s syndrome infertility result from a failure of gonadal development or testis determination, endocrinopathies, well- (partial or complete X-chromosome known genetic syndromes, and numerical and structural chromosomal abnormalities (translocations, monosomy), have streak (remnant) gonads deletions and inversions). with no oocytes. Many Turner’s syndrome cases with ovarian failure seem to be caused (Kleinfelter syndrome) males, as well as the SRY protein; not surprisingly, this by loss of the short arm of the X-chromo- XO and XX female mice and XY male region is highly conserved among differ- some2. Among the candidate Turner’s syn- mice, it was concluded that the Y chromo- ent species32. HMG box-containing pro- drome ovarian failure genes are ZFX, some was male determining26–28. teins typically bind and significantly bend BMP15, UBE1 and USP9X (ref. 2). An Subsequent chromosomal and genetic DNA and function as transcription fac- absence of Zfx in mice results in a loss of studies of humans and mice with sex tors or facilitators of transcription. germ cells and subfertility in both sexes47; reversal syndromes and infertility revealed Several genes upstream and downstream loss of BMP15 in sheep causes a block at the that many XX males have translocations of of SRY in the sex determination pathway primary follicle stage and infertility48. a small piece of the Y chromosome, so that are now known (reviewed in ref. 23). For Studies in XO mice suggest that abnormal the sex-determining region Y gene (SRY) example, XY female sex reversal correlates chromosomal segregation contributes to results in testis development. Similarly, XY with a duplication of the human X-linked germ cell problems49, indicating that multi- females often have inactivating mutations gene DAX1 (ref. 33) or haploinsufficiency ple factors are responsible for these human in the SRY gene, resulting in the develop- of the autosomal SOX9 gene32,34–36. ovarian abnormalities. ment of ovaries29,30.