1. Introduction
According to the definition of the World Health Organization, infertility means that a couple cannot become pregnant naturally within 1 year after normal intercourse without contraception (Ford, Reference Ford2010). Here, 10–15% of gestational-age couples in the world are infertile, and male infertility accounts for 50%. Teratozoospermia may be the main cause of male infertility (Coutton et al., Reference Coutton, Escoffier, Martinez, Arnoult and Ray2015). Teratozoospermia is a disease in which there a high proportion of abnormal sperm counts in semen, which greatly impairs male fertility and is a common cause of fertilization failure. It is characterized by defects in sperm morphology. The percentage of sperm with normal morphology is less than 4%, mainly divided into head, neck, middle, and/or tail defects (Yatsenko et al., Reference Yatsenko, O’Neil, Roy, Arias-Mendoza, Chen, Murthy, Lamb and Matzuk2012). Abnormal sperm morphology can lead to decreased vitality and affect fertility (Sha et al., Reference Sha, Ding, Wu, Lin, Wang, Ji and Li2017a).
The pathogenesis of teratozoospermia is still unclear, and it is generally believed to be related to factors such as inflammation, oxidative stress, obesity, and genetics. With the continuous improvement in research methods, studies have shown that sperm is regulated by genes in the process of formation. Gene deletion, mutation, and abnormal expression may all lead to the formation of abnormal sperm. Therefore, identifying the related genes that cause teratozoospermia and clarifying its mechanism of action have important practical significances for the occurrence, development, diagnosis, and treatment of teratozoospermia.
2. Overview of teratospermia
2.1 Definition of teratozoospermia
Sperm morphological defects can be divided into head defects, neck and mid-segment defects, main-segment defects, and excess residual cytoplasm.
Malformation of the sperm head includes globozoospermia, macrospermia, and needle spermatozoa:
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(1) Globozoospermia: Globozoospermia is described as the absence of an acrosomal cap in round-headed spermatozoa (Ghédir et al., Reference Ghédir, Ibala-Romdhane, Okutman, Viot, Saad and Viville2016). The main feature of this disease is that the round-headed sperm lacking acrosome cannot enter into the oocyte during fertilization (Ghédir et al., Reference Ghédir, Braham, Viville, Saad and Ibala-Romdhane2019).
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(2) Macrozoospermia: Macrozoospermia is a rare sperm abnormality characterized by the presence of nearly 100% abnormal spermatozoa. This is a rare but serious genetic disease that causes male infertility due to an oversized irregular head, abnormal midpiece and acrosome, and multiple flagella in the ejaculate (Ray et al., Reference Ray, Touré, Metzler-Guillemain, Mitchell, Arnoult and Coutton2017).
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(3) Needle spermatozoa: In addition to the types of head deformities described above, there are also small head deformities, tapered heads, and irregular shapes (Dehghanpour et al., Reference Dehghanpour, Tabibnejad, Fesahat, Yazdinejad and Talebi2017; Jiang et al., Reference Jiang, Wang, Zhang, Xiao, Zhang, Liu, Xu, Li and Shen2019a).
Sperm tail deformities include sperm neck defects and tail deformities:
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(1) Acephalic spermatozoa: Acephalic spermatozoa is a common type of teratozoospermia in humans and one of the causes of mammalian sterility (Martínez-Rodríguez et al., Reference Martínez-Rodríguez, Alvarez, López-Urueña, Gomes-Alves, Anel-López, Tizado, Anel and de Paz2015).
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(2) Malformation of the tail:
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(i) Multiple morphological abnormalities of the flagella (MMAF): MMAF is defined as the presence of sperm in ejaculation with abnormal morphology of the flagella, for example, a flagellate, short, curved, coiled, and irregular flagella. Up to 20% of MMAF cases have a genetic origin (Ben Khelifa et al., Reference Ben Khelifa, Coutton, Zouari, Karaouzène, Rendu, Bidart, Yassine, Pierre, Delaroche, Hennebicq, Grunwald, Escalier, Pernet-Gallay, Jouk, Thierry-Mieg, Touré, Arnoult and Ray2014).
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(ii) Dysplasia of the fibrous sheath (DFS): DFS refers to the ‘short-tail’ or ‘stump’ defect of the flagella of the hairs. It is characterized by male infertility and abnormally shaped flagella (Dávila Garza and Patrizio, Reference Dávila Garza and Patrizio2013).
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(iii) Primary ciliary dyskinesia (PCD): PCD, also called ‘Kartagener’s syndrome’, belongs to a heterogeneous group of hereditary autosomal-recessive inherited diseases, which are characterized by reduced or no activity of cilia (Sironen et al., Reference Sironen, Shoemark, Patel, Loebinger and Mitchison2020).
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2.2 Clinical effect of teratozoospermia
2.2.1 The effect of teratospermia on infertility
Teratozoospermia can cause infertility due to abnormal sperm structure, and most teratozoospermia are caused by abnormal head acrosomes. One study (Liu et al., Reference Liu, Stewart and Baker2003) found that the zona pellucida-induced acrosome reaction was significantly positively correlated with morphologically normal sperm. The sperm–zona pellucida binding rate was significantly reduced in teratozoospermia patients (Liu and Baker, Reference Liu and Baker2003). Malformed sperm will affect the structure of glycosyl and glycosyl-binding proteins on the surface of sperm, and affect the ability of sperm to fix to the zona pellucida, so that sperm cannot penetrate the zona pellucida and egg membrane to achieve fertilization.
2.2.2 The effect of teratozoospermia on abortion
A recent study (Rondanino et al., Reference Rondanino, Duchesne, Escalier, Jumeau, Verhaeghe, Peers, Mitchell and Rives2015) showed that abnormal sperm morphology and chromosomal abnormalities in patients with decapitation sperm syndrome (lack of tail and only head) would directly affect ICSI pregnancy failure.
3. Research on related genes of teratozoospermia
The aetiology of teratozoospermia is complex and closely related to many factors. Large numbers of studies have shown that there is a close relationship between genetic abnormalities and teratozoospermia. Some evidence ha suggested that genetic factors, such as gene mutation, gene deletion, and abnormal gene expression, can cause various special types of teratozoospermia, which are summarized in the following sections. Abnormalities of spermatozoa gene are listed in Table 1.
Classification of teratozoospermia for genetic abnormalities
3.1 Gene deletion
3.1.1 Globozoospermia
3.1.1.1 PICK1 gene
The human PICK1 gene is located on chromosome 22 and contains 13 exons, encoding a 415 amino acid protein (Liu et al., Reference Liu, Shi and Lu2010; Modarres et al., Reference Modarres, Tanhaei, Tavalaee, Ghaedi, Deemeh and Nasr-Esfahani2016). The PICK1 protein contains a PDZ (PSD-95, Dlg, and ZO1) domain and a Bin, amphiphysin, and Rvs (BAR) domain (He et al., Reference He, Xia, Tsang, Chow and Xia2015), and is important for protein and vesicle trafficking. It is highly expressed in round spermatids and localizes to the vesicles between the Golgi apparatus and the acrosomes (Xiao et al., Reference Xiao, Kam, Shen, Jin, Wang, Lee, Jiang and Xia2009). The lipid binding of the PICK1 BAR domain is positively regulated by the PDZ domain and negatively regulated by the C-terminal acid domain. A homozygous missense mutation (G198A) in the C-terminal domain, which destroys a PvuII site, eventually resulted in round-headed sperm formation (Liu et al., Reference Liu, Shi and Lu2010; Modarres et al., Reference Modarres, Tanhaei, Tavalaee, Ghaedi, Deemeh and Nasr-Esfahani2016). In addition, PICK1 participates in vesicle transport between the Golgi apparatus and the acrosome, and the PDZ domain regulates the transport process. Deletion of PICK1 affects the transport of precursor particles, which indirectly leads to insufficient fusion of precursor particles, in turn affecting acrosome formation. Alternatively, the lack of PICK1 may cause membrane proteins to mismatch into precursor particles (Xiao et al., Reference Xiao, Kam, Shen, Jin, Wang, Lee, Jiang and Xia2009; He et al., Reference He, Xia, Tsang, Chow and Xia2015).
3.1.1.2 SIRT1 gene
Sirtuins are a family of NAD+-dependent enzymes (Haigis and Sinclair, Reference Haigis and Sinclair2010). Mammals possess seven Sir2 type genes (SirT), namely Sirt1 to Sirt7 (Verdin et al., Reference Verdin, Hirschey, Finley and Haigis2010). In a study using mice, deletion mutations in the Sirt1 gene disrupted the sperm cell LC3 and Atg7 deacetylation, affecting the redistribution of LC3 from the nucleus to the cytoplasm and activation of autophagy. The deletion of Sirt1 prevents LC3 from being recruited to Golgi-derived vesicles.
3.1.1.3 GOPC gene
GOPC (Golgi-associated PDZ- and coiled-coil motif-containing protein) is abundantly found in the trans-Golgi region in round spermatids (Suzuki-Toyota et al., Reference Suzuki-Toyota, Ito, Toyama, Maekawa, Yao, Noda, Iida and Toshimori2007; Yatsenko et al., Reference Yatsenko, O’Neil, Roy, Arias-Mendoza, Chen, Murthy, Lamb and Matzuk2012). In mice, GOPC collaborates with PICK1 to participate in vesicle transport in acrosomes. Both Pick1-knockout and Gopc-knockout mice show acrosome fragmentation during spermiogenesis (Jiang et al., Reference Jiang, Liang, Gao, Liu, Han, Wang and Zhang2019b). GOPC interacts with CCDC62 in mouse testes and is regulated by CCDC62. Upon deletion of the Gopc gene in the mouse, vesicles are transported from the Golgi apparatus and the acrosome in the sperm disappear completely (Li et al., Reference Li, Li, Lin, Yang, Huang, Wu, Chen, Yang, Luo, Guo, Chen, Wang, Ma, Gu, Mou, Jiang, Xia and Gui2017). These findings indicated that GOPC plays an important role in vesicle transport and acrosome fusion. It is also involved in the formation of abnormal sperms (Yao et al., Reference Yao, Ito, Natsume, Sugitani, Yamanaka, Kuretake, Yanagida, Sato, Toshimori and Noda2002; Wang et al., Reference Wang, Wan, Li, Liu, Chen, Wang, Yang, Tang, Zhang, Duan, Zhao, Gao and Li2014).
3.1.1.4 GBA2 gene
The GBA2 gene (12.36 kb) is located on the human chromosome 9p13.3, and encodes β-glucosidase. GBA2 is the only enzyme known to have true β-glucosidase activity in the endoplasmic reticulum (ER). Studies have shown that lipid transport is inhibited in round-headed sperm in mice lacking GBA2 (Roy and Lin et al., Reference Roy, Lin and Matzuk2006). The round-headed spermatozoa produced by such mice have abnormal acrosomes and show the accumulation of glycosylceramide, which may prevent acrosome formation (Walden et al., Reference Walden, Sandhoff, Chuang, Yildiz, Butters, Dwek, Platt and van der Spoel2007).
3.1.1.5 PCI gene
The PCI (serpin peptidase inhibitor; 11.73 kb) gene is located on human chromosome 14q32.1, and encodes a serine protease inhibitor. In contrast with serine protease, PCI is a serine protease inhibitor (serpin), including protein C (PC) and plasminogen activators (Odet et al., Reference Odet, Guyot, Leduque and Le Magueresse-Battistoni2004). Male mice lacking PCI are unable to resist proteolytic activity, and unrestricted proteolytic activity may directly or indirectly disrupt the Sertoli cell barrier (Uhrin et al., Reference Uhrin, Dewerchin, Hilpert, Chrenek, Schöfer, Zechmeister-Machhart, Krönke, Vales, Carmeliet, Binder and Geiger2000). Sperm acrosomes are also destroyed in the absence of PCI (Elisen et al., Reference Elisen, van Kooij, Nolte, Marquart, Lock, Bouma and Meijers1998).
3.1.1.6 CREM gene
The CREM (cyclic AMP responsive element modulator) gene is located at chromosome 10p11.21, and has a length of 86.12 kb (Liu et al., Reference Liu, Wu and Wang2013). The CREM protein is thought to be important for mammalian spermatogenesis (Kramer et al., Reference Kramer, Koornneef, de Laat and van den Eijnden-van Raaij1991). All male mice lacking the CREM protein are infertile, because sperm do not complete the differentiation process normally (Blendy et al., Reference Blendy, Kaestner, Weinbauer, Nieschlag and Schütz1996; Krausz and Sassone-Corsi, Reference Krausz and Sassone-Corsi2005), leaving the sperm at the round sperm stage (Krausz and Sassone-Corsi, Reference Krausz and Sassone-Corsi2005).
3.1.1.7 TH2A and TH2B genes
TH2A (393 bp) and TH2B (470 bp) (testicular histone H2A and H2B, respectively) genes are both located on rat 17p11. TH2A and TH2B are highly expressed during spermatogenesis and play a vital role in the change of chromatin structure during spermatogenesis; they induce an open chromatin structure (Huynh et al., Reference Huynh, Shinagawa and Ishii2016). During spermatogenesis, chromatin in round spermatozoa condenses because most of the histones are replaced by transition proteins and then by PRM. Studies have shown that disruption of Th2a and Th2b can lead to sperm defects, especially affecting the two steps involved in the normal release of adhesin between meiosis I and meiosis II and histone replacement during spermatogenesis, leading to changes in sperm morphology (Shinagawa et al., Reference Shinagawa, Huynh, Takagi, Tsukamoto, Tomaru, Kwak, Dohmae, Noguchi and Ishii2015).
3.1.1.8 HRB gene
HRB is an HIV-1 Rev binding/interacting protein and an important cofactor in the Rev export pathway (Kierszenbaum et al., Reference Kierszenbaum, Tres, Rivkin, Kang-Decker and van Deursen2004). Studies have shown that the Hrb gene is expressed in large amounts during spermatogenesis. The transformation of the Hrb gene-deficient round spermatids into testicular spermatozoa (spermatogenesis) is significantly disrupted. The resulting mutant microtubules do not have elongated spermatids, and only contain round-headed sperm cells that lack acrosome structure (Kang-Decker et al., Reference Kang-Decker, Mantchev, Juneja, McNiven and van Deursen2001). At the same time, HRB interacts with the EPS15 homology domain (EH) protein and, as part of the EH network, it plays an important role in vesicle classification. If their sperm lacks acrosomes, defects such as multiple tails, amorphous heads, and multiple nuclei appear (Kopp et al., Reference Kopp, Sukura, Tuunainen, Gustavsson, Parvinen and Andersson2007).
3.1.1.9 Csnk2a2 gene
Csnk2a2 is considered to be one of the pathogenic genes associated with round-headed sperm (Pirrello et al., Reference Pirrello, Machev, Schimdt, Terriou, Ménézo and Viville2005). The Csnk2a2 gene is located on mouse chromosome 8, and the human homolog is located on 16q21. This gene belongs to the casein kinase II gene family, which encodes a protein that is associated with the nuclear matrix during the later stages of spermatogenesis. Studies have found that Csnk2a2 is preferentially expressed in the late stage of spermatogenesis. Homozygous mice with the Csnk2a2 gene deletion are infertile due to oligospermia and round-headed spermatozoa, and their sperm acrosomes often detach from the sperm nucleus and disappear during spermatogenesis. Currently, the heterodimeric partner Csnk2b of the Csnk2a2 gene is also considered to be a candidate gene for balospermia.
3.1.2 Macrozoospermia
3.1.2.1 AURKC gene
The AURKC (Aurora Kinase C) gene is located on 19q13.3-qter. It contains seven exon sequences and is highly expressed in the testes (Jedidi et al., Reference Jedidi, Ouchari and Yin2018). Human AURKC and mouse AURKC have 82.1% amino acid identity in the kinase domain, 68.8% amino acid identity in the N-terminal domain, and only 26.7% identity in the C-terminal domain. This difference in amino acid content indicates specific differences between the species (Quartuccio and Schindler, Reference Quartuccio and Schindler2015). A 1-bp deletion in the AURKC gene (c.144delC) interferes with the CPC positioning and results in a metaphase chromosomal misalignment. This deletion produces a frameshift that causes a change from leucine to tryptophan codon at amino acid 49. The 22 missense residues after the frameshift are translation stop codons that cause protein truncation and block meiosis, resulting in the appearance of tetraploids and a large number of flagella (Chianese et al., Reference Chianese, Fino, Riera Escamilla, López Rodrigo, Vinci, Guarducci, Daguin, Muratori, Tamburrino, Lo Giacco, Ars, Bassas, Costa, Pisatauro, Noci, Coccia, Provenzano, Ruiz-Castañé and Giglio2015; Fellmeth et al., Reference Fellmeth, Ghanaim and Schindler2016; Ray et al., Reference Ray, Touré, Metzler-Guillemain, Mitchell, Arnoult and Coutton2017). Studies have shown that, in the cohort of patients with megaspermia, the frequency of the AURKC mutation was 78.6% (11/14); the c.144delC mutant accounted for 91% of the mutant alleles, and the rest were p.Y248 (Ounis et al., Reference Ounis, Zoghmar, Coutton, Rouabah, Hachemi, Martinez, Martinez, Bellil, Khelifi, Arnoult, Fauré, Benbouhedja, Rouabah and Ray2015).
3.1.3 Needle spermatozoa
3.1.3.1 SPATA6 gene
The SPATA6 gene is an evolutionarily conserved testis-specific gene and also a spermatogenesis-related gene. The SPATA6 gene consists of 15 exons, encoding a protein of 488 amino acid residues. It encodes the proteins required for the formation of the segmented column and the head, two major structures of the sperm junction that are essential for connecting the developing flagella to the head during late spermatogenesis. It is specifically expressed in haploid germ cells, and SPATA6 deletion may lead to needle spermatozoa or azoospermia. Yu et al. (Reference Yu, Vanhorne and Oko2009) studied the mouse animal model of headless sperm; proteomic results found that the SPATA6 gene was involved in the transport of myosin microfilaments and, to some extent, affected the function of motor proteins in sperm cell myosin. Thereby affecting the transformation of sperm cells into sperm with normal shape and function, and finally generating needle sperm or headless sperm.
3.1.4 Acephalic spermatozoa
3.1.4.1 SPATA6 gene
The SPATA6 gene deletion impairs the construction of segmented columns based on myosin-based microfilament transport. Partial or complete lack of segmented columns during the development of connectors that extend sperm cells or cause sperm to become part of a segmented column results in headless sperm (Shang et al., Reference Shang, Yan, Tang, Liu, Xiao, Guo, Yuan, Chen, Jiang, Guo, Qiao and Li2018; Abu-Halima et al., Reference Abu-Halima, Ayesh, Hart, Alles, Fischer, Hammadeh, Keller, Huleihel and Meese2019; Sujit et al., Reference Sujit, Singh, Trivedi, Singh, Gupta and Rajender2020).
3.1.4.2 ODF1 gene
The ODF1 (dense outer fibre of sperm tail 1) gene is located on human chromosome 8q22.3, with a total length of 9.40 kb. In mice, the Odf1 gene is a single-copy gene located in the B2–C region of chromosome 15. ODF1 has been assigned as a small heat shock protein (sHSP), leading it to be renamed HSPB10 (Yang et al., Reference Yang, Grzmil, Meinhardt and Hoyer-Fender2014). ODF1 defects lead to abnormal spermatogenesis, with some developmental defects in the implant plate and thin-layered fibres (Hetherington et al., Reference Hetherington, Schneider, Scott, DeKretser, Muller, Hondermarck, Velkov and Baker2017). The protein complex consisting of the core proteins ODF2/ODF1/CCDC42 forms a rigid scaffold that is essential for the formation of the connector and sperm tail. In the absence of any of these proteins, the rigid scaffold is destroyed, resulting in the failure of the formation of the connection complex and the sperm tail, resulting in sperm deformity (Tapia Contreras and Hoyer-Fender, Reference Tapia Contreras and Hoyer-Fender2019).
3.1.4.3 HOOK1 gene
The HOOK1 (Hook homolog 1) gene is located on human chromosome 1p32.1, it contains 22 exons, and is 61.52 kb in length. In the sperm of the azh mutant mice, the deletion of two exons in the Hook1 gene leads to the production of truncated proteins that can affect the intra-manchette transport processes, resulting in abnormal sperm heads, detached tails, and aberrant manchette positioning (Zhou et al., Reference Zhou, Du, Qin, Hu, Huang, Bao, Han, Mansouri and Xu2009; Schwarz et al., Reference Schwarz, Prieler, Schmid, Grzmil and Neesen2017; Tapia Contreras and Hoyer-Fender, Reference Tapia Contreras and Hoyer-Fender2019).
3.1.4.4 OAZ3 gene
OAZt/OAZ3 is a member of the ornithine decarboxylase anti-enzyme family and is only expressed in the haploid germ cells of the testis. The OAZ3 gene contains five exons and four introns, and the sequence of the exons is the same as the cDNA sequence (Ike et al., Reference Ike, Yamada, Tanaka, Nishimune and Nozaki2002). Sperm head–tail separation was observed in Oaz3-knockout mice. It is speculated that a lack of Oaz3 may cause the accumulation of amines, which affects the structure of sperm junctions (Tokuhiro et al., Reference Tokuhiro, Isotani, Yokota, Yano, Oshio, Hirose, Wada, Fujita, Ogawa, Okabe, Nishimune and Tanaka2009). By studying the protein p12 expressed by Oaz3 in rats, it was found that p12 plays a role in signal transduction by regulating protein phosphatase and may play a role in the assembly of the head–tail-coupling apparatus (HTCA), without the p12 head–tail connection being fragile. The ultrastructure of the separated head and tail appeared normal. Fractures occurred on the base plate and the struts, indicating that the connector was fragile (Ruan et al., Reference Ruan, Cheng, Ou, Oko and van der Hoorn2011).
3.1.4.5 SPEM1 gene
SPEMI (spermatid maturation 1) is one of the spermiogenesis-essential genes and is exclusively expressed in the testis. Spem1-knockout mice display a ‘head-bent-back’ phenotype and show cytoplasmic droplet-like remnants translocated to the junction between the head and the neck (Bao et al., Reference Bao, Zhang, Zheng, Xu and Yan2010). In the absence of SPEM1, many proteins originally degraded by the ubiquitin-proteasome system may be retained, resulting in abnormal sperm cell development and cytoplasmic abnormalities in the later stages (Bao et al., Reference Bao, Zhang, Zheng, Xu and Yan2010).
3.1.5 MMAF
3.1.5.1 DNAH1 gene
DNAH1 (MIM #603332) was the first gene formally identified in humans with mutations that caused an MMAF phenotype and male sterility (Touré et al., Reference Touré, Martinez, Kherraf, Cazin, Beurois, Arnoult, Ray and Coutton2021). DNAH1 encodes for inner-arm heavy-chain dynein, which is an axonemal component. This component is needed in spermatozoa to form the inner dynein arms, which are distributed as three molecular complexes in groups of 3–2–2 in three different types of inner arms IDA1–IDA3. The absence of DNAH1 eliminates the anchoring sites of radial spokes 3, which results in weakened adhesion of the two central singlet microtubules, leading to MMAF and dysplasia of the fibrous sheath (DFS) (Sha et al., Reference Sha, Yang, Mei, Ji, Wang, Ding, Li and Yang2017b).
3.1.5.2 CFAP69 gene
CFAP69 (formerly known as c7orf63; GenBank: NM_001039706) is located on chromosome 7 and contains 23 exons encoding a predicted 941-amino acid protein (A5D8W1) (Dong et al., Reference Dong, Amiri-Yekta, Martinez, Saut, Tek, Stouvenel, Lorès, Karaouzène, Thierry-Mieg, Satre, Brouillet, Daneshipour, Hosseini, Bonhivers, Gourabi, Dulioust, Arnoult and Touré2018). This gene encodes the protein of cilia and flagella-associated protein 69, which contains a WD repeat domain (He et al., Reference He, Li, Wu, Lv, Liu, Liu, Zhu, Li, Fang, Yang, Cheng, Zhang, Tan, Chen, Tang, Song, Wang, Zha and Wang2019). Cfap69 gene deletion causes MMAF in mice. Furthermore, it was shown that CFAP69 may be involved in sperm tail biogenesis and CPC assembly through flagellin transport (Nsota Mbango et al., Reference Nsota Mbango, Coutton, Arnoult, Ray and Touré2019).
3.2 Gene mutation
3.2.1 Globozoospermia
3.2.1.1 SPATA16 gene
SPATA16 (spermatogenesis-associated 16, also known as NYD-SP12) is located on chromosome 3q26.32 and is composed of 11 exons encoding a highly conserved protein of 569 amino acids, which contains a tetratricopeptide repeat (TPR [MIM 602259]) domain (Dam et al., Reference Dam, Koscinski, Kremer, Moutou, Jaeger, Oudakker, Tournaye, Charlet, Lagier-Tourenne, van Bokhoven and Viville2007; Jedidi et al., Reference Jedidi, Ouchari and Yin2018). SPATA16 is localized in the Golgi apparatus and proacrosomal granules (Karaca et al., Reference Karaca, Yilmaz, Kanten, Kervancioglu, Solakoglu and Kervancioglu2014). It is involved in the transport of proacrosomal granules to the acrosome in the round and elongated spermatids, thereby playing an essential role in acrosome formation during spermatogenesis (Chianese et al., Reference Chianese, Fino, Riera Escamilla, López Rodrigo, Vinci, Guarducci, Daguin, Muratori, Tamburrino, Lo Giacco, Ars, Bassas, Costa, Pisatauro, Noci, Coccia, Provenzano, Ruiz-Castañé and Giglio2015; Bracke et al., Reference Bracke, Peeters, Punjabi, Hoogewijs and Dewilde2018). SPATA16 gene mutation has been demonstrated to show autosomal-recessive inheritance 10. The deletion of its exons due to gene mutation can destroy the TPR domain, leading to interference with protein–protein interaction and aberrant meiosis, finally resulting in globozoospermia (Dam et al., Reference Dam, Koscinski, Kremer, Moutou, Jaeger, Oudakker, Tournaye, Charlet, Lagier-Tourenne, van Bokhoven and Viville2007; Ghédir et al., Reference Ghédir, Braham, Viville, Saad and Ibala-Romdhane2019).
3.2.1.2 ZPBP1 gene
ZPBP1 (zona pellucida binding protein 1/sp38) is localized to the acrosomal membrane (Yatsenko et al., Reference Yatsenko, O’Neil, Roy, Arias-Mendoza, Chen, Murthy, Lamb and Matzuk2012). In mice, Zpbp1 is 184 kb long, has eight exon configurations and is located on chromosome 11 (Lin et al., Reference Lin, Roy, Yan, Burns and Matzuk2007). The zona pellucida binding protein gene knockout (Zpbp12/2) shows that an acrosomal protein (IAM38/Sp38/ZPBP1) can play a critical architectural role during acrosomal formation (Yu et al., Reference Yu, Vanhorne and Oko2009). In mice, the sperm containing the ZPBP1 gene knockout shows acrosome fragmentation, disrupted Sertoli–spermatid junctions, and defective sperm head morphology (Liu et al., Reference Liu, Shi and Lu2010). This feature is similar to that observed in human teratozoospermia. Studies have shown that abnormal head morphology may be due to a lack of functional ZPBP1 protein or due to the negative effects of the mutant ZPBP1 protein and other acrosome proteins required for sperm head formation (Yatsenko et al., Reference Yatsenko, O’Neil, Roy, Arias-Mendoza, Chen, Murthy, Lamb and Matzuk2012).
3.2.1.3 PRM genes
Protamines (PRMs) are arginine-rich and cysteine-rich small basic proteins that play a role in packaging paternal genomes. PRMs are conserved in some species and are only found in mature spermatozoa (Aydos et al., Reference Aydos, Hekmatshoar, Altınok, Özkan, Şakirağaoğlu, Karadağ, Kaplan, Ilgaz, Taşpınar, Yükselen, Sunguroğlu and Aydos2018). Humans and mice have three different protamine genes: Prm1, Prm2, and Prm3. In mice, PRM1 is an arginine- and cysteine-rich protein of 50 amino acids, whereas PRM2 is rich in histidine and contains 106 amino acids (Takeda et al., Reference Takeda, Yoshinaga, Furushima, Takamune, Li, Abe, Aizawa and Yamamura2016). The PRM1 and PRM2 genes are located on chromosome 16p13.2, spanning a 28.5-kb region, and play a critical role in spermatid differentiation (Tüttelmann et al., Reference Tüttelmann, Křenková, Römer, Nestorovic, Ljujic, Stambergová, Macek, Macek, Nieschlag, Gromoll and Simoni2010; Ganguly et al., Reference Ganguly, Gaur, Kumar, Mandal, Kumar, Singh, Kumar and Sharma2013). Both genes contain a single intron. In the process of spermatogenesis, the compaction of sperm chromatin requires testicular-specific nuclear proteins called transition proteins and PRM. The transition proteins convert sperm nuclear proteins from histones to PRM. Deletion or mutation of the PRM gene results in protamine deficiency in the sperm nucleus. As a result, sperms appear as round-headed sperms or show other head morphological abnormalities. In addition, abnormal ratios of protamine P1 and P2 can also cause abnormal morphology of the sperm head (Steger et al., Reference Steger, Fink, Klonisch, Bohle and Bergmann2002; Ravel et al., Reference Ravel, Chantot-Bastaraud, El Houate, Berthaut, Verstraete, De Larouziere, Lourenço, Dumaine, Antoine, Mandelbaum, Siffroi and McElreavey2007; Tüttelmann et al., Reference Tüttelmann, Křenková, Römer, Nestorovic, Ljujic, Stambergová, Macek, Macek, Nieschlag, Gromoll and Simoni2010; Ganguly et al., Reference Ganguly, Gaur, Kumar, Mandal, Kumar, Singh, Kumar and Sharma2013; Utsuno et al., Reference Utsuno, Miyamoto, Oka and Shiozawa2014).
3.2.2 Acephalic spermatozoa
3.2.2.1 PRSS21 gene
Serine proteases play an important role in sperm production, maturation, and functional capacity. Several lines of evidence have suggested that one or more trypsin-like serine proteases are essential for successful fertilization in mammals (Swegen et al., Reference Swegen, Smith, Gibb, Curry and Aitken2019). PRSS21 (also known as testosterone, esp-1, tryptase 4, and TESP5) is a tryptase-like protease that is abundantly expressed by male germ cells and sperm. PRSS21 is a unique gene family in the homologous region of the chromosome and is located at 16p13.3 on the human chromosome and on chromosome 17 in mice (Netzel-Arnett et al., Reference Netzel-Arnett, Bugge, Hess, Carnes, Stringer, Scarman, Hooper, Tonks, Kay and Antalis2009). PRSS21 plays an important role in guiding the maturation of sperm cells in the epididymis and the process of sperm fertilization. A sperm lacking PRSS21 shows the characteristics of decreased motility, angulation, and fragility of the sperm neck (Liu et al., Reference Liu, Liang, Gao, Jiang, He, Han, Olfati, Manthari, Wang and Zhang2019). In a study on bull sperm, the PRSS21 (serine protease 21) gene, along with TEX14, was found to be involved in the production of male gametes. This process encodes a cell surface-anchored serine protease called testis protein, which is expressed in large quantities in the pre-meiotic stage. However, sperm lacking PRSS21 showed some defects during epididymal transport (Stafuzza et al., Reference Stafuzza, Silva, Silva, Costa Filho, Barbosa, Macedo, Lobo and Baldi2020). Prss21-mutant mouse sperm showed an increased tendency to go out. PRSS21 is highly expressed on the surface of round and elongated sperm in the testis in both humans and mice, and remains associated with sperm tails throughout the epididymal tract. PRSS21 deficiency produces headless sperm and hairpin-like structures (Dehghanpour et al., Reference Dehghanpour, Tabibnejad, Fesahat, Yazdinejad and Talebi2017).
3.2.2.2 IFT88 gene
IFT88 (intraflagellar transport protein 88) is a core anterograde or type B trafficking protein. It can transport proteins to the tip of primary cilia, so it plays a vital role in the assembly process of cilia (Coveney et al., Reference Coveney, Collins, Mc Fie, Chanalaris, Yamamoto and Wann2018). Studies have shown that IFT88 is localized in precursor vesicles, and that there is a complete actin-based Va-Rab27a/Rab27b vesicle transport system in Ift88-mutant mouse, which may facilitate acrosome–acrosome and tail development. In the presence of a normal Ift88, the development of the acrosome–acrosome enzyme complex was not significantly disrupted in the mutant. Although the actin-based transport system can partially offset the defects of the microtubule-based transport system and the IFT88 protein, this compensation cannot prevent sperm from forming abnormal heads. Similar to myosin Va and Rab27/Rab27b, GMAP210 and IFT88 also seem to be involved in HTCA and tail transport (Kierszenbaum et al., Reference Kierszenbaum, Rivkin, Tres, Yoder, Haycraft, Bornens and Rios2011b). Therefore, the Ift88 gene can participate in the development of the sperm acrosome, HTCA and tail. If Ift88 is mutated, the mouse sperm head will be abnormal and the sperm will have no tail (Pandey et al., Reference Pandey, Yadav, Vishvkarma, Singh, Maikhuri, Rajender and Gupta2019).
3.2.2.3 Cntrob gene
Centrobin (also known as NIP2 or LIP8) is a centrosome component and the corresponding gene is designated Cntrob (Entrez Gene ID: 303240; Rat Genome Database ID: 1307488) (Ogungbenro et al., Reference Ogungbenro, Tena, Gaboriau, Lalor, Dockery, Philipp and Morrison2018). The hd mutation in WHD rats was induced by retroviral element mutagenesis into intron 10 of the Cntrob gene on rat chromosome 10. The transcription of the hd allele produces exons containing retrotransposon origin. For multiple mRNA species, these exons cause truncated centrin proteins at the C-terminus. Centrin-truncated proteins may interfere with the shape of sperm heads, HTCA organization and anchoring, and sperm tail development (Liska et al., Reference Liska, Gosele, Rivkin, Tres, Cardoso, Domaing, Krejcí, Snajdr, Lee-Kirsch, de Rooij, Kren, Krenová, Kierszenbaum and Hubner2009).
3.2.3 MMAF
3.2.3.1 CFAP43 and CFAP44 genes
The CFAP43 gene has 37 exons and encodes a protein of 1665 amino acids. The CFAP44 gene has 35 exons and encodes a protein of 1854 amino acids. Both genes contain WD repeat domains and are mainly expressed in humans (Sha et al., Reference Sha, Wang, Xu, Su, Cui, Mei, Huang, Chen, He, Ji, Bao, Yang, Li and Li2019). Mice models lacking the CFAP43 and CFAP44 orthologous proteins showed male sterility and an MMAF phenotype similar to that observed in humans. Gene changes in CFAP43 and CFAP44 can result in abnormalities in cilia and flagella (Touré et al., Reference Touré, Martinez, Kherraf, Cazin, Beurois, Arnoult, Ray and Coutton2021) and CFAP43 and CFAP44 mutations induce severe axonemal disorganization (Coutton et al., Reference Coutton, Vargas, Amiri-Yekta, Kherraf, Ben Mustapha, Le Tanno, Wambergue-Legrand, Karaouzène, Martinez, Crouzy, Daneshipour, Hosseini, Mitchell, Halouani, Marrakchi, Makni, Latrous, Kharouf, Deleuze and Boland2018).
3.2.4 PCD gene
Different PCD gene mutations lead to the loss of the inner dynein arms of the cilia due to the disorder of the microtubule structure, or the lack of outer and inner dynein arms. Multiple genes have been associated with PCD, including CCNO, MCIDAS, CFAP298, CFAP300, DNAAF1, DNAAF2, DNAAF3, DNAAF4, DNAAF5, LRRC6, PIH1D3, SPAG1, ZMYND10, CCDC103, DNAH11, DNAH5, DNAH9, DNAI1, DNAI2, DNAL1, MNS1, NME8, TTC25, DNAH6, CCDC39, CCDC40, CCDC65, DRC1, GAS8, DNAJB13, RSPH1, RSPH3, RSPH4A, RSPH9, WDR66, STK36, HYDIN, GAS2L2, and LRRC56 (Sironen et al., Reference Sironen, Shoemark, Patel, Loebinger and Mitchison2020).
3.3 Abnormal gene expression
3.3.1 Globozoospermia
3.3.1.1 DPY19L2 gene
DPY19L2 is a testis-specific protein with nine transmembrane domains. The corresponding gene is located on chromosome 12q14.2, has 22 exons, two flanking low copy repeat (LCR) sequences, and an identity of 96.5% (Elinati et al., Reference Elinati, Kuentz, Redin, Jaber, Vanden Meerschaut, Makarian, Koscinski, Nasr-Esfahani, Demirol, Gurgan, Louanjli, Iqbal, Bisharah, Pigeon, Gourabi, De Briel, Brugnon, Gitlin and Grillo2012). Four LCRs are localized at the end of DPY19L2. These repeats are ∼25 and 9 kb at the 3′-end and 62 and 77 kb at the 5´-end of DPY19L2, respectively (Ghazavi et al., Reference Ghazavi, Peymani, Hashemi, Ghaedi and Nasr-Esfahani2019). In contrast with SPATA16 (Koscinski et al., Reference Koscinski, Elinati, Fossard, Redin, Muller, Velez de la Calle, Schmitt, Ben Khelifa, Ray, Kilani, Barratt and Viville2011), the DPY19L2 mutation may disrupt only the spermiogenesis process and not germ cell proliferation and meiosis. There is evidence that the lack of DPY19L2 leads to a lack of related proteins, resulting in the instability of the connection between the dense nuclear layer and the roof and the nuclear membrane in sperms. Furthermore, the acrosome and the manchette fail to be linked to the nucleus, leading to the disruption of vesicular trafficking, failure of sperm nuclear shaping, and eventually the elimination of the unbound acrosomal vesicle (Chianese et al., Reference Chianese, Fino, Riera Escamilla, López Rodrigo, Vinci, Guarducci, Daguin, Muratori, Tamburrino, Lo Giacco, Ars, Bassas, Costa, Pisatauro, Noci, Coccia, Provenzano, Ruiz-Castañé and Giglio2015). The loss of the acrosome occurs in the final stage of sperm formation (Ounis et al., Reference Ounis, Zoghmar, Coutton, Rouabah, Hachemi, Martinez, Martinez, Bellil, Khelifi, Arnoult, Fauré, Benbouhedja, Rouabah and Ray2015). Therefore, a lack of DPY19L2 affects the formation of the sperm acrosome and hinders sperm head elongation (Modarres et al., Reference Modarres, Tanhaei, Tavalaee, Ghaedi, Deemeh and Nasr-Esfahani2016).
3.3.2 Acephalic spermatozoa
3.3.2.1 SUN gene
In mammals, the SUN-domain proteins are characterized by a conserved C-terminal region of ∼175 amino acids and the presence of at least one transmembrane domain (Yang and Adham et al., Reference Yang, Adham, Meinhardt and Hoyer-Fender2018). At least five SUN-domain proteins have been reported, and three of these genes in mice have been named Sun1, Sun2, and Sun3. Two other SUN-domain proteins were originally named rat sperm-associated antigen 4 (SPAG4) and SPAG4-like (SPAG4L), and then later named Sun4 and Sun5 (Shang et al., Reference Shang, Zhu, Wang, Ouyang, Dong, Liu, Zhao, Cui, Ma, Zhang, Yang, Guo, Liu, Yuan, Gao, Guo, Sun, Cao and Li2017). Spag4 is mainly present in the testis and pancreas, and specifically expresses SPAG4 (SUN4) in the testis. It is expressed in the sperm and axons and interacts with external dense fibrin 1 during sperm head formation and rat sperm tail elongation. In Spag4-deficient mice, the function of the LINC complex was impaired. Furthermore, the absence of SPAG4 inhibits the formation of the HTCA, eventually resulting in the production of headless sperm (Kierszenbaum et al., Reference Kierszenbaum, Rivkin and Tres2011a; Zhu et al., Reference Zhu, Liu, Wang, Yang, Zhang, Wu, Zhang, He, Zhang, Zhou, Wei, Shang, Wang, Zhang, Ouyang, Sun, Cao and Li2018; Li et al., Reference Li, Wu, Huang, Yang and Xing2019; Yeh et al., Reference Yeh, Wang, Wee, Chen, Chiang, Kuo and Lin2019) SUN5 is a transmembrane protein with 379 amino acids located in the inner membrane (INM). It consists of a C-terminal N-terminal nucleoplasmic region, a transmembrane helix, a coiled-coil region, and a SUN domain. SUN5 is synthesized in the ER, then transported to the Golgi apparatus, reaches the nuclear envelope (NE), attaches to the INM, and finally moves to the junction between the head and the tail of the sperm. During sperm division, most of the head remains in the seminiferous epithelium when it separates from the flagella (Zhu et al., Reference Zhu, Liu, Wang, Yang, Zhang, Wu, Zhang, He, Zhang, Zhou, Wei, Shang, Wang, Zhang, Ouyang, Sun, Cao and Li2018; Li et al., Reference Li, Wu, Huang, Yang and Xing2019). In the absence of SUN5, effective anchoring is lost in the last step of spermatogenesis, due to which the last part of the sperm cytoplasm cannot be removed in time, and the top of the sperm will carry part of the cytoplasm. Moreover, flagella cannot be anchored properly, mitochondrial sheaths cannot be aligned properly, and axon assembly is affected. Typical cytoplasmic droplets can be found in normal ejaculated sperm or headless sperm (Pasch et al., Reference Pasch, Link, Beck, Scheuerle and Alsheimer2015; Zhu et al., Reference Zhu, Wang, Yang, Zhang, Wu, Zhang, Zhang, He, Zhou, Wei, Gecz and Cao2016; Elkhatib et al., Reference Elkhatib, Paci, Longepied, Saias-Magnan, Courbière, Guichaoua, Lévy, Metzler-Guillemain and Mitchell2017; Shang et al., Reference Shang, Yan, Tang, Liu, Xiao, Guo, Yuan, Chen, Jiang, Guo, Qiao and Li2018).
3.3.2.2 GAT1 gene
GAT1 (γ-aminobutyric acid transporter I) is located on human chromosome 3p25.3 and the full length of the gene is 46.52 kb. GAT1 is the primary neuronal transport protein in the rodent brain among the four GATs (GAT1–GAT4) (Hu et al., Reference Hu, Zhang, Ma, Jiang, Yang, Li, Yu Chi, Fei and Guo2004). The testis and sperm of transgenic mice overexpressing GAT1 were significantly abnormal. In human sperm, GABA can induce an acrosomal reaction (Zhang et al., Reference Zhang, Gui, Yuan, Bian and Guo2009).
3.4 Abnormal gene duplication
3.4.1 Globozoospermia
3.4.1.1 AR gene
The AR protein (androgen receptor) mediates androgen action by determining male sexual differentiation, initiation and promotion of spermatogenesis, and the growth of accessory sex organs (Bhanmeechao et al., Reference Bhanmeechao, Srisuwatanasagul, Prapaiwan and Ponglowhapan2018). The AR gene is a single-copy sequence composed of eight exons located on chromosome Xq11–12 (Milatiner et al., Reference Milatiner, Halle, Huerta, Margalioth, Cohen, Ben-Chetrit, Gal, Mimoni and Eldar-Geva2004). The AR gene contains two polymorphic trinucleotide repeat loci: [CAG]n, which encodes a polyglutamine bundle, and [GGC]n, which encodes a polyglycine bundle. Both loci are located in exon 1, and the trans-activation domain of the exon encoding the receptor protein contains a CAG repeat that is translated into polyglutamine. Glutamine repeats are polymorphic, with normal individuals showing 9–36 repeats. A CAG repeat length of 49 in AR was positively correlated with teratozoospermia (Tut et al., Reference Tut, Ghadessy, Trifiro, Pinsky and Yong1997).
3.5 Clinical application of gene detection of spermatozoa deformity
Different teratozoospermia patients have genetic differences. De Braekeleer et al. (Reference De Braekeleer, Nguyen, Morel and Perrin2015) reviewed the literature and showed that more than 90% of sperm with megacephalospermia were aneuploid, mainly diploid; Ounis et al. (Reference Ounis, Zoghmar, Coutton, Rouabah, Hachemi, Martinez, Martinez, Bellil, Khelifi, Arnoult, Fauré, Benbouhedja, Rouabah and Ray2015) found a homozygous aurora in Algerian megacephalospermia patients The mutation rate of the kinase C gene (AURKC) was 79%; 18 typical infertile patients with macrocephaly in Morocco were all homozygous for the c.144delC mutation in the AURKC gene (El Kerch et al., Reference El Kerch, Lamzouri, Laarabi, Zahi, Ben Amar and Sefiani2011), so the AURKC gene detection is recommended for patients with macrocephaly. The proportion of male infertility caused by round-headed spermatozoa was less than 0.1% (Perrin et al., Reference Perrin, Coat, Nguyen, Talagas, Morel, Amice and De Braekeleer2013), and several studies support that DPY19L2 deletion is the main reason, accounting for 19% of patients (Koscinski et al., Reference Koscinski, Elinati, Fossard, Redin, Muller, Velez de la Calle, Schmitt, Ben Khelifa, Ray, Kilani, Barratt and Viville2011; Elinati et al., Reference Elinati, Kuentz, Redin, Jaber, Vanden Meerschaut, Makarian, Koscinski, Nasr-Esfahani, Demirol, Gurgan, Louanjli, Iqbal, Bisharah, Pigeon, Gourabi, De Briel, Brugnon, Gitlin and Grillo2012; Zhu et al., Reference Zhu, Gong, Lin and Lu2013). Homozygous deletion of DPY19L2 blocks sperm head elongation and acrosome formation. The incidence of homozygous DPY19L2 in patients with round-headed spermatozoa is 100%. Zhu et al. (Reference Zhu, Gong, Lin and Lu2013) reported 15 genetically independent Chinese patients with round-headed spermatozoa, four were homozygous for the DPY19L2 deletion, five were heterozygous for a point mutation, one was heterozygous for one allele deletion, and the other was heterozygous for one allele. One allele had no mutation; 60% of patients had biallelic DPY19L2 sequence variants. Therefore, DPY19L2 should be sequenced for diagnosis in patients with round-headed spermatozoa. In addition, the SPATA6 gene has been implicated in spiculospermia; Cntrob is a new candidate gene for the currently unexplained genotype of easily decapitated sperm syndrome in humans.
Gene abnormality is an important cause of teratozoospermia, but in the actual clinical diagnosis, the diagnosis at the gene level is still lacking. At present, the genetic research that causes sperm deformity is still in theory. The relationship between the genes and their upstream and downstream genes is not clear. In addition, it is not entirely clear whether the genes that cause deformed sperm act by regulating other genes to cause sperm deformities or directly through proteins.
4. Summary and outlook
Spermatogenesis is a complex process. Although there have been many reports of sperm abnormalities, many mechanisms remain unclear, and clinical applications are still in the developmental stage. Studying the molecular genetics of abnormal sperm can help guide clinicians toward more effective treatments for their patients. With the deepening of research, the molecular mechanism of teratozoospermia will become more and more clear, which will help the gene diagnosis and gene therapy of teratozoospermia in the clinic.
Acknowledgements
This research was financially supplied through the Science and Technology Development Programme of Jilin Provincial of China (no. 20190201148JC), the Education Department of Jilin Province (JJKH20210503KJ), and Jilin Provincial Development and Reform Commission Project (2023C037-4).
Conflicts of interest
The authors declare no conflicts of interest.
