Hisashi Narimatsu (Director), the Research Center for Medical Glycoscience (RCMG) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Ryoji Chubachi), and Nobuyoshi Takasaki (Invited Senior Researcher), Functional Glycoproteomics Team, RCMG, AIST, have discovered that human GALNTL5 gene expressed exclusively in human testis is an essential gene required for normal spermatogenesis, in collaboration with Ichikawa General Hospital of Tokyo Dental College, RIKEN, and Chiba University. GALNTL5 is a completely new type of gene called a glycosyltransferase-like gene, for its resemblance to the pp-GalNAc-T glycosyltransferase genes.

The analysis of Galntl5-deficient mice revealed that this gene is essential for normal spermatogenesis. A patient diagnosed with asthenozoospermia, one of the causes of male infertility, has been found with a mutation of the GALNTL5 gene. These results will contribute to the development of methods to judge precisely the cause of male infertility, which are expected to become an essential technology for selection of appropriate infertility treatment in the near future.

The details of this discovery have been published on-line in the Proceedings of the National Academy of Sciences.

Glycosyltransferase-like genes found in mice and humans and expressed only in the testis

In Japan, the ratio of couples suffering from infertility has risen to one out of every six, and it has been found that approximately 80% of male infertility is caused by poor sperm motility (asthenozoospermia). However, assisted reproductive techniques, such as in vitro fertilization, are being conducted at medical sites without clarifying the cause of asthenozoospermia. On the other hand, the molecular mechanism of mammalian spermatogenesis, including humans, still remains unknown in many aspects. The elucidation of the spermatogenesis mechanism, including the human GALNTL5 gene discovered in this study, is expected to lead to the identification of the cause of male infertility and the selection of appropriate infertility treatment methods.

RCMG of AIST has discovered many new glycosyltransferase genes from the human genome and has elucidated the in vivo functions of the individual glycosyltransferase genes and their relationships to various disorders. GALNTL5 was one of the discoveries; however, its function had been unclear for ten years due to lack of the enzyme activity as a glycosyltransferase. As mice have a gene homologous to GALNTL5, the study was conducted on mice with mutated Galntl5 gene aiming to identify the in vivo functions of the GALNTL5 protein.

This study was supported by "Medical Glycomics Project (FY2006 - 2011)" commissioned by the New Energy and Industrial Technology Development Organization.

The newly identified human GALNTL5 gene is a glycosyltransferase-like gene that expresses only in the human testis and closely resembles a pp-GalNAc-T glycosyltransferase gene. To elucidate its functions, Galntl5-deficient mice were created, and it was found that heterozygous Galntl5-deficient male mice (Ht mice) become infertile with symptoms similar to human asthenozoospermia. Furthermore, the investigation of the protein compositions of sperm from the Ht mice revealed drastic reductions of a glycolytic enzyme protein necessary to generate energy for the sperm motility (HXK) and acrosome proteins that contribute to insemination of the ovum (tACE, NSF) compared to normal spermatozoa (Wt) (Fig. 1).

Figure 1 : Various protein aberrations observed in sperm of the gene-mutated mice

Since immotile sperm and aberrations of various sperm proteins were observed in the Ht mice, a study was conducted on sperm from asthenozoospermia patients to search for the case caused by the mutation of the GALNTL5 gene. As shown in Fig. 2A, sperm from patient No. 9 (enclosed in red) indicated reductions of the multiple proteins caused by the mutation of human GALNTL5, similar to Fig. 1. The sperm and blood cells of this patient indicated hetero deficiency of a single base of the GALNTL5 gene (Fig. 2B, C). This gene mutation is presumed to be inherited from the mother.

Figure 2 : A case of human GALNTL5 mutation in asthenozoospermia patients

With the development of the technology to easily and swiftly identify GALNTL5 mutation, AIST is now proceeding with establishment of the appropriate curative methods. Furthermore, several other genes are also believed to be involved in the onset of asthenozoospermia. AIST will work to identify the other causative genes aiming for development of simple and rapid identification technology. This method is considered to become a requisite diagnostic technology for selection of appropriate reproductive techniques by clarifying the causative gene mutation of asthenozoospermia.