Description of Research Expertise

Research Interests
Epigenetics in reproduction, spermatogonial stem cell self-renewal, silencing of retrotransposons, piRNA (small non-coding RNA) biogenesis, m6A RNA modification, regulation of meiosis, DNA recombination, chromosome segregation, DNA double-strand break repair, chromosome synapsis, male infertility in humans.

Key words: meiosis, piRNA, homologous recombination, synaptonemal complex, spermatogonial stem cell, retrotransposon, and male infertility

Wang Lab website
https://www.vet.upenn.edu/wang

Description of Research
Our group focuses on the study of spermatogonial stem cells and meiosis. Spermatogonial stem cells are the adult male germline stem cells and responsible for life-long production of sperm. Meiosis, a cell division unique to germ cells, allows the reciprocal exchange of genetic material between paternal and maternal genomes. Meiosis generates the genetic diversity necessary for evolution of species. Abnormality in meiosis is a leading cause of birth defects and infertility. Our research interests include self-renewal of spermatogonial stem cells, molecular genetics of chromosomal synapsis, DNA double-strand break repair, homologous recombination, retrotansposon silencing in germ cells, piRNA biogenesis, and genetic causes of male infertility in humans. Functional characterization of a number of new genes in our laboratory has uncovered novel regulatory mechanisms underlying key biological processes unique to germ cells. On one hand, our studies provide molecular insights into the development of germ cells in mice. On the other hand, these mouse studies have important implications for understanding the genetic causes of male infertility in humans and developing novel male contraceptives.

Epigenetic control of spermatogonial stem cells
Self-renewal of spermatogonial stem cells is vital to lifelong production of male gametes and thus fertility. However, the underlying mechanisms remain enigmatic. We have discovered that DOT1L, the sole H3K79 methyltransferase, is required for spermatogonial stem cell self-renewal. Mice lacking DOT1L fail to maintain spermatogonial stem cells, characterized by a sequential loss of germ cells from spermatogonia to spermatids and ultimately a Sertoli cell only syndrome. Inhibition of DOT1L reduces the stem cell activity after transplantation. Our findings identify an essential function for DOT1L in adult stem cells and provide an epigenetic paradigm for regulation of spermatogonial stem cells.

piRNAs, chromatin remodelers, and retrotransposon silencing

Piwi-interacting RNAs (piRNAs) are a diverse class of small non-coding RNAs implicated in the silencing of transposable elements and the safeguarding of genome integrity. In mammals, male germ cells express two genetically and developmentally distinct populations of piRNAs in pre-pachytene and pachytene germ cells, respectively. Pre-pachytene piRNAs are mostly derived from retrotransposons and required for their silencing. In contrast, pachytene piRNAs originate from about one hundred genomic clusters. We find that MOV10L1 is required for biogenesis of both pre-pachytene and pachytene piRNAs. MOV10L1 is a 5′-to-3′ RNA helicase. MOV10L1 binds to piRNA precursors to initiate primer processing coupled with elements of local secondary structures such as G quadruplexes. We show that MOV10L1 acts upstream of Piwi proteins in the primary processing of piRNAs and is a master regulator of the piRNA pathway in mammals.

We find that TEX15 interacts with MILI, a PIWI protein, and is required for retrotranspson silencing in germ cells. TEX15 serves as a bridge factor between the piRNA pathway and the epigenetic silencing machinery of retrotransposons. MORC2, a DNA-binding ATPase, forms a complex with MORC1 and is essential for retrotransposon silencing in germ cells. Therefore, we have uncovered multiple molecular mechanisms in silencing of retrotransposons and protection of genome integrity in germ cells.

X-linked male infertility

We have identified TEX11 as the first X chromosome-encoded meiosis-specific factor in mammals. In principle, meiosis-specific genes could be located anywhere in the genome. However, no mouse sex chromosome-linked mutants with meiosis-specific defects had been reported, leading to the perception that meiosis-specific factors are rarely if ever encoded by the sex chromosomes. We were the first to clone Tex11, an X-linked germ cell-specific gene. By ablating the function of Tex11 in mice, we have demonstrated that Tex11 is essential for meiosis and fertility in males. Our findings have important implications for male infertility in humans, which accounts for about half of the cases of infertility among couples. An estimated 15% of couples are affected by infertility worldwide. Genetic screening of a large cohort of idiopathic infertile men reveals that TEX11 mutations, including frameshift and splicing acceptor site mutations, cause infertility in 1% of azoospermic men. Collectively, our studies demonstrated that the X chromosome plays a disproportionately eminent role in male fertility, challenging the dogma that the X chromosome is a female chromosome. Furthermore, we find that TEX11 protein levels modulate genome-wide recombination rates in both sexes. These studies indicate that TEX11 alleles affecting expression level or substituting single amino acids may contribute to variations in recombination rates between sexes and among individuals in humans.

Homologous recombination and chromosome synapsis

During meiosis, homologous chromosomes undergo synapsis and recombination. The arrangement of homologous chromosomes is tightly regulated by the synaptonemal complex (SC). SYCP2 is an integral component of SCs in mammals. Our genetic and cell biological studies demonstrate that SYCP2 is required for the formation of SCs and chromosomal synapsis (Yang et al., J Cell Biol 2006). We also find that TEX11 interacts with SYCP2 and is a novel constituent of meiotic nodules involved in recombination. TEX11 promotes both synapsis and recombination, and thus may provide a physical link between these two fundamental meiotic processes.

We performed a genome-wide proteomics screen and identified 51 knonwn and putative meiotic chromatin-associated proteins. We have functionally characterized a number of these proteins: MEIOB, SCML2, and SKP1. MEIOB forms a complex with RPA and SPATA22 and is essential for meiotic recombination. Polycomb protein SCML2 associates with USP7 and counteracts histone H2A ubiquitination in the XY chromatin during male meiosis. SKP1 localizes to the synaptonemal complex. SKP1 is essential for chromosomal synapsis, meiotic recombination, and prophase I to metaphase I transition.

For rotation projects and postdoctoral positions, please contact Jeremy Wang.

Lab personnel:

Fang Yang, senior scientist
Sora Yoon, research associate
Zhenlong Kang, postdoctoral fellow
Ankit Jaiswal, postdoctoral fellow
Yiyun Zhang, graduate student
Cong Liu, graduate student
Siyu Chen, graduate student