Teresa Lee

Formation of a zygote is coupled with extensive epigenetic reprogramming to enable appropriate inheritance of histone methylation and prevent developmental delays. In Caenorhabditis elegans, this reprogramming is mediated by the H3K4me2 demethylase SPR-5 and the H3K9 methyltransferase, MET-2. In contrast, the H3K36 methyltransferase MES-4 maintains H3K36me2/3 at germline genes between generations to facilitate re-establishment of the germline. To determine whether the MES-4 germline inheritance pathway antagonizes spr-5; met-2 reprogramming, we examined the interaction between these two pathways. We found that the developmental delay of spr-5; met-2 mutant progeny is associated with ectopic H3K36me3 and the ectopic expression of MES-4-targeted germline genes in somatic tissues. Furthermore, the developmental delay is dependent upon MES-4 and the H3K4 methyltransferase, SET-2. We propose that MES-4 prevents crucial germline genes from being repressed by antagonizing maternal spr-5; m...

During active transcription, the COMPASS complex methylates histone H3 at lysine 4 (H3K4me). In Caenorhabditis elegans, mutations in COMPASS subunits, including WDR-5, extend lifespan and enable the inheritance of increased lifespan in wild-type descendants. Here we show that the increased lifespan of wdr-5 mutants is itself a transgenerational trait that manifests after eighteen generations and correlates with changes in the heterochromatin factor H3K9me2. Additionally, we find that wdr-5 mutant longevity and its inheritance requires the H3K9me2 methyltransferase MET-2 and can be recapitulated by a mutation in the putative H3K9me2 demethylase JHDM-1. These data suggest that lifespan is constrained by reduced H3K9me2 due to transcription-coupled H3K4me. wdr-5 mutants alleviate this burden, extending lifespan and enabling the inheritance of increased lifespan. Thus, H3K9me2 functions in the epigenetic establishment and inheritance of a complex trait. Based on this model, we propose t...

Like breadcrumbs in the forest, cotranscriptionally acquired histone methylation acts as a memory of prior transcription. Because it can be retained through cell divisions, transcriptional memory allows cells to coordinate complex transcriptional programs during development. However, if not reprogrammed properly during cell fate transitions, it can also disrupt cellular identity. In this review, we discuss the consequences of failure to reprogram histone methylation during three crucial epigenetic reprogramming windows: maternal reprogramming at fertilization, embryonic stem cell (ESC) differentiation, and the continuous maintenance of cell identity in differentiated cells. In addition, we discuss how following the wrong breadcrumb trail of transcriptional memory provides a framework for understanding how heterozygous loss-of-function mutations in histone-modifying enzymes may cause severe neurodevelopmental disorders.

Participation in research provides personal and professional benefits for undergraduates. However, some students face institutional barriers that prevent their entry into research, particularly those from underrepresented groups who may stand to gain the most from research experiences. Course-based undergraduate research experiences (CUREs) effectively scale research availability, but many only last for a single semester, which is rarely enough time for a novice to develop proficiency. To address these challenges, we present the Pipeline CURE, a framework that integrates a single research question throughout a biology curriculum. Students are introduced to the research system-in this implementation, C. elegans epigenetics research-with their first course in the major. After revisiting the research system in several subsequent courses, students can choose to participate in an upper-level research experience. In the Pipeline, students build resilience via repeated exposure to the same research system. Its iterative, curriculum-embedded approach is flexible enough to be implemented at a range of institutions using a variety of research questions. By uniting evidence-based teaching methods with ongoing scientific research, the Pipeline CURE provides a new model for overcoming barriers to participation in undergraduate research.

Transvection is broadly defined as the ability of one locus to affect its homologous locus in trans. Although it was first discovered in the 1950s, there are only two known cases in mammals. Here, we report another instance of mammalian transvection induced by the Cre/LoxP system, which is widely used for conditional gene targeting in the mouse. We attempted to use the germline-expressed Vasa-Cre transgene to engineer a mouse mutation, but observe a dramatic reduction of LoxP recombination in mice that inherit an already deleted LoxP allele in trans. A similar phenomenon has previously been observed with another Cre that is expressed during meiosis: Sycp-1-Cre. This second example of LoxP inhibition in trans reinforces the conclusion that certain meiotically expressed Cre alleles can initiate transvection in mammals. However, unlike the previous example, we find that the inhibition of LoxP recom-bination is not due to DNA methylation. In addition, we demonstrate that LoxP inhibition is easily alleviated by adding an extra generation to our crossing scheme. This finding confirms that the LoxP sites are inhibited via an epigenetic mechanism, and provides a method for the use of other Cre transgenes associated with a similar LoxP inhibition event. Furthermore, the abrogation of LoxP inhibition by the simple addition of an extra generation in our crosses establishes a unique mouse system for future studies to uncover the mechanism of transvection in mammals.

Evolution and its mechanisms of action are concepts that unite all aspects of biology, but remain some of the most difficult for students to understand. To address this challenge, we designed a hands-on activity that introduces fundamental mechanisms of evolutionary change: natural selection, genetic drift, and gene flow. In small groups, students use a population of sticky notes to reveal the consequences of each mechanism on phenotype frequency. In a follow-up homework assignment, students then explore how changes in phenotype frequency reflect changes in allele frequency in the population. This activity is suitable for anyone learning the basics of evolution, from high-school through the undergraduate level. We have provided detailed instructions, in-class worksheets, follow-up homework, and extensions that allow the activity to be simplified or made more complex as needed. In our own classrooms, we have observed that the concrete and collaborative nature of this activity enables students to deepen their understanding of the mechanisms through which evolution occurs. We have designed this study such that, in completing this activity, we hope to offer students the opportunity to confront potential misconceptions about evolution and gain a solid foundation for future explorations in the discipline.

Critically shortened telomeres can be subjected to DNA repair events that generate end-to-end chromosome fusions. The resulting dicentric chromosomes can enter breakage-fusion-bridge cycles, thereby impeding elucidation of the structures of the initial fusion events and a mechanistic understanding of their genesis. Current models for the molecular basis of fusion of critically shortened, uncapped telomeres rely on PCR assays that typically capture fusion breakpoints created by direct ligation of chromosome ends. Here we use independent approaches that rely on distinctive features of Caenorhabditis elegans to study the frequency of direct end-to-end chromosome fusion in telomerase mutants: (1) holocentric chromosomes that allow for genetic isolation of stable end-to-end fusions and (2) unique subtelomeric sequences that allow for thorough PCR analysis of samples of genomic DNA harboring multiple end-to-end fusions. Surprisingly, only a minority of end-to-end fusion events resulted from direct end joining with no additional genome rearrangements. We also demonstrate that deficiency for the C. elegans Ku DNA repair heterodimer does not affect telomere length or cause synthetic effects in the absence of telomerase.

In C. elegans, the H3K36 methyltransferase, MES-4, helps establish germ cell fate by maintaining H3K36me2/3 at germline genes between generations. Previously, we showed that the H3K4me2 demethylase, SPR-5, and the H3K9 methyltransferase, MET-2, reprogram histone methylation at fertilization to prevent the ectopic expression of germline genes in somatic tissues. Together, this indicates that SPR-5 and MET-2 maternal reprogramming may antagonize MES-4 to establish germline versus soma. Here, we show that spr-5; met-2 mutant progeny have a severe developmental delay that is associated with the ectopic maintenance of H3K36me2/3 at MES-4 targeted germline genes in somatic tissues, and the ectopic expression of these genes. We further show that the developmental delay and the ectopic expression are dependent upon MES-4. Thus, we propose that SPR-5, MET-2, and MES-4 balance inherited histone methylation to establish germline versus soma. Without this balance, the inappropriate transcription of germline genes in somatic tissues causes developmental delay.