FANCD2 is required for the repression of germline transposable elements

Research progress of the Fanconi anemia pathway and premature ovarian insufficiency†

The Fanconi anemia pathway is a key pathway involved in the repair of deoxyribonucleic acidinterstrand crosslinking damage, which chiefly includes the following four modules: lesion recognition, Fanconi anemia core complex recruitment, FANCD2-FANCI complex monoubiquitination, and downstream events (nucleolytic incision, translesion synthesis, and homologous recombination). Mutations or deletions of multiple Fanconi anemia genes in this pathway can damage the interstrand crosslinking repair pathway and disrupt primordial germ cell development and oocyte meiosis, thereby leading to abnormal follicular development. Premature ovarian insufficiency is a gynecological clinical syndrome characterized by amenorrhea and decreased fertility due to decreased oocyte pool, accelerated follicle atresia, and loss of ovarian function in women <40 years old. Furthermore, in recent years, several studies have detected mutations in the Fanconi anemia gene in patients with premature ovarian insufficiency. In addition, some patients with Fanconi anemia exhibit symptoms of premature ovarian insufficiency and infertility. The Fanconi anemia pathway and premature ovarian insufficiency are closely associated.

Regulation and function of R-loops at repetitive elements

R-loops are atypical, three-stranded nucleic acid structures that contain a stretch of RNA:DNA hybrids and an unpaired, single stranded DNA loop. R-loops are physiological relevant and can act as regulators of gene expression, chromatin structure, DNA damage repair and DNA replication. However, unscheduled and persistent R-loops are mutagenic and can mediate replication-transcription conflicts, leading to DNA damage and genome instability if left unchecked. Detailed transcriptome analysis unveiled that 85% of the human genome, including repetitive regions, hold transcriptional activity. This anticipates that R-loops management plays a central role for the regulation and integrity of genomes. This function is expected to have a particular relevance for repetitive sequences that make up to 75% of the human genome. Here, we review the impact of R-loops on the function and stability of repetitive regions such as centromeres, telomeres, rDNA arrays, transposable elements and triplet repeat expansions and discuss their relevance for associated pathological conditions.

FAAP100 is required for the resolution of transcription-replication conflicts in primordial germ cells

BACKGROUND

The maintenance of genome stability in primordial germ cells (PGCs) is crucial for the faithful transmission of genetic information and the establishment of reproductive reserve. Numerous studies in recent decades have linked the Fanconi anemia (FA) pathway with fertility, particularly PGC development. However, the role of FAAP100, an essential component of the FA core complex, in germ cell development is unexplored.

RESULTS

We find that FAAP100 plays an essential role in R-loop resolution and replication fork protection to counteract transcription-replication conflicts (TRCs) during mouse PGC proliferation. FAAP100 deletion leads to FA pathway inactivation, increases TRCs as well as cotranscriptional R-loops, and contributes to the collapse of replication forks and the generation of DNA damage. Then, the activated p53 signaling pathway triggers PGC proliferation defects, ultimately resulting in insufficient establishment of reproductive reserve in both sexes of mice.

CONCLUSIONS

Our findings suggest that FAAP100 is required for the resolution of TRCs in PGCs to safeguard their genome stability.

Global expression pattern of genes containing positively selected sites in European anchovy (Engraulis encrasicolus L.) may shed light on teleost reproduction

European anchovy is a multiple-spawning and highly fecundate pelagic fish with high economic and ecological significance. Although fecundity is influenced by nutrition, temperature and weight of spawners, high reproductive capacity is related to molecular processes in the ovary. The ovary is an essential and complex reproductive organ composed of various somatic and germ cells, which interact to facilitate the development of the ovary and functional oocytes. Revealing the ovarian transcriptome profile of highly fecundate fishes provides insights into oocyte production in teleosts. Here we use a comprehensive tissue-specific RNA sequencing which yielded 102.3 billion clean bases to analyze the transcriptional profiles of the ovary compared with other organs (liver, kidney, ovary, testis, fin, cauda and gill) and juvenile tissues of European anchovy. We conducted a comparative transcriptome and positive selection analysis of seven teleost species with varying fecundity rates to identify genes potentially involved in oogenesis and oocyte development. Of the 2,272 single copies of orthologous genes found, up to 535 genes were under positive selection in European anchovy and these genes are associated with a wide spectrum of cellular and molecular functions, with enrichments such as RNA methylation and modification, ribosome biogenesis, DNA repair, cell cycle processing and peptide/amide biosynthesis. Of the 535 positively selected genes, 55 were upregulated, and 45 were downregulated in the ovary, most of which were related to RNA and DNA transferase, developmental transcription factors, protein kinases and replication factors. Overall, our analysis of the transcriptome level in the ovarian tissue of a teleost will provide further insights into molecular processes and deepen our genetic understanding of egg production in highly fecund fish.

UBE2T resolves transcription-replication conflicts and protects common fragile sites in primordial germ cells

The proper development of primordial germ cells (PGCs) is an essential prerequisite for gametogenesis and mammalian fertility. The Fanconi anemia (FA) pathway functions in maintaining the development of PGCs. FANCT/UBE2T serves as an E2 ubiquitin-conjugating enzyme that ubiquitylates the FANCD2-FANCI complex to activate the FA pathway, but its role in the development of PGCs is not clear. In this study, we found that Ube2t knockout mice showed defects in PGC proliferation, leading to severe loss of germ cells after birth. Deletion of UBE2T exacerbated DNA damage and triggered the activation of the p53 pathway. We further demonstrated that UBE2T counteracted transcription-replication conflicts by resolving R-loops and stabilizing replication forks, and also protected common fragile sites by resolving R-loops in large genes and promoting mitotic DNA synthesis to maintain the genome stability of PGCs. Overall, these results provide new insights into the function and regulatory mechanisms of the FA pathway ensuring normal development of PGCs.

DNA repair protein FANCD2 has both ubiquitination-dependent and ubiquitination-independent functions during germ cell development

When DNA interstrand crosslink lesions occur, a core complex of Fanconi anemia proteins promotes the ubiquitination of FANCD2 and FANCI, which recruit downstream factors to repair the lesion. However, FANCD2 maintains genome stability not only through its ubiquitination-dependent but also its ubiquitination-independent functions in various DNA damage response pathways. Increasing evidence suggests that FANCD2 is essential for fertility, but its ubiquitination-dependent and ubiquitination-independent roles during germ cell development are not well characterized. In this study, we analyzed germ cell development in Fancd2 KO and ubiquitination-deficient mutant (Fancd2^K559R/K559R) mice. We showed that in the embryonic stage, both the ubiquitination-dependent and ubiquitination-independent functions of FANCD2 were required for the expansion of primordial germ cells and establishment of the reproductive reserve by reducing transcription-replication conflicts and thus maintaining genome stability in primordial germ cells. Furthermore, we found that during meiosis in spermatogenesis, FANCD2 promoted chromosome synapsis and regulated crossover formation independently of its ubiquitination, but that both ubiquitinated and nonubiquitinated FANCD2 functioned in programmed double strand break repair. Finally, we revealed that on meiotic XY chromosomes, H3K4me2 accumulation required ubiquitination-independent functionality of FANCD2, while the regulation of H3K9me2 and H3K9me3 depended on FANCD2 ubiquitination. Taken together, our findings suggest that FANCD2 has distinct functions that are both dependent on and independent of its ubiquitination during germ cell development.

Epigenetics, DNA damage, and aging

Over the course of a human lifespan, genome integrity erodes, leading to an increased abundance of several types of chromatin changes. The abundance of DNA lesions (chemical perturbations to nucleotides) increases with age, as does the number of genomic mutations and transcriptional disruptions caused by replication or transcription of those lesions, respectively. At the epigenetic level, precise DNA methylation patterns degrade, likely causing increasingly stochastic variations in gene expression. Similarly, the tight regulation of histone modifications begins to unravel. The genomic instability caused by these mechanisms allows transposon element reactivation and remobilization, further mutations, gene dysregulation, and cytoplasmic chromatin fragments. This cumulative genomic instability promotes cell signaling events that drive cell fate decisions and extracellular communications known to disrupt tissue homeostasis and regeneration. In this Review, we focus on age-related epigenetic changes and their interactions with age-related genomic changes that instigate these events.

Retrotransposons in the Mammalian Male Germline

Retrotransposons are a subset of DNA sequences that constitute a large part of the mammalian genome. They can translocate autonomously or non-autonomously, potentially jeopardizing the heritable germline genome. Retrotransposons coevolved with the host genome, and the germline is the prominent battlefield between retrotransposons and the host genome to maximize their mutual fitness. Host genomes have developed various mechanisms to suppress and control retrotransposons, including DNA methylation, histone modifications, and Piwi-interacting RNA (piRNA), for their own benefit. Thus, rapidly evolved retrotransposons often acquire positive functions, including gene regulation within the germline, conferring reproductive fitness in a species over the course of evolution. The male germline serves as an ideal model to examine the regulation and evolution of retrotransposons, resulting in genomic co-evolution with the host genome. In this review, we summarize and discuss the regulatory mechanisms of retrotransposons, stage-by-stage, during male germ cell development, with a particular focus on mice as an extensively studied mammalian model, highlighting suppression mechanisms and emerging functions of retrotransposons in the male germline.

The neglected members of the family: non-BRCA mutations in the Fanconi anemia/BRCA pathway and reproduction

BACKGROUND

BReast CAncer (BRCA) genes are extensively studied in the context of fertility and reproductive aging. BRCA proteins are part of the DNA repair Fanconi anemia (FA)/BRCA pathway, in which more than 20 proteins are implicated. According to which gene is mutated and which interactions are lost owing to the mutation, carriers and patients with monoallelic or biallelic FA/BRCA mutations exhibit very different phenotypes, from overt FA to cancer predisposition or no pathological implications. The effect of the so far neglected non-BRCA FA mutations on fertility also deserves consideration.

OBJECTIVE AND RATIONALE

As improved treatments allow a longer life expectancy in patients with biallelic FA mutations and overt FA, infertility is emerging as a predominant feature. We thus reviewed the mechanisms for such a manifestation, as well as whether they also occur in monoallelic carriers of FA non-BRCA mutations.

SEARCH METHODS

Electronic databases PUBMED, EMBASE and CENTRAL were searched using the following term: 'fanconi' OR 'FANC' OR 'AND' 'fertility' OR 'pregnancy' OR 'ovarian reserve' OR 'spermatogenesis' OR 'hypogonadism'. All pertinent reports in the English-language literature were retrieved until May 2021 and the reference lists were systematically searched in order to identify any potential additional studies.

OUTCOMES

Biallelic FA mutations causing overt FA disease are associated with premature ovarian insufficiency (POI) occurring in the fourth decade in women and with primary non-obstructive azoospermia (NOA) in men. Hypogonadism in FA patients seems mainly associated with a defect in primordial germ cell proliferation in fetal life. In recent small, exploratory whole-exome sequencing studies, biallelic clinically occult mutations in the FA complementation group A (Fanca) and M (Fancm) genes were found in otherwise healthy patients with isolated NOA or POI, and also monoallelic carrier status for a loss-of-function mutation in Fanca has been implicated as a possible cause for POI. In those patients with known monoallelic FA mutations undergoing pre-implantation genetic testing, poor assisted reproduction outcomes are reported. However, the mechanisms underlying the repeated failures and the high miscarriage rates observed are not fully known.

WIDER IMPLICATIONS

The so far 'neglected' members of the FA/BRCA family will likely emerge as a relevant focus of investigation in the genetics of reproduction. Several (rather than a single) non-BRCA genes might be implicated. State-of-the-art methods, such as whole-genome/exome sequencing, and further exploratory studies are required to understand the prevalence and mechanisms for occult FA mutations in infertility and recurrent miscarriage.

Focal Point of Fanconi Anemia Signaling

Among human genetic diseases, Fanconi Anemia (FA) tops all with its largest number of health complications in nearly all human organ systems, suggesting the significant roles played by FA genes in the maintenance of human health. With the accumulated research on FA, the encoded protein products by FA genes have been building up to the biggest cell defense signaling network, composed of not only 22+ FA proteins but also ATM, ATR, and many other non-FA proteins. The FA D2 group protein (FANCD2) and its paralog form the focal point of FA signaling to converge the effects of its upstream players in response to a variety of cellular insults and simultaneously with downstream players to protect humans from contracting diseases, including aging and cancer. In this review, we update and discuss how the FA signaling crucially eases cellular stresses through understanding its focal point.

Factors Regulating the Activity of LINE1 Retrotransposons

LINE-1 (L1) is a class of autonomous mobile genetic elements that form somatic mosaicisms in various tissues of the organism. The activity of L1 retrotransposons is strictly controlled by many factors in somatic and germ cells at all stages of ontogenesis. Alteration of L1 activity was noted in a number of diseases: in neuropsychiatric and autoimmune diseases, as well as in various forms of cancer. Altered activity of L1 retrotransposons for some pathologies is associated with epigenetic changes and defects in the genes involved in their repression. This review discusses the molecular genetic mechanisms of the retrotransposition and regulation of the activity of L1 elements. The contribution of various factors controlling the expression and distribution of L1 elements in the genome occurs at all stages of the retrotransposition. The regulation of L1 elements at the transcriptional, post-transcriptional and integration into the genome stages is described in detail. Finally, this review also focuses on the evolutionary aspects of L1 accumulation and their interplay with the host regulation system.

LncRNA Landscape of Coronary Atherosclerosis Reveals Differentially Expressed LncRNAs in Proliferation and Migration of Coronary Artery Smooth Muscle Cells

We aimed to investigate differentially expressed long non-coding RNAs (lncRNAs) and messenger RNAs (mRNAs) in atherosclerosis and validate the expression of lncRNAs and co-expressed target genes in proliferation and migration models of human coronary artery smooth muscle cells (HCASMCs). Ten coronary artery specimens from a subject who died from a heart attack were employed. The pathological analysis was analyzed by hematoxylin and eosin (H&E) staining, and the lncRNAs and mRNAs were identified by RNA sequencing. Bioinformatic analyses were performed to predict possible mechanisms. The proliferation and migration of HCASMCs were induced with oxidized low-density lipoprotein (ox-LDL). Differentially expressed lncRNAs and mRNAs were validated by quantitative real-time polymerase chain reaction (qRT-PCR). In this study, 68 lncRNAs and 222 mRNAs were identified differentially expressed in atherosclerosis. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses showed that the Fanconi anemia pathway may be involved in atherosclerosis. GON4L was found to be the co-localized target gene of LNC_000439, and 14 genes had high correlations with the expression of seven lncRNAs. In addition, nine lncRNA–miRNA–mRNA networks were constructed, and 53 co-expressed gene modules were detected with weighted gene co-expression network analysis (WGCNA). LNC_000684, LNC_001046, LNC_001333, LNC_001538, and LNC_002115 were downregulated, while LNC_002936 was upregulated in proliferation and migration models of HCASMCs. In total, six co-expressed mRNAs were upregulated in HCASMCs. This study suggests that the differentially expressed lncRNAs identified by RNA sequencing and validated in smooth muscle cells may be a target for regulating HCASMC proliferation and migration in atherosclerosis, which will provide a new diagnostic basis and therapeutic target for the treatment of cardiovascular diseases.

Single cell ATAC-Seq reveals cell type-specific transcriptional regulation and unique chromatin accessibility in human spermatogenesis

During human spermatogenesis, germ cells undergo dynamic changes in chromatin organization/re-packaging and in transcriptomes. In order to better understand the underlying mechanism(s), scATAC-Seq of 5376 testicular cells from 3 normal men were performed. Data were analyzed in parallel with the scRNA-Seq data of human testicular cells. Ten germ cell types associated with spermatogenesis and 6 testicular somatic cell types were identified, along with 142 024 peaks located in promoter, genebody and CpG Island. We had examined chromatin accessibility of all chromosomes, with chromosomes 19 and 17 emerged as the leading chromosomes that displayed high chromatin accessibility. In accessible chromatin regions, transcription factor (TF)-binding sites were identified and specific motifs with high frequencies at different spermatogenesis stages were detected, including CTCF, BORIS, NFY, DMRT6, EN1, ISL1 and GLI3. Two most notable observations were noted. First, TLE3 was specifically expressed in differentiating spermatogonia. Second, PFN4 was found to be involved in actin cytoskeletal organization during meiosis. More important, unique regions upstream of PFN4 and TLE3 were shown to display high accessibility, illustrating their significance in supporting human spermatogenesis.