Focused ultrasound-mediated brain genome editing.

Gene editing in the brain has been challenging because of the restricted transport imposed by the blood-brain barrier (BBB). Current approaches mainly rely on local injection to bypass the BBB. However, such administration is highly invasive and not amenable to treating certain delicate regions of the brain. We demonstrate a safe and effective gene editing technique by using focused ultrasound (FUS) to transiently open the BBB for the transport of intravenously delivered CRISPR/Cas9 machinery to the brain.

Widespread genomic breaks generated by activation-induced cytidine deaminase are prevented by homologous recombination.

Activation-induced cytidine deaminase (AID) is required for somatic hypermutation and immunoglobulin class switching in activated B cells. Because AID has no known target-site specificity, there have been efforts to identify non-immunoglobulin AID targets. We show here that AID acts promiscuously, generating widespread DNA double-strand breaks (DSBs), genomic instability and cytotoxicity in B cells with less homologous recombination ability. We demonstrate that the homologous-recombination factor XRCC2 suppressed AID-induced off-target DSBs, promoting B cell survival. Finally, we suggest that aberrations that affect human chromosome 7q36, including XRCC2, correlate with genomic instability in B cell cancers. Our findings demonstrate that AID has promiscuous genomic DSB-inducing activity, identify homologous recombination as a safeguard against off-target AID action, and have implications for genomic instability in B cell cancers.

Activation-induced cytidine deaminase-initiated off-target DNA breaks are detected and resolved during S phase.

Activation-induced cytidine deaminase (AID) initiates DNA double-strand breaks (DSBs) in the IgH gene (Igh) to stimulate isotype class switch recombination (CSR), and widespread breaks in non-Igh (off-target) loci throughout the genome. Because the DSBs that initiate class switching occur during the G1 phase of the cell cycle, and are repaired via end joining, CSR is considered a predominantly G1 reaction. By contrast, AID-induced non-Igh DSBs are repaired by homologous recombination. Although little is known about the connection between the cell cycle and either induction or resolution of AID-mediated non-Igh DSBs, their repair by homologous recombination implicates post-G1 phases. Coordination of DNA breakage and repair during the cell cycle is critical to promote normal class switching and prevent genomic instability. To understand how AID-mediated events are regulated through the cell cycle, we have investigated G1-to-S control in AID-dependent genome-wide DSBs. We find that AID-mediated off-target DSBs, like those induced in the Igh locus, are generated during G1. These data suggest that AID-mediated DSBs can evade G1/S checkpoint activation and persist beyond G1, becoming resolved during S phase. Interestingly, DSB resolution during S phase can promote not only non-Igh break repair, but also Ig CSR. Our results reveal novel cell cycle dynamics in response to AID-initiated DSBs, and suggest that the regulation of the repair of these DSBs through the cell cycle may ensure proper class switching while preventing AID-induced genomic instability.

Efficient in vivo neuronal genome editing in the mouse brain using nanocapsules containing CRISPR-Cas9 ribonucleoproteins.

Genome editing of somatic cells via clustered regularly interspaced short palindromic repeats (CRISPR) offers promise for new therapeutics to treat a variety of genetic disorders, including neurological diseases. However, the dense and complex parenchyma of the brain and the post-mitotic state of neurons make efficient genome editing challenging. In vivo delivery systems for CRISPR-Cas proteins and single guide RNA (sgRNA) include both viral vectors and non-viral strategies, each presenting different advantages and disadvantages for clinical application. We developed non-viral and biodegradable PEGylated nanocapsules (NCs) that deliver preassembled Cas9-sgRNA ribonucleoproteins (RNPs). Here, we show that the RNP NCs led to robust genome editing in neurons following intracerebral injection into the healthy mouse striatum. Genome editing was predominantly observed in medium spiny neurons (>80%), with occasional editing in cholinergic, calretinin, and parvalbumin interneurons. Glial activation was minimal and was localized along the needle tract. Our results demonstrate that the RNP NCs are capable of safe and efficient neuronal genome editing in vivo.

Nuclear positioning, higher-order folding, and gene expression of Mmu15 sequences are refractory to chromosomal translocation.

Nuclear localization influences the expression of certain genes. Chromosomal rearrangements can reposition genes in the nucleus and thus could impact the expression of genes far from chromosomal breakpoints. However, the extent to which chromosomal rearrangements influence nuclear organization and gene expression is poorly understood. We examined mouse progenitor B cell lymphomas with a common translocation, der(12)t(12;15), which fuses a gene-rich region of mouse chromosome 12 (Mmu 12) with a gene-poor region of mouse chromosome 15 (Mmu 15). We found that sequences 2.3 Mb proximal and 2.7 Mb distal to the der(12)t(12;15) breakpoint had different nuclear positions measured relative to the nuclear radius. However, their positions were similar on unrearranged chromosomes in the same tumor cells and normal progenitor B cells. In addition, higher-order chromatin folding marked by three-dimensional gene clustering was not significantly altered for the 7 Mb of Mmu 15 sequence distal to this translocation breakpoint. Translocation also did not correspond to significant changes in gene expression in this region. Thus, any changes to Mmu 15 structure and function imposed by the der(12)t(12;15) translocation are constrained to sequences near (<2.5 Mb) the translocation junction. These data contrast with those of certain other chromosomal rearrangements and suggest that significant changes to Mmu 15 sequence are structurally and functionally tolerated in the tumor cells examined.

Complex oncogenic translocations with gene amplification are initiated by specific DNA breaks in lymphocytes.

Chromosomal instability is a hallmark of many tumor types. Complex chromosomal rearrangements with associated gene amplification, known as complicons, characterize many hematologic and solid cancers. Whereas chromosomal aberrations, including complicons, are useful diagnostic and prognostic cancer markers, their molecular origins are not known. Although accumulating evidence has implicated DNA double-strand break repair in suppression of oncogenic genome instability, the genomic elements required for chromosome rearrangements, especially complex lesions, have not been elucidated. Using a mouse model of B-lineage lymphoma, characterized by complicon formation involving the immunoglobulin heavy chain (Igh) locus and the c-myc oncogene, we have now investigated the requirement for specific genomic segments as donors for complex rearrangements. We now show that specific DNA double-strand breaks, occurring within a narrow segment of Igh, are necessary to initiate complicon formation. By contrast, neither specific DNA breaks nor the powerful intronic enhancer Emu are required for complicon-independent oncogenesis. This study is the first to delineate mechanisms of complex versus simple instability and the first to identify specific chromosomal elements required for complex chromosomal aberrations. These findings will illuminate genomic cancer susceptibility and risk factors.

A mouse model of conditional lipodystrophy.

Lipodystrophies are syndromes of adipose tissue degeneration associated with severe defects in lipid and glucose homeostasis. We report here the generation and analysis of Pparg(ldi), a targeted allele that confers conditional dominant lipodystrophy in mice. The Pparg(ldi) allele was generated by insertion of the Tet activator (tTA) and a tTA-regulated Flag-Pparg1 transgene into the Pparg gene. Unexpectedly, tTA elicits mild lipodystrophy, insulin resistance, and dyslipidemia, and the Flag-PPARgamma1 transgene surprisingly exacerbates these traits. Doxycycline can both completely prevent and reverse these phenotypes, providing a mouse model of inducible lipodystrophy. Embryonic fibroblasts from either Pparg(ldi/+) or the phenotypically similar aP2-nSrebp1c (Sr) transgenic mice undergo robust adipogenesis, suggesting that neither strain develops lipodystrophy because of defective adipocyte differentiation. In addition, Pparg(ldi/+) adipose tissue shares extensive gene expression aberrations with that of Sr mice, authenticating the phenotype at the molecular level and revealing a common expression signature of lipodystrophic fat. Thus, the Pparg(ldi/+) mouse provides a conditional animal model for studying lipodystrophy and its associated physiology and gene expression.