Expansion of a G4C2 repeat in the C9orf72 gene is associated with familial Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD). To investigate the underlying mechanisms of repeat instability, which occurs both somatically and intergenerationally, we created a novel mouse model of familial ALS/FTD that harbors 96 copies of G4C2 repeats at a humanized C9orf72 locus. In mouse embryonic stem cells, we observed two modes of repeat expansion. First, we noted minor increases in repeat length per expansion event, which was dependent on a mismatch repair pathway protein Msh2. Second, we found major increases in repeat length per event when a DNA double- or single-strand break (DSB/SSB) was artificially introduced proximal to the repeats, and which was dependent on the homology-directed repair (HDR) pathway. In mice, the first mode primarily drove somatic repeat expansion. Major changes in repeat length, including expansion, were observed when SSB was introduced in one-cell embryos, or intergenerationally without DSB/SSB introduction if G4C2 repeats exceeded 400 copies, although spontaneous HDR-mediated expansion has yet to be identified. These findings provide a novel strategy to model repeat expansion in a non-human genome and offer insights into the mechanism behind C9orf72 G4C2 repeat instability.

Immunoglobulin E (IgE) plays an important role in allergic diseases. Nevertheless, the source of IgE serological memory remains controversial. We reexamined the mechanism of serological memory in allergy using a dual reporter system to track IgE+ plasma cells in mice. Short-term allergen exposure resulted in the generation of IgE+ plasma cells that resided mainly in secondary lymphoid organs and produced IgE that was unable to degranulate mast cells. In contrast, chronic allergen exposure led to the generation of long-lived IgE+ plasma cells that were primarily derived from sequential class switching of IgG1, accumulated in the bone marrow, and produced IgE capable of inducing anaphylaxis. IgE+ plasma cells were found in the bone marrow of human allergic, but not nonallergic donors, and allergen-specific IgE produced by these cells was able to induce mast cell degranulation when transferred to mice. These data demonstrate that long-lived IgE+ bone marrow plasma cells arise during chronic allergen exposure and establish serological memory in both mice and humans.

Immunoglobulin E (IgE) is a central player in the development and progression of allergic diseases. Allergen-specific IgE binds to FcɛRI on mast cells and basophils and, when crosslinked by allergen, causes these cells to degranulate and release inflammatory mediators of the allergic response. Identifying the source of IgE serological memory is key to understanding atopic diseases and the development of novel therapies. We used mouse models of both short-term (< 4 weeks) and chronic (> 15 weeks) allergen exposure and re-examined the mechanism of serological memory in allergy using a dual-reporter system to track IgE plasma cells (PCs). Short-term house dust mite (HDM) exposure resulted in the generation of IgE plasmablasts that mainly resided in secondary lymphoid organs and produced IgE that was unable to induce mast cell degranulation in response to allergen. In contrast, chronic exposure to HDM resulted in IgE plasma cells (PCs) that arose from sequential class switching of IgG1 memory B cells, accumulated in the bone marrow (BM) and produced IgE of sufficient affinity to support allergen-induced anaphylaxis when transferred to naïve mice. Most importantly, we identified IgE PCs in the BM of human allergic, but not non-allergic donors, and showed that allergen-specific IgE produced by these cells can cause mast cell degranulation when transferred to naïve mice that express human FcɛR1a. Our results demonstrate that long-lived IgE BMPCs arise during chronic allergen exposure and maintain serological memory in both mice and humans.