Murine Model of Bloom Syndrome Demonstrates Radiation Survivability Altered By Gut Microbiota

Bloom Syndrome (BSyn) is a rare autosomal recessive disorder caused by biallelic germline variants in BLM, a key genome stabilizer responsible for double-strand break repair. Patients with BSyn commonly present with short stature, elongated facial structure, sun-induced skin rashes, and increased predisposition for developing cancers at young ages, due to genomic instability. Because traditional anti-cancer treatments such as chemotherapy and radiation cause damage to DNA, patients with Bloom Syndrome are prone to severe side effects, such as prolonged hematologic suppression, infections, and secondary cancers.

In this study, we used a BSyn mouse model with an extra copy of exon 3, which causes a frameshift and truncates the BLM protein, in order to identify the impact of Blm deficiency in hematopoietic stem cell regeneration. We observed significant differences in 30-day survival after 950 cGy total body irradiation with only 5% of Bloom mice surviving at 30 days compared to 24% of wildtype mice. Peripheral blood counts at day 7 after 950 cGy total body irradiation revealed that Bloom mice had significantly decreased white blood cell and platelet counts compared to wildtype mice, suggesting increased hematotoxicity caused by the Bloom defect.

Given increasing evidence that the gut microbiome regulates hematopoiesis, we hypothesized that DNA repair defects seen in BSyn affect gut microbiota, which in turn may regulate hematologic response to DNA damage induced by radiation injury. To first identify any microbiome differences at baseline, we collected fecal samples from Bloom mice and wildtype littermate controls. DNA was extracted and subjected to shotgun sequencing. Data was compared against >127K whole microbial reference genomes using OneCodex. Beta diversity analysis using Bray-Curtis dissimilarity revealed distinct differences between the diversity of Bloom and wildtype gut microbe species at baseline. 2-way ANOVA table analysis identified a significant difference between the relative abundances of the Lachnospiracaea family, specifically the Acetatifactor muris species, in baseline wildtype compared to Bloom gut microbiomes. This established that there are significant baseline gut microbiome differences in Bloom mice compared to wildtype mice and that these differences could be involved in regulating hematopoietic stem cell regeneration after radiation.

Interestingly, when irradiated Bloom mice were co-housed with non-irradiated wildtype mice, the Bloom mice had the same median survival as the wildtype group. This led us to hypothesize that introducing the gut microbiota of healthy wildtype mice to Bloom mice could ameliorate effects of lethal irradiation. We then performed fecal microbial transplant (FMT) after lethal irradiation, where Bloom mice were given a suspension of fecal pellets collected from wildtype mice on days 1 and 4 post-radiation, by oral gavage. We found that Bloom mice treated with FMT had a trend toward increased white blood cell count and increased absolute neutrophil count at 7 days post-radiation. Bone marrow examination revealed increased stem and progenitor cell frequency in Bloom mice treated with FMT. Colony forming assay showed statistically significant increased total colonies from Bloom mice treated with FMT compared to untreated Bloom mice, suggesting FMT improved proliferation and differentiation ability of hematopoietic progenitors.

Further studies are underway to identify the mechanism by which the gut microbiome may be regulating hematopoietic stem cell regeneration after lethal radiation in Bloom mice.