Thymine DNA glycosylase modulates DNA damage response and gene expression by base excision repair-dependent and independent mechanisms.

Thymine DNA glycosylase (TDG) is a base excision repair (BER) enzyme, which is implicated in correction of deamination-induced DNA mismatches, the DNA demethylation process and regulation of gene expression. Because of these pivotal roles associated, it is crucial to elucidate how the TDG functions are appropriately regulated in vivo. Here, we present evidence that the TDG protein undergoes degradation upon various types of DNA damage, including ultraviolet light (UV). The UV-induced degradation of TDG was dependent on proficiency in nucleotide excision repair and on CRL4CDT2 -mediated ubiquitination that requires a physical interaction between TDG and DNA polymerase clamp PCNA. Using the Tdg-deficient mouse embryonic fibroblasts, we found that ectopic expression of TDG compromised cellular survival after UV irradiation and repair of UV-induced DNA lesions. These negative effects on cellular UV responses were alleviated by introducing mutations in TDG that impaired its BER function. The expression of TDG induced a large-scale alteration in the gene expression profile independently of its DNA glycosylase activity, whereas a subset of genes was affected by the catalytic activity of TDG. Our results indicate the presence of BER-dependent and BER-independent functions of TDG, which are involved in regulation of cellular DNA damage responses and gene expression patterns.

PRDM14 promotes active DNA demethylation through the ten-eleven translocation (TET)-mediated base excision repair pathway in embryonic stem cells.

Ten-eleven translocation (TET) proteins oxidize 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC). 5fC and 5caC can be excised and repaired by the base excision repair (BER) pathway, implicating 5mC oxidation in active DNA demethylation. Genome-wide DNA methylation is erased in the transition from metastable states to the ground state of embryonic stem cells (ESCs) and in migrating primordial germ cells (PGCs), although some resistant regions become demethylated only in gonadal PGCs. Understanding the mechanisms underlying global hypomethylation in naive ESCs and developing PGCs will be useful for realizing cellular pluripotency and totipotency. In this study, we found that PRDM14, the PR domain-containing transcriptional regulator, accelerates the TET-BER cycle, resulting in the promotion of active DNA demethylation in ESCs. Induction of Prdm14 expression transiently elevated 5hmC, followed by the reduction of 5mC at pluripotency-associated genes, germline-specific genes and imprinted loci, but not across the entire genome, which resembles the second wave of DNA demethylation observed in gonadal PGCs. PRDM14 physically interacts with TET1 and TET2 and enhances the recruitment of TET1 and TET2 at target loci. Knockdown of TET1 and TET2 impaired transcriptional regulation and DNA demethylation by PRDM14. The repression of the BER pathway by administration of pharmacological inhibitors of APE1 and PARP1 and the knockdown of thymine DNA glycosylase (TDG) also impaired DNA demethylation by PRDM14. Furthermore, DNA demethylation induced by PRDM14 takes place normally in the presence of aphidicolin, which is an inhibitor of G1/S progression. Together, our analysis provides mechanistic insight into DNA demethylation in naive pluripotent stem cells and developing PGCs.

The successful treatment of microscopic polyangiitis associated with non-tuberculous mycobacterial-pulmonary disease.

While the incidence and prevalence of non-tuberculous mycobacterial-pulmonary disease (NTM-PD) are increasing and microscopic polyangiitis (MPA) is common in East Asian countries, case reports of MPA associated with NTM-PD are limited. A 72-year-old male receiving treatment for NTM-PD with antibiotics was referred to our hospital with fever and arthralgia that developed a few months previously. A blood test revealed the presence of the myeloperoxidase antineutrophil cytoplasmic antibody (MPO-ANCA) and renal impairment. Based on a pathological examination of renal tissue, which showed crescentic glomerulonephritis, the patient was diagnosed with MPA. Due to acute kidney injury and strongly positive MPO-ANCA, pulse steroid therapy was initiated followed by intravenous rituximab (RTX). The patient also received plasmapheresis (14 sessions). Renal dysfunction was reversed. MPA associated with NTM-PD is extremely rare and, thus, there is currently no established treatment. Our patient was diagnosed with MPA based on the findings of renal biopsy while receiving treatment for NTM-PD. RTX and plasmapheresis combined with systemic glucocorticoid therapy were initiated before these clinical conditions had fully recovered. Although MPA secondary to NTM-PD may be more refractory to treatment than primary MPA in the presence of a very low interferon-gamma (IFN-γ) level, this case was successfully treated with steroids, RTX, and plasmapheresis.