The variant form of human xeroderma pigmentosum syndrome (XPV) is caused by a deficiency in DNA polymerase η (Pol η) that enables replication through sunlight-induced pyrimidine dimers. We report high-resolution crystal structures of human Pol η at four consecutive steps during DNA synthesis through cis-syn cyclobutane thymine dimers. Pol η acts like a molecular splint to stabilize damaged DNA in a normal B-form conformation. An enlarged active site accommodates the thymine dimer with excellent stereochemistry for two-metal ion catalysis. Two residues conserved among Pol η orthologs form specific hydrogen bonds with the lesion and the incoming nucleotide to assist translesion synthesis. Based on the structures, eight Pol η missense mutations causing XPV can be rationalized as undermining the “molecular splint” or perturbing the active-site alignment. The structures also shed light on the role of Pol η in replicating through D loop and DNA fragile sites.

Native human DNA polymerase η, DNA and dATP were crystallized at pH 6.0 without Mg2+. The polymerization reaction was initiated by exposing crystals to 1 mM Mg2+ at pH 7.0, and stopped at various time points by freezing at 77 K for crystallographic analysis. The substrates and two Mg2+ are aligned for reaction within 40 seconds, but the new bond begins to form at 80 seconds. Structures at reaction times from 80s to 300s reveal a mixture of decreasing substrate and increasing product of the nucleotidyl-transfer reaction. In sequence, the nucleophile 3′-OH is deprotonated, the deoxyribose at the primer end converts from C2′-endo to C3′-endo to avoid steric clashes, and the nucleophile and the α-phosphate of dATP approach each other to form the new bond. A third Mg2+ ion, which arrives with the new bond and stabilizes the intermediate state, may be an unappreciated feature of the two-metal-ion mechanism.

Major depressive disorder is a serious mental disorder that profoundly affects an individual's quality of life. Although the aetiologies underlying this disorder remain unclear, an increasing attention has been focused on the influence imposed by psychological stress over depression. Despite limited animal models of psychological stress, significant progress has been made as to be explicated in this review to elucidate the physiopathology underlying depression and to treat depressive symptoms. Therefore, we will review classical models along with new methods that will enrich our knowledge of this disorder.