It is widely accepted that the specific actions of transcriptional regulatory proteins are mediated through their selective association with other protein factors. Such interactions allow transcription factors to distinguish relevant target sequences from the many fortuitous binding sites in the genome and confer highly precise transcriptional regulatory properties. Selective protein-protein interactions are thought to be particularly important for specifying the actions of homeodomaincontaining transcriptional regulatory proteins. Homeoproteins are notorious for their promiscuous DNA binding specificities, which contrast with their highly selective biological functions. It is therefore presumed that specificity is achieved through their interactions with other protein factors. Protein-protein interactions are likely to be particularly important for specifying the transcriptional activities of Msx homeoproteins. The murine Msx family includes three members, two of which (Msx1 and Msx2) have been well characterized with respect to their DNA binding and transcriptional properties (3-5, 40, 43) and one of which (Msx3) has been recently described (14, 33). The homeodomain sequences of Msx proteins are highly conserved (Ͼ90%), and Msx proteins also share several other conserved features, including nearly identical sequences that flank the homeodomain (the extended homeodomain [EHD]) and three other regions of similarity located N terminal and C terminal of the homeodomain (Msx homology regions) (see Fig. 1). In addition, the DNA binding specificities of Msx1 and Msx2 are virtually identical, and both proteins function as transcriptional repressors (4, 40). Moreover, Msx1 and Msx2 share an unusual feature in which repression is mediated through interactions with other protein factors rather than binding to homeodomain DNA sites (4,5,43). Therefore, the Msx1 homeodomain interacts directly with the TATA-binding protein (TBP), and the residues in the homeodomain that mediate this interaction are also required for repression by Msx1 (43). However, the ability of Msx proteins to regulate specific target genes undoubtedly requires additional, as yet undefined, interactions with protein factors that exhibit tissuerestricted expression and promoter-specific activities.The embryonic expression patterns of Msx genes, as well as the phenotypic consequences of targeted disruption of Msx1, are consistent with a role for Msx proteins in inductive signaling between epithelial and mesenchymal tissues. Msx genes are expressed primarily in regions of epithelial-mesenchymal interactions, such as the limb bud, tooth, heart, and neural tube (2,4,6,9,13,18,22,23,25,31), and targeted disruption of Msx1 leads to significant defects in the development of craniofacial structures (32). Moreover, a role for Msx proteins in active morphogenesis is further suggested by the lack of Msx1 expression in cells undergoing terminal differentiation (35,40) and by the restricted expression of Msx1 transcripts during periods of rapid cellular proliferatio...

Failing to repair DNA double-strand breaks by either nonhomologous end joining (NHEJ) or homologous recombination (HR) poses a threat to genome integrity, and may have roles in the onset of aging and age-related diseases. Recent work indicates an age-related decrease of NHEJ efficiency in mouse models, but whether NHEJ and HR change with age in humans and the underlying mechanisms of such a change remain uncharacterized. Here, using 50 eyelid fibroblast cell lines isolated from healthy donors at the age of 16-75 years, we demonstrate that the efficiency and fidelity of NHEJ, and the efficiency of HR decline with age, leading to increased IR sensitivity in cells isolated from old donors. Mechanistic analysis suggests that decreased expression of XRCC4, Lig4 and Lig3 drives the observed, age-associated decline of NHEJ efficiency and fidelity. Restoration of XRCC4 and Lig4 significantly promotes the fidelity and efficiency of NHEJ in aged fibroblasts. In contrast, essential HR-related factors, such as Rad51, do not change in expression level with age, but Rad51 exhibits a slow kinetics of recruitment to DNA damage sites in aged fibroblasts. Further rescue experiments indicate that restoration of XRCC4 and Lig4 may suppress the onset of stress-induced premature cellular senescence, suggesting that improving NHEJ efficiency and fidelity by targeting the NHEJ pathway holds great potential to delay aging and mitigate aging-related pathologies. Cell Death and Differentiation (2016) 23, 1765-1777; doi:10.1038/cdd.2016.65; published online 8 July 2016Aging in mammals is a complex biological process, characterized by several major hallmarks.1,2 Of all the features associated with aging, a gradual destabilization of genome integrity is perhaps the most fundamental as increased genomic instability may lead to other age-associated phenotypes such as cellular senescence and stem cell exhaustion. Indeed, for the past several decades, numerous studies have indicated that DNA mutations and chromosomal rearrangements gradually accumulate with age.3-6 Of all types of DNA lesions, which may contribute to the gradual loss of genetic information during aging, DNA double-strand breaks (DSBs) are the most hazardous to cells as unrepaired or inappropriately repaired DSBs can cause insertions, deletions and chromosomal rearrangements. Using different analysis approaches, several studies have demonstrated that aging is often associated with the accumulation of DNA DSBs in various organs and tissues in mammals such as mice and humans. [7][8][9][10][11] Moreover, a recent study provides direct evidence that an induction of DNA DSBs in genomes causes aging in mouse livers.12 However, why DNA DSBs accumulate with age remains an open question. A number of studies indicate that it may be a consequence of a progressing imbalance between DNA damage and the efficiency of the molecular machinery that catalyzes DNA repair. 7,9,11 Two major pathways, nonhomologous end joining (NHEJ) and homologous recombination (HR) evolved to repair DNA DSBs. NHEJ is...