Hair cycling is modulated by factors both intrinsic and extrinsic to hair follicles. Cycling defects lead to conditions such as aging-associated alopecia. Recently, we demonstrated that mouse skin exhibits regenerative hair waves, reflecting a coordinated regenerative behavior in follicle populations. Here, we use this model to explore the regenerative behavior of aging mouse skin. Old mice (>18 months) tracked over several months show that with progressing age, hair waves slow down, wave propagation becomes restricted, and hair cycle domains fragment into smaller domains. Transplanting aged donor mouse skin to a young host can restore donor cycling within a 3 mm range of the interface, suggesting that changes are due to extracellular factors. Therefore, hair stem cells in aged skin can be reactivated. Molecular studies show that extra-follicular modulators Bmp2, Dkk1, and Sfrp4 increase in early anagen. Further, we identify follistatin as an extra-follicular modulator, which is highly expressed in late telogen and early anagen. Indeed, follistatin induces hair wave propagation and its level decreases in aging mice. We present an excitable medium model to simulate the cycling behavior in aging mice and illustrate how the interorgan macroenvironment can regulate the aging process by integrating both "activator" and "inhibitor" signals.

Azathioprine (AZA) is commonly used for many autoimmune disorders; however, the limitation of its clinical use is due to potential toxicities, including severe leukopenia. Recent studies have identified genetic NUDT15 variants strongly associated with AZA-induced leukopenia in Asian patients. This study aimed to investigate the strength of above genetic association and evaluate the usefulness of prospective screening of the NUDT15 variants to prevent AZA-induced leukopenia in Chinese patients. AZA-induced leukopenia in patients with autoimmune disorders were enrolled from multiple medical centers in Taiwan/China between 2012 and 2017 to determine the strength of genetic association of NUDT15 or TPMT variants by whole exome sequencing (WES). Furthermore, a prospective study was conducted between 2018 and 2021 to investigate the incidence of AZA-induced leukopenia with and without genetic screening. The WES result showed the genetic variants of NUDT15 R139C (rs116855232) (P = 3.7 × 10^-25 , odds ratio (OR) = 21.7, 95% confidence interval (95% CI) = 12.1-38.8) and NUDT15 rs746071566 (P = 4.2 × 10^-9 , OR = 7.1, 95% CI = 3.7-13.7), but not TPMT, were associated with AZA-induced leukopenia and NUDT15 R139C variant shows the highest sensitivity with 92.5%. Furthermore, the targeted screening of 1,013 participants for NUDT15 R139C enabled those identified as carriers to use alternative immunosuppressants. This strategy resulted in a significant decrease in the incidence of AZA-induced leukopenia compared with historical incidence (incidence rate = from 7.6% decreased to 0.4%; P = 9.3 × 10^-20 ). In conclusion, the NUDT15 R139C variant was strongly associated with AZA-induced leukopenia in Chinese patients. The genetic screening of NUDT15 R139C followed by use of alternative immunosuppressants in identified carriers effectively decreased the incidence of AZA leukopenia for patients with autoimmune disorders.