In a study published last month in Nature Communications, researchers from Tokyo Medical and Dental University (TMDU) have revealed that growing donor skin in another species yields surprisingly robust and functional skin grafts.

The gold standard for treating burn wounds is autologous skin grafting, a process in which sheets of skin containing both the epidermis (the outer layer) and the dermis (the deeper layer) are transferred from other parts of the patient’s body to cover the wounded area. However, for large wounds it can be difficult to harvest enough skin from the limited donor sites. Split-thickness grafts that contain mostly epidermis with only some of dermis can be used to cover larger areas, but they do not include features like hair and sweat glands, and are more prone to shrinkage and scarring.

“As alternatives to autologous skin grafts, artificial skin substitutes including cultured epidermis and reconstituted skins have been developed,” says the lead author of the study Dr. Hisato Nagano. “These options are inferior, though, as cultured epidermis can only be used for shallow wounds, and the engraftment rate of reconstituted skins is low.”

To provide proof-of-concept for a new way to produce autologous skin grafts, the researchers generated skin grafts by introducing a mutation into mouse fetuses that made them unable to grow mature epidermis. These fetuses were then injected with mouse stem cells and allowed to develop normally until birth, when their skin growth was analyzed.

“The results were very surprising,” explains Dr. Naoaki Mizuno, the corresponding author. “Not only were the chimeric mice born covered with large patches of skin derived from the injected cells, but these patches also survived up to 3 months when grafted onto mature mice, and even grew fur.”

Intriguingly, injecting the same mutated mouse embryos with human skin cells yielded similar results: as the mice developed in utero, they grew sheets of human skin that mimicked the structure and organization of mature epidermis.

“Our findings suggest that semi-autologous skin grafts containing hair follicles and other skin appendages can be generated in vivo and engrafted successfully,” says Dr. Hiromitsu Nakauchi, senior author.

Given that mouse embryos can only grow small amounts of skin, the next step would be to scale up the process to larger animals with a longer gestation period to generate large human skin grafts. This approach, which involves generating only skin tissue, could help avoid ethical concerns about using human-animal chimeras to produce organs for medical use.

The 2020 Nobel Prize in Chemistry was awarded to Jennifer Doudna and Emmanuelle Charpentier for developing a groundbreaking yet simple way to edit DNA, the “blueprint” of living organisms. While their discovery opened new avenues for research, the accuracy of the method and safety concerns about “cutting” both strands of DNA limited its use for gene therapy treatments. As such, research has been underway to develop tools that do not have these drawbacks.

The prime editing system is one such tool, a molecule complex consisting of two components. One component is the prime editor, which combines a SpCas9 protein, used in the first CRISPR-Cas gene editing technology, and a reverse transcriptase, an enzyme that transcribes RNA into DNA. The second component is the prime editing guide RNA (pegRNA), a modified guide RNA that identifies the target sequence within the DNA and encodes the desired edit. In this complex, the prime editor works like a “word processor,” accurately replacing genomic information. The tool has already been successfully implemented in living cells of organisms such as plants, zebrafish, and mice. However, precisely how this molecule complex executes each step of the editing process has not been clear, mostly due to a lack of information on its spatial structure.

“We became curious about how the unnatural combination of proteins Cas9 and reverse transcriptase work together,” says Shuto, the first author of the paper.

The research team used cryogenic electron microscopy, an imaging technique that makes observations possible at a near-atomic scale. The method required samples to be in glassy ice to protect them from the potential damage by the electron beams, posing some additional challenges.

“We found the prime editor complex to be unstable under experimental conditions,” explains Shuto. “So, it was very challenging to optimize the conditions for the complex to stay stable. For a long time, we could only determine the structure of Cas9.”

Finally overcoming the challenges, the researchers succeeded in determining the three-dimensional structure of the prime editor complex in multiple states during reverse transcription on the target DNA. The structures revealed that the reverse transcriptase bound to the RNA-DNA complex that formed along the “part” of the Cas9 protein associated with DNA cleavage, the splitting of a single strand of the double helix. While performing the reverse transcription, the reverse transcriptase maintained its position relative to the Cas9 protein. The structural and biochemical analyses also indicated that the reverse transcriptase could lead to additional, undesired insertions.

These findings have opened new avenues for both basic and applied research. So, Shuto lays out the next steps.

“Our structure determination strategy in this study can also be applied to prime editors composed of a different Cas9 protein and reverse transcriptase. We want to utilize the newly obtained structural information to lead to the development of improved prime editors.”

The study will be published on May 29 in JAMA Network Open. It is the latest publication from the Apple Women’s Health Study, a longitudinal study of menstrual cycles, gynecological conditions, and overall women’s health conducted by Harvard Chan School, the National Institute of Environmental Health Sciences, and Apple.

“Our findings can lead to a better understanding of menstrual health across the lifespan and how our lived environment impacts this critical vital sign,” said co-principal investigator Shruthi Mahalingaiah, assistant professor of environmental, reproductive, and women’s health at Harvard Chan School.

While previous studies have shown trends towards earlier menarche over the past five decades, data has been limited on how these trends present within different racial groups and socioeconomic statuses. Additionally, few studies have had sufficient data to identify any trends regarding time to menstrual cycle regularity.

The researchers used the Apple Women’s Health Study’s large, diverse dataset to fill this research gap. Participants who enrolled in the study between November 2018 and March 2023 — 71,341 in total — self-reported the age at which they first began menstruating and their race and socioeconomic status. The researchers divided the participants into five age brackets: born between 1950-1969, 1970-1979, 1980-1989, 1990-1999, and 2000-2005. Ages of menarche were defined as early (younger than 11 years old), very early (younger than 9), and late (ages 16 and above). A subset of participants (61,932) self-reported the time it took for their menstrual cycle to become regular and were divided into five categories: up to two years, between three and four years, longer than five years, hasn’t become regular, or became regular with use of hormones. Another subset (9,865) provided their body mass index (BMI) at their age of menarche.

The study found that as birth year increased (meaning younger participants), average age at menarche decreased and time from menarche to menstrual cycle regularity increased. Among participants born from 1950-1969, the average age at menarche was 12.5 years, and the rates of early and very early menarche were 8.6% and 0.6%, respectively. Among participants born from 2000-2005, the average age of menarche was 11.9 years, and the rates of early and very early menarche were 15.5% and 1.4%, respectively. Across the two groups, the percentage of participants who reached menstrual cycle regularity within two years of menarche decreased from 76% to 56%. The researchers observed that these trends were present among all sociodemographic groups but were most pronounced among the participants who identified as Black, Hispanic, Asian, or mixed race, and who rated themselves as belonging to a low socioeconomic status.

The findings showed that BMI at age of menarche could explain part of the trend toward periods starting earlier — in other words, that childhood obesity, a risk factor for early puberty and a growing epidemic in the U.S., could be a contributing factor to earlier menarche. Other possible factors that might explain the trend include dietary patterns, psychological stress and adverse childhood experiences, and environmental factors such as endocrine-disrupting chemicals and air pollution.

“Continuing to investigate early menarche and its drivers is critical,” said corresponding author Zifan Wang, postdoctoral research fellow in Harvard Chan School’s Department of Environmental Health. “Early menarche is associated with higher risk of adverse health outcomes, such as cardiovascular disease and cancer. To address these health concerns — which our findings suggest may begin to impact more people, with disproportionate impact on already disadvantaged populations — we need much more investment in menstrual health research.”

The authors noted some limitations to the study, including that it relies heavily on retrospective self-reporting.

Other Harvard Chan School authors included Gowtham Asokan, Jukka-Pekka Onnela, Michelle Williams, Russ Hauser, and Brent Coull.

The study was made possible by funding from Apple, Inc. and the National Institutes of Health (grant Z01ES103333).