Chronological Age vs Biological Age: Can Aging be Reversed?

Blog post by Jasmin Skinner

While our chronological age is on a fixed trajectory, biological age is a far more variable determination of health. Two individuals could be born simultaneously, but their bodies and subsequently, their cells, could be vastly different in age due to a variety of life circumstances. Disease, drug treatment, lifestyle changes, and environmental exposures have all been observed to influence biological age, however, these changes are not absolute (1,2). Interest in the malleability of biological age has been increasing as the technology and information capable of measuring biological age have improved substantially (3,4). Additionally, as human longevity has been steadily increasing since the turn of the last century, understanding the mechanisms behind age-related illnesses has become an increasingly significant public health dilemma (2,3). Having a simplified summary of one’s current health status could prove extremely beneficial for both health care professionals and individuals alike, but how does one measure their biological age?

DNA methylation (DNAm) clocks are the primary way to measure one’s biological age, as methylation of specific pairs of nucleotides predictably change as one ages chronologically. Methylation is a long term regulator of epigenetic gene expression; by transferring a methyl group on cytosine, it can recruit or inhibit proteins from binding to DNA, thereby preventing transcription. The most frequently methylated cytosines are those that precede a guanine nucleotide, known as CpG sites. As methylation of these CpG sites occurs at a predictable rate, DNAm clocks are able to produce a measure of biological age and morbidity/mortality risk, based on both CpG methylation and other phenotypic measures of aging (1,5). 

Since the advent of the first human DNAm clock, a variety of mouse and improved human DNAm clocks have been developed. The first generation of DNAm clocks sought to simply identify physiological markers of aging, while also attempting to approximate an individual’s chronological age based on their epigenetics (1). Second-generation human DNAm clocks vastly improved upon earlier models by integrating numerous phenotypic and biochemical measurements of aging, which are then compared to the individual’s chronological age to get an estimate of their true biological age (4). This has allowed researchers to develop specialized clocks that are better equipped to assess a particular age-related parameter, such as future age prediction, age-related functional deterioration, or disease-related progression (4). However, each DNA methylation clock is calibrated differently, and could yield different estimates of biological age, depending on the clock used (1).

As measurements of biological age can fluctuate quite significantly, the question arises of whether biological age is truly a fixed number, or something capable of fluctuating throughout one’s lifetime. Recent reports of biological age reversal in particular disease states, such as diabetes mellitus, have shown promise as appropriate diabetic treatment effectively reduced biological age (6).  Additionally, a methylation-supported diet and lifestyle change demonstrated up to an 11-year decrease in age, showing the significant potential for altering biological age (7). Given the intertwined nature of chronic stress and accelerated aging, Poganik et al. sought to investigate fluctuations in biological age, in both a murine model and using clinical data, to further elucidate the variability of biological age in response to a variety of physiological stressors (1,8).

In the mouse model experiments, parabiosis was used, in which two mice were surgically attached resulting in a shared circulatory system. This method has been previously shown to reduce DNA methylation age in murine models, and can provide insight as to how the circulatory system affects biological age (1). To evaluate interactions between blood and age, heterochronic (3 month old and 20 month old) mouse pairs were compared to isochronic (2 three-month-old mice) pairs. Mice were surgically attached and maintained for three months, after which point a subset was euthanized for analysis, and the other subset were surgically separated and allowed to recover for 2 months before euthanization and analysis. Additionally, to determine the impact of pregnancy on biological age, male and female mice were housed together and, after successful pregnancy and birth, pups were euthanized to allow the female mice to recover without needing to nurse (1). 

In the human studies, DNA methylation data was obtained from blood samples, or from publicly available human methylation datasets. For the COVID-19 study, blood samples were obtained from individuals who were admitted to an intensive care unit in Boston, Massachusetts. These blood samples were centrifuged to derive a buffy coat from which DNA methylation profiling could be performed. To further elucidate whether physiological stress affects biological age, methylation data from pregnant patients, as well as patients undergoing elective and emergency surgery was obtained from the NCBI Gene Expression Omnibus (GEO), a publically funded genomics data repository, as well as from a few academic collaborators. In addition to the DNA methylation analysis, metabolite and gene expression profiling analysis was performed in a variety of different organs to provide additional data for determining biological age (1).

After comparing mouse DNA methylation data to the methylation expected at their chronological age, it was determined that heterochronic parabiosis brought about a reversible increase in biological age in the liver, heart, brain, kidney, and adipose tissue of the younger mouse. This was further validated by an increase in gene expression signatures of aging and age-related metabolites that disappeared upon recovery, demonstrating a transient increase in biological age at both a transcriptomic, metabolomic, and DNAm level (1). 

These reversible changes were also investigated in pregnant mice, as this highly physiologically stressful event results in several hallmarks of aging, including increased incidence of age-related diseases like heart disease or diabetes. Blood sample testing revealed that biological age increased after mice became pregnant, and decreased after birth, further validating that biological age temporarily increases in response to physiological stress (1).

Figure 1: Pattern of biological aging and recovery after experiencing physiological stress. Original image inspired by graphical abstract from Poganik et al., 2023.