Physical Function Effects of Live Video Group Exercise Interventions for Older Adults: A Systematic Review and Veteran's Gerofit Group Case Study.

Background: Literature on telehealth interventions for older adults has been primarily on asynchronous interventions. During the COVID-19 pandemic, older adult exercise programs transitioned to an online format. This systematic review and case study examines the effectiveness of older adult live video exercise group interventions on physical health with insights from a Los Angeles VA program, Gerofit. Methods: PubMed was searched for live video older adult exercise groups from database inception to November 2021. All eligible studies included assessments of physical health and were limited to participants with an average age of 65 years or greater. Ten Veterans, who had participated in both in-person and virtual Gerofit sessions, were surveyed in the case study. Results: Nine studies met the inclusion criteria. Four studies included an equivalent in-person group as a comparator to the live video group and reported no significant between-group differences in outcomes, including energy expenditure and 6-minute walking distance test (6MWD). The other five studies reported statistically significant in-group improvement in outcomes including isokinetic knee strength. Case study participants reported similar attendance rates and perceived benefits, such as improved balance, when comparing virtual and in-person sessions. Discussion: Live video exercise groups in older adults demonstrated an improvement in physical function that was not statistically different from the comparison in-person sessions with the added benefit of averaging a higher attendance rate, providing initial support for the use of live video in older adult exercise programs. Insights from the case study supplement this by demonstrating older adults' positive attitude on these groups.

The educational affordances of external clinician observation of GP trainee consultations.

AIM: There is widespread agreement about the importance of direct observation of trainee practice by clinical supervisors. Less is known about observation by observers external to the supervisory team. We explored the educational affordances of external observation of GP trainee consultations for educational and assessment purposes, from the perspectives of both observers and trainees. METHOD: GP medical educators, who were scheduled to observe sessions of GP trainee consultations, were recruited as participant observers. They completed field notes, reflective memos and a focus group discussion, and conducted post-observation interviews with trainees, focused on trainee experiences of accessing educational input from supervisors and observers during the sessions. Thematic analysis was guided by constructivist realism and socio-cultural theory. RESULTS: A total of 23 observation sessions (131 observed patient consultations), 33 trainee interviews and 23 observer reflections were completed. External observers embraced teaching, coaching and pastoral opportunities, rather than being 'flies on the wall', despite also having research and assessment roles. They gained useful insights into the challenges of obtaining and providing in-consultation supervisory assistance and provided some in-consultation guidance themselves. Both trainees and observers experienced the sessions as valuable and collegial. Trainees appreciated post-consultation feedback conversations: topics included consultation challenges, managing uncertainty and variation in clinical practice. Patient expectations and pre-existing relationships influenced the distinctive interpersonal dynamic of the externally observed consultation. CONCLUSIONS: The educational affordances and interpersonal dynamics of external observation differ from supervisor observation. We recommend wider use in clinical training of observers who cross between educator and clinician communities.

Mitochondrial DNA deletion mutations increase exponentially with age in human skeletal muscle.

BACKGROUND: Mitochondrial DNA (mtDNA) deletion mutations lead to electron transport chain-deficient cells and age-induced cell loss in multiple tissues and mammalian species. Accurate quantitation of somatic mtDNA deletion mutations could serve as an index of age-induced cell loss. Quantitation of mtDNA deletion molecules is confounded by their low abundance in tissue homogenates, the diversity of deletion breakpoints, stochastic accumulation in single cells, and mosaic distribution between cells. AIMS: Translate a pre-clinical assay to quantitate mtDNA deletions for use in human DNA samples, with technical and biological validation, and test this assay on human subjects of different ages. METHODS: We developed and validated a high-throughput droplet digital PCR assay that quantitates human mtDNA deletion frequency. RESULTS: Analysis of human quadriceps muscle samples from 14 male subjects demonstrated that mtDNA deletion frequency increases exponentially with age-on average, a 98-fold increase from age 20-80. Sequence analysis of amplification products confirmed the specificity of the assay for human mtDNA deletion breakpoints. Titration of synthetic mutation mixtures found a lower limit of detection of at least 0.6 parts per million. Using muscle DNA from 6-month-old mtDNA mutator mice, we measured a 6.4-fold increase in mtDNA deletion frequency (i.e., compared to wild-type mice), biologically validating the approach. DISCUSSION/CONCLUSIONS: The exponential increase in mtDNA deletion frequency is concomitant with the known muscle fiber loss and accelerating mortality that occurs with age. The improved assay permits the accurate and sensitive quantification of deletion mutations from DNA samples and is sufficient to measure changes in mtDNA deletion mutation frequency in healthy individuals across the lifespan and, therefore, patients with suspected mitochondrial diseases.

Good help: a model for providing in-consultation supervision of general practice trainees.

Timely clinical supervision of trainee consultations plays a key role in ensuring the safety of patients under the care of general practice trainees, and in trainee learning and professional development. Trainee requests for assistance during their consultations present supervisors with a number of challenges, however, and a number of factors act as barriers to, or reduce the utility of, this in-consultation assistance from the trainee's perspective. Face-to-face supervision in the presence of the patient presents particular challenges and opportunities. It is important to address barriers to trainee help-seeking and improve both trainee and supervisor skills in promoting safe, effective and efficient in-consultation supervision. We introduce a model (ß-LACTAM) to assist supervisors in planning and delivering their face-to-face in-consultation supervision. The recent evidence which informed the development of this model is outlined, and some preliminary findings from a pilot of ß-LACTAM in Australian general practice training are presented.

Flags for seeking help: making supervisor expectations of general practice trainee help-seeking explicit.

Timely supervisor input into patient care plays a key role in ensuring the safety of patients under the care of general practice trainees. Current models of clinical supervision for trainees in both hospital and general practice training have, however, been criticised for placing too much onus on the trainee to request assistance, despite the many known barriers for trainees to do so. An important barrier to general practice trainee help-seeking is trainee uncertainty about when and how their clinical supervisor expects them to seek this assistance. We introduce a tool, 'Flags for Seeking Help', which was modified from an existing checklist, to assist supervisors to tailor their input to the care of their trainee's patients. The tool aims to make supervisor expectations of trainees explicit, including when trainees should request assistance during consultations (rather than defer this until more convenient opportunities) and when this assistance should be face-to-face (rather than by phone or messaging systems). Our aim is to reduce the barriers for trainees to request in-consultation and face-to-face supervision, in particular, when it is indicated. We outline the evidence which informed the development of the tool, and present some preliminary findings from a pilot in Australian general practice training.

Getting good help: a guide for reflection, debriefing and feedback conversations about in-consultation supervision.

Timely supervisor input to the care of their trainees' patients plays a key role in ensuring the safety of patients under the care of general practice trainees. Supervisor responses to trainee calls for assistance are also important for trainee learning and professional identity formation. The in-consultation supervisory encounter in general practice training is, however, a complex social space with multiple trainee, supervisor and patient agendas. Trainee requests for assistance during their consultations are known to present general practitioner supervisors with a number of challenges. From the trainee's perspective, a safe learning environment is essential during these supervisory interactions. A number of factors may act as barriers to, or reduce the usefulness of, in-consultation assistance in particular, resulting in trainees being less likely to seek such assistance on future occasions. It is therefore important to improve both trainee and supervisor skills in safe, effective and efficient in-consultation supervision. Making time for trainee and supervisor conversations about their help-seeking and help provision may uncover opportunities for improving skills, aligning agendas and enhancing outcomes. Finding time for debriefing, reflection and effective feedback conversations may be challenging, however, and opportunities for trainees to provide feedback to their supervisors are known to be particularly limited. We introduce a Debriefing, Reflection and Feedback Guide which is designed to prompt reflection, and structure effective and efficient debriefing and reciprocal feedback conversations. We outline the evidence which informed the development of the Guides, and present some preliminary findings from a pilot in Australian general practice training.

An innovative educational clinical experience promoting geriatric exercise.

Formal educational training in physical activity promotion is relatively sparse throughout the medical education system. The authors describe an innovative clinical experience in physical activity directed at medical clinicians on a geriatrics rotation. The experience consists of a single 2 1/2 hour session, in which learners are partnered with geriatric patients engaged in a formal supervised exercise program. The learners are guided through an evidence-based exercise regimen tailored to functional status. This experience provides learners with an opportunity to interact with geriatric patients outside the hospital environment to counterbalance the typical geriatric rotation in which geriatric patients are often seen in clinics or hospitals. In this experience, learners are exposed to fit and engaged geriatric patients successfully living in the community despite chronic or disabling conditions. A survey of 105 learners highlighted positive responses to the experience, with 96% of survey respondents indicating that the experience increased their confidence in their ability to serve as advocates for physical activity for older adults, and 89.5% of responders to a follow-up survey indicating that the experience changed their perception of geriatric patients. Modifications to the experience, implemented at partnering facilities are described. The positive feedback from this experience warrants consideration for implementation in other settings.

Mobility Improvements are Found in Older Veterans After 6-Months of Gerofit Regardless of BMI Classification.

Veterans represent a unique population of older adults as they are more likely to self-report disability and be overweight or obese compared to the general population. We sought to compare changes in mobility function across the obesity spectrum in older Veterans participating in six-months of Gerofit, a clinical exercise program. 270 Veterans completed baseline, three, and six-month functional assessment and were divided post-hoc into groups: normal weight, overweight, and obese. Physical function assessment included: ten-meter walk time, six-minute walk distance, 30-second chair stands, and eight-foot up-and-go time. No significant weight x time interactions were found for any measure. However, significant (P<0.02) improvements were found for all mobility measures from baseline to three-months and maintained at six-months. Six-months of participation in Gerofit, if enacted nationwide, appears to be one way to improve mobility and function in older Veterans at high risk for disability regardless of weight status.

Variational methods for fitting complex Bayesian mixed effects models to health data.

We consider approximate inference methods for Bayesian inference to longitudinal and multilevel data within the context of health science studies. The complexity of these grouped data often necessitates the use of sophisticated statistical models. However, the large size of these data can pose significant challenges for model fitting in terms of computational speed and memory storage. Our methodology is motivated by a study that examines trends in cesarean section rates in the largest state of Australia, New South Wales, between 1994 and 2010. We propose a group-specific curve model that encapsulates the complex nonlinear features of the overall and hospital-specific trends in cesarean section rates while taking into account hospital variability over time. We use penalized spline-based smooth functions that represent trends and implement a fully mean field variational Bayes approach to model fitting. Our mean field variational Bayes algorithms allow a fast (up to the order of thousands) and streamlined analytical approximate inference for complex mixed effects models, with minor degradation in accuracy compared with the standard Markov chain Monte Carlo methods.

Young-onset colorectal cancer in New South Wales: a population-based study.

OBJECTIVE: Australia has the highest incidence of colorectal cancer (CRC) in the world. The incidence of young-onset CRC (yCRC) is increasing in developed nations. Our aim was to determine the incidence of yCRC in New South Wales, the demographic and clinico-pathological characteristics of these patients, and their survival. DESIGN, SETTING, PARTICIPANTS: A population-based cohort study of all cases of CRC diagnosed in NSW during 2001-2008. Data on newly diagnosed cases of CRC were obtained from the NSW Central Cancer Registry; mortality data were obtained from the NSW Registry of Births Deaths and Marriages (to 2012). The characteristics and tumour-related factors of patients under 50 years of age (yCRC) were compared with those for patients aged 50 years or more. MAIN OUTCOME MEASURES: Current incidence of yCRC and trends in incidence; 5-year cancer-specific survival rates and risks of death compared with older patients. RESULTS: 32 178 patients were diagnosed with CRC, including 2001 (6.2%) with yCRC. The incidence of yCRC was unchanged across the study period (2001, 13.7 cases per 100 000 population; 2008, 11.8 per 100 000; P = 0.26). Rectal cancer was more frequent in yCRC than in older patients (34.4% v 26.0%), as was distant disease (21.2% v 15.3%). However, 5-year cancer-specific survival was greater for patients with yCRC (68.8%; 95% CI, 66.2-71.2%) than for older patients (66.3%; 95% CI, 65.6-67.0%; P < 0.001). CONCLUSIONS: The incidence of yCRC did not increase in NSW during 2001-2008. Despite more advanced disease at presentation, cancer-specific survival was better than for older patients with CRC.

Streamlined mean field variational Bayes for longitudinal and multilevel data analysis.

Streamlined mean field variational Bayes algorithms for efficient fitting and inference in large models for longitudinal and multilevel data analysis are obtained. The number of operations is linear in the number of groups at each level, which represents a two orders of magnitude improvement over the naïve approach. Storage requirements are also lessened considerably. We treat models for the Gaussian and binary response situations. Our algorithms allow the fastest ever approximate Bayesian analyses of arbitrarily large longitudinal and multilevel datasets, with little degradation in accuracy compared with Markov chain Monte Carlo. The modularity of mean field variational Bayes allows relatively simple extension to more complicated scenarios.

A pilot validation of multi-echo based echo-planar correlated spectroscopic imaging in human calf muscles.

A current limitation of MR spectroscopic imaging of multiple skeletal muscles is prolonged scan duration. A significant reduction in the total scan duration using the echo-planar correlated spectroscopic imaging (EP-COSI) sequence was accomplished using two bipolar readout trains with different phase-encoded echoes for one of two spatial dimensions within a single repetition time (TR). The second bipolar readout was used for spatially encoding the outer k-space, whereas the first readout was used for the central k-space only. The performance of this novel sequence, called multi-echo based echo-planar correlated spectroscopic imaging (ME-EPCOSI), was demonstrated by localizing specific key features in calf muscles and bone marrow of 11 healthy volunteers and five subjects with type 2 diabetes (T2D). A 3 T MRI-MRS scanner equipped with a transmit-receive extremity coil was used. Localization of the ME-EPCOSI sequence was in good agreement with the earlier single-readout based EP-COSI sequence and the required scan time was reduced by a factor of two. In agreement with an earlier report using single-voxel based 2D MRS, significantly increased unsaturated pools of intramyocellular lipid (IMCL) and extramyocellular lipid (EMCL) and decreased IMCL and EMCL unsaturation indices (UIs) were observed in the soleus and tibialis anterior muscle regions of subjects with T2D compared with healthy controls. In addition, significantly decreased choline content was observed in the soleus of T2D subjects compared with healthy controls. Multi-voxel characterization of IMCL and EMCL ratios and UI in the calf muscle may be useful for the non-invasive assessment of altered lipid metabolism in the pathophysiology of T2D.

FoxG1 interacts with Bmi1 to regulate self-renewal and tumorigenicity of medulloblastoma stem cells.

Brain tumors represent the leading cause of childhood cancer mortality, of which medulloblastoma (MB) is the most frequent malignant tumor. Recent studies have demonstrated the presence of several MB molecular subgroups, each distinct in terms of prognosis and predicted therapeutic response. Groups 1 and 2 are characterized by relatively good clinical outcomes and activation of the Wnt and Shh pathways, respectively. In contrast, groups 3 and 4 ("non-Shh/Wnt MBs") are distinguished by metastatic disease, poor patient outcome, and lack a molecular pathway phenotype. Current gene expression platforms have not detected brain tumor-initiating cell (BTIC) self-renewal genes in groups 3 and 4 MBs as BTICs typically comprise a minority of tumor cells and may therefore go undetected on bulk tumor analyses. Since increasing BTIC frequency has been associated with increasing tumor aggressiveness and poor patient outcome, we investigated the subgroup-specific gene expression profile of candidate stem cell genes within 251 primary human MBs from four nonoverlapping MB transcriptional databases (Amsterdam, Memphis, Toronto, Boston) and 74 NanoString-subgrouped MBs (Vancouver). We assessed the functional relevance of two genes, FoxG1 and Bmi1, which were significantly enriched in non-Shh/Wnt MBs and showed these genes to mediate MB stem cell self-renewal and tumor initiation in mice. We also identified their transcriptional regulation through reciprocal promoter occupancy in CD15+ MB stem cells. Our work demonstrates the application of stem cell data gathered from genomic platforms to guide functional BTIC assays, which may then be used to develop novel BTIC self-renewal mechanisms amenable to therapeutic targeting.

Risk of cardiovascular disease among postmenopausal women with prior pregnancy loss: the women's health initiative.

PURPOSE: Metabolic, hormonal, and hemostatic changes associated with pregnancy loss (stillbirth and miscarriage) may contribute to the development of cardiovascular disease (CVD) in adulthood. This study evaluated prospectively the association between a history of pregnancy loss and CVD in a cohort of postmenopausal women. METHODS: Postmenopausal women (77,701) were evaluated from 1993-1998. Information on baseline reproductive history, sociodemographic, and CVD risk factors were collected. The associations between 1 or 2 or more miscarriages and 1 or more stillbirths with occurrence of CVD were evaluated using multiple logistic regression. RESULTS: Among 77,701 women in the study sample, 23,538 (30.3%) reported a history of miscarriage; 1,670 (2.2%) reported a history of stillbirth; and 1,673 (2.2%) reported a history of both miscarriage and stillbirth. Multivariable-adjusted odds ratio (OR) for coronary heart disease (CHD) for 1 or more stillbirths was 1.27 (95% CI, 1.07-1.51) compared with no stillbirth; for women with a history of 1 miscarriage, the OR=1.19 (95% CI, 1.08-1.32); and for 2 or more miscarriages the OR=1.18 (95% CI, 1.04-1.34) compared with no miscarriage. For ischemic stroke, the multivariable odds ratio for stillbirths and miscarriages was not significant. CONCLUSIONS: Pregnancy loss was associated with CHD but not ischemic stroke. Women with a history of 1 or more stillbirths or 1 or more miscarriages appear to be at increased risk of future CVD and should be considered candidates for closer surveillance and/or early intervention; research is needed into better understanding the pathophysiologic mechanisms behind the increased risk of CVD associated with pregnancy loss.

Associations of appendicular lean mass and abdominal adiposity with insulin resistance in older adults: A cross-sectional study.

Loss of lean muscle mass and accumulation of adipose tissue are changes associated with aging. Previous studies have documented various components of body composition as predictors for insulin resistance. The objective of this study was to investigate whether components of body composition-appendicular lean mass (ALM) and/or abdominal fat mass (AFM)-correlate with insulin resistance in older men and women. This was a cross-sectional study of 92 older men and women. Weight was classified according to body mass index (BMI)-normal (BMI <25), overweight (BMI 25-30), and obese (BMI >30). All body composition data was determined by dual energy x-ray absorptiometry (DEXA), and insulin resistance was assessed by the homeostatic model assessment of insulin resistance (HOMA-IR). Multivariable regression models with two-way interaction terms were employed to assess whether the associations between components of body composition and log HOMA varied by BMI categories. Adjusted regression showed that log HOMA was significantly associated with AFM (estimate ± standard error: 0.055 ± 0.026) and ALM (0.057 ± 0.029) for the overweight participants (p-values <0.05). Additionally, the adjusted associations between log HOMA and ALM were significantly greater for participants who were either obese or overweight compared to those with a normal BMI (p<0.002). Less consistent relationships were observed between insulin resistance and abdominal fat mass across BMI categories, whereas more consistent associations were observed between insulin resistance and appendicular lean mass in individuals with greater BMI. Further research is needed to clarify if lipid deposition within muscle tissue promotes muscle dysfunction and thereby increases risk for insulin resistance.

Longitudinal analysis of physical function in older adults: The effects of physical inactivity and exercise training.

Lack of exercise contributes to systemic inflammation and is a major cause of chronic disease. The long-term impact of initiating and sustaining exercise in late life, as opposed to sustaining a sedentary lifestyle, on whole-body health measures such as physical performance is not well known. This is an exploratory study to compare changes in physical performance among older adults initiating exercise late in life versus inactive older adults. Data from two observational cohorts were included in this analysis, representing two activity groups. The Active group cohort comprises older adults (n = 318; age 72.5 ± 7.2 years) enrolled in a supervised exercise program, "Gerofit." The inactive group comprises older adults (n = 146; age 74.5 ± 5.5 years) from the Italian study "Act on Ageing" (AOA) who self-reported being inactive. Participants in both groups completed physical performance battery at baseline and 1-year including: 6-min walk test, 30-s chair stand, and timed up-and-go. Two-sample t-tests measured differences between Gerofit and AOA at baseline and 1-year across all measures. Significant between-group effects were seen for all performance measures (ps = 0.001). The AOA group declined across all measures from baseline to 1 year (range -18% to -24% change). The Gerofit group experienced significant gains in function for all measures (range +10% to +31% change). Older adults who initiated routine, sustained exercise were protected from age-related declines in physical performance, while those who remained sedentary suffered cumulative deficits across strength, aerobic endurance, and mobility. Interventions to reduce sedentary behaviors and increase physical activity are both important to promote multi-system, whole-body health.

Smoking and diabetes: does the increased risk ever go away?

Recent studies reported that smoking cessation leads to higher short-term risk of type 2 diabetes than continuing to smoke. However, the duration of increased diabetes risk following smoking cessation needs further investigation. We followed 135,906 postmenopausal women aged 50-79 years enrolled in the Women's Health Initiative between September 1, 1993, and December 31, 1998, over an average of 11 years to examine the association between smoking cessation and risk of diabetes using Cox proportional hazard multivariable-adjusted regression models. Compared with that for never smokers, the risk for diabetes was significantly elevated in current smokers (hazard ratio = 1.28, 95% confidence interval: 1.20, 1.36) but was even higher in women who quit smoking during the first 3 years of follow-up (hazard ratio = 1.43, 95% confidence interval: 1.26, 1.63). Among former smokers, the risk of diabetes decreased significantly as the time since quitting increased and was equal to that of never smokers following a cessation period of 10 years. In new quitters with low cumulative exposure (<20 pack-years), diabetes risk was not elevated following smoking cessation. In conclusion, the risk of diabetes in former smokers returns to that in never smokers 10 years after quitting, and even more quickly in lighter smokers.

Polo-like kinase 1 inhibition kills glioblastoma multiforme brain tumor cells in part through loss of SOX2 and delays tumor progression in mice.

Glioblastoma multiforme (GBM) ranks among the deadliest types of cancer and given these new therapies are urgently needed. To identify molecular targets, we queried a microarray profiling 467 human GBMs and discovered that polo-like kinase 1 (PLK1) was highly expressed in these tumors and that it clustered with the proliferative subtype. Patients with PLK1-high tumors were more likely to die from their disease suggesting that current therapies are inactive against such tumors. This prompted us to examine its expression in brain tumor initiating cells (BTICs) given their association with treatment failure. BTICs isolated from patients expressed 110-470 times more PLK1 than normal human astrocytes. Moreover, BTICs rely on PLK1 for survival because the PLK1 inhibitor BI2536 inhibited their growth in tumorsphere cultures. PLK1 inhibition suppressed growth, caused G(2) /M arrest, induced apoptosis, and reduced the expression of SOX2, a marker of neural stem cells, in SF188 cells. Consistent with SOX2 inhibition, the loss of PLK1 activity caused the cells to differentiate based on elevated levels of glial fibrillary acidic protein and changes in cellular morphology. We then knocked glial fibrillary acidic protein (GFAP) down SOX2 with siRNA and showed that it too inhibited cell growth and induced cell death. Likewise, in U251 cells, PLK1 inhibition suppressed cell growth, downregulated SOX2, and induced cell death. Furthermore, BI2536 delayed tumor growth of U251 cells in an orthotopic brain tumor model, demonstrating that the drug is active against GBM. In conclusion, PLK1 level is elevated in GBM and its inhibition restricts the growth of brain cancer cells.

Nanopore sequencing identifies a higher frequency and expanded spectrum of mitochondrial DNA deletion mutations in human aging.

Mitochondrial DNA (mtDNA) deletion mutations cause many human diseases and are linked to age-induced mitochondrial dysfunction. Mapping the mutation spectrum and quantifying mtDNA deletion mutation frequency is challenging with next-generation sequencing methods. We hypothesized that long-read sequencing of human mtDNA across the lifespan would detect a broader spectrum of mtDNA rearrangements and provide a more accurate measurement of their frequency. We employed nanopore Cas9-targeted sequencing (nCATS) to map and quantitate mtDNA deletion mutations and develop analyses that are fit-for-purpose. We analyzed total DNA from vastus lateralis muscle in 15 males ranging from 20 to 81 years of age and substantia nigra from three 20-year-old and three 79-year-old men. We found that mtDNA deletion mutations detected by nCATS increased exponentially with age and mapped to a wider region of the mitochondrial genome than previously reported. Using simulated data, we observed that large deletions are often reported as chimeric alignments. To address this, we developed two algorithms for deletion identification which yield consistent deletion mapping and identify both previously reported and novel mtDNA deletion breakpoints. The identified mtDNA deletion frequency measured by nCATS correlates strongly with chronological age and predicts the deletion frequency as measured by digital PCR approaches. In substantia nigra, we observed a similar frequency of age-related mtDNA deletions to those observed in muscle samples, but noted a distinct spectrum of deletion breakpoints. NCATS-mtDNA sequencing allows the identification of mtDNA deletions on a single-molecule level, characterizing the strong relationship between mtDNA deletion frequency and chronological aging.