Estimating Glycemia From HbA1c and CGM: Analysis of Accuracy and Sources of Discrepancy

OBJECTIVE

To examine the accuracy of different periods of continuous glucose monitoring (CGM), hemoglobin A1c (HbA1c), and their combination for estimating mean glycemia over 90 days (AG90).

RESEARCH DESIGN AND METHODS

We retrospectively studied 985 CGM periods of 90 days with <10% missing data from 315 adults (86% of whom had type 1 diabetes) with paired HbA1c measurements. The impact of mean red blood cell age as a proxy for nonglycemic effects on HbA1c was estimated using published theoretical models and in comparison with empirical data. Given the lack of a gold standard measurement for AG90, we applied correction methods to generate a reference (eAG90) that we used to assess accuracy for HbA1c and CGM.

RESULTS

Using 14 days of CGM at the end of the 90-day period resulted in a mean absolute error (95th percentile) of 14 (34) mg/dL when compared with eAG90. Nonglycemic effects on HbA1c led to a mean absolute error for average glucose calculated from HbA1c of 12 (29) mg/dL. Combining 14 days of CGM with HbA1c reduced the error to 10 (26) mg/dL. Mismatches between CGM and HbA1c >40 mg/dL occurred more than 5% of the time.

CONCLUSIONS

The accuracy of estimates of eAG90 from limited periods of CGM can be improved by averaging with an HbA1c-based estimate or extending the monitoring period beyond ∼26 days. Large mismatches between eAG90 estimated from CGM and HbA1c are not unusual and may persist due to stable nonglycemic factors.

Hematologic setpoints are a stable and patient-specific deep phenotype

The complete blood count is an important screening tool for healthy adults and is the most commonly ordered test at periodic physical exams. However, results are usually interpreted relative to one-size-fits-all reference intervals, undermining the goal of precision medicine to tailor medical care to the needs of individual patients based on their unique characteristics. Here we show that standard complete blood count indices in healthy adults have robust homeostatic setpoints that are patient-specific and stable, with the typical healthy adult's set of 9 blood count setpoints distinguishable from 98% of others, and with these differences persisting for decades. These setpoints reflect a deep physiologic phenotype, enabling improved detection of both acquired and genetic determinants of hematologic regulation, including discovery of multiple novel loci via GWAS analyses. Patient-specific reference intervals derived from setpoints enable more accurate personalized risk assessment, and the setpoints themselves are significantly correlated with mortality risk, providing new opportunities to enhance patient-specific screening and early intervention. This study shows complete blood count setpoints are sufficiently stable and patient-specific to help realize the promise of precision medicine for healthy adults.

Genetic reversal of the globin switch concurrently modulates both fetal and sickle hemoglobin and reduces red cell sickling

We previously reported initial clinical results of post-transcriptional gene silencing of BCL11A expression (NCT03282656) reversing the fetal to adult hemoglobin switch. A goal of this approach is to increase fetal hemoglobin (HbF) expression while coordinately reducing sickle hemoglobin (HbS) expression. The resulting combinatorial effect should prove effective in inhibiting HbS polymerization at lower physiologic oxygen values thereby mitigating disease complications. Here we report results of exploratory single-cell analysis of patients in which BCL11A is targeted molecularly and compare results with cells of patients treated with hydroxyurea (HU), the current standard of care. We use single-cell assays to assess HbF, HbS, oxygen saturation, and hemoglobin polymer content in RBCs for nine gene therapy trial subjects (BCLshmiR, median HbF% = 27.9) and compare them to 10 HU-treated subjects demonstrating high and comparable levels of HbF (HU High Responders, median HbF% = 27.0). All BCL11A patients achieved the primary endpoint for NCT03282656, which was defined by an absolute neutrophil count greater than or equal to 0.5 × 109 cells/L for three consecutive days, achieved within 7 weeks following infusion. Flow cytometric assessment of single-RBC HbF and HbS shows fewer RBCs with high HbS% that would be most susceptible to sickling in BCLshmiR vs. HU High Responders: median 42% of RBCs with HbS%>70% in BCLshmiR vs. 61% in HU High Responders (p = 0.004). BCLshmiR subjects also demonstrate more RBCs resistant to HbS polymerization at lower physiologic oxygen tension: median 32% vs. 25% in HU High Responders (p = 0.006). Gene therapy-induced BCL11A down-regulation reverses the fetal-to-adult hemoglobin switch and induces RBCs with higher HbF%, lower HbS%, and greater resistance to deoxygenation-induced polymerization in clinical trial subjects compared with a cohort of highly responsive hydroxyurea-treated subjects.

Computer vision quantitation of erythrocyte shape abnormalities provides diagnostic, prognostic, and mechanistic insight

Examination of red blood cell (RBC) morphology in peripheral blood smears can help diagnose hematologic diseases, even in resource-limited settings, but this analysis remains subjective and semiquantitative with low throughput. Prior attempts to develop automated tools have been hampered by their poor reproducibility and limited clinical validation. Here, we present a novel, open-source machine-learning approach (denoted as RBC-diff) to quantify abnormal RBCs in peripheral smear images and generate an RBC morphology differential. RBC-diff cell counts showed high accuracy for single-cell classification (mean AUC, 0.93) and quantitation across smears (mean R2, 0.76 compared with experts, interexperts R2, 0.75). RBC-diff counts were concordant with the clinical morphology grading for 300 000+ images and recovered the expected pathophysiologic signals in diverse clinical cohorts. Criteria using RBC-diff counts distinguished thrombotic thrombocytopenic purpura and hemolytic uremic syndrome from other thrombotic microangiopathies, providing greater specificity than clinical morphology grading (72% vs 41%; P < .001) while maintaining high sensitivity (94% to 100%). Elevated RBC-diff schistocyte counts were associated with increased 6-month all-cause mortality in a cohort of 58 950 inpatients (9.5% mortality for schist. >1%, vs 4.7% for schist; <0.5%; P < .001) after controlling for comorbidities, demographics, clinical morphology grading, and blood count indices. RBC-diff also enabled the estimation of single-cell volume-morphology distributions, providing insight into the influence of morphology on routine blood count measures. Our codebase and expert-annotated images are included here to spur further advancement. These results illustrate that computer vision can enable rapid and accurate quantitation of RBC morphology, which may provide value in both clinical and research contexts.

Microfluidic study of retention and elimination of abnormal red blood cells by human spleen with implications for sickle cell disease

The spleen clears altered red blood cells (RBCs) from circulation, contributing to the balance between RBC formation (erythropoiesis) and removal. The splenic RBC retention and elimination occur predominantly in open circulation where RBCs flow through macrophages and inter-endothelial slits (IESs). The mechanisms underlying and interconnecting these processes significantly impact clinical outcomes. In sickle cell disease (SCD), blockage of intrasplenic sickled RBCs is observed in infants splenectomized due to acute splenic sequestration crisis (ASSC). This life-threatening RBC pooling and organ swelling event is plausibly triggered or enhanced by intra-tissular hypoxia. We present an oxygen-mediated spleen-on-a-chip platform for in vitro investigations of the homeostatic balance in the spleen. To demonstrate and validate the benefits of this general microfluidic platform, we focus on SCD and study the effects of hypoxia on splenic RBC retention and elimination. We observe that RBC retention by IESs and RBC-macrophage adhesion are faster in blood samples from SCD patients than those from healthy subjects. This difference is markedly exacerbated under hypoxia. Moreover, the sickled RBCs under hypoxia show distinctly different phagocytosis processes from those non-sickled RBCs under hypoxia or normoxia. We find that reoxygenation significantly alleviates RBC retention at IESs, and leads to rapid unsickling and fragmentation of the ingested sickled RBCs inside macrophages. These results provide unique mechanistic insights into how the spleen maintains its homeostatic balance between splenic RBC retention and elimination, and shed light on how disruptions in this balance could lead to anemia, splenomegaly, and ASSC in SCD and possible clinical manifestations in other hematologic diseases.

The Effects of Daily Prednisone and Tocilizumab on Hemoglobin A1c During the Treatment of Giant Cell Arteritis

OBJECTIVE

To study the longitudinal effects of both glucocorticoids and tocilizumab, an interleukin-6 receptor inhibitor, on hemoglobin A_1c (HbA_1c ) levels during glucocorticoid tapering.

METHODS

We analyzed patients with complete data from the Tocilizumab in Giant Cell Arteritis (GiACTA) trial to investigate the impact of both glycemic and nonglycemic factors on changes in HbA_1c levels over the 52-week trial. Giant cell arteritis (GCA) patients were randomized to receive either tocilizumab or placebo in addition to glucocorticoids. We used a multivariable mixed-effects model to evaluate associations of HbA_1c level with daily glucocorticoid dose, randomization to receive tocilizumab, and red blood cell count in patients with and those without diabetes mellitus at baseline, over 52 weeks.

RESULTS

In 209 patients, the median HbA_1c level decreased by 0.50% (P < 0.01) in the group that received both tocilizumab and glucocorticoids (tocilizumab/glucocorticoid) and by 0.10% (P < 0.01) in the glucocorticoid-only group. Randomization to tocilizumab/glucocorticoid was associated with lower HbA_1c (β = -0.209% in those without diabetes, P < 0.01; β = -0.290% in those with diabetes, P = 0.23). These changes had a sizable impact on glucose tolerance classification: 42.5% of patients in the tocilizumab/glucocorticoid group improved from prediabetes status to normal, compared to only 12.5% of patients treated with glucocorticoids alone. Daily glucocorticoid dose was associated with HbA_1c level in patients with baseline diabetes (β = 0.018%/mg, P < 0.01) and those without baseline diabetes (β = 0.005%/mg, P < 0.01).

CONCLUSION

Tocilizumab treatment was associated with a substantial reduction in HbA_1c level, independent of glucocorticoid exposure, which may be achieved through a combination of glycemic and nonglycemic effects.

Set Norm and Equivariant Skip Connections: Putting the Deep in Deep Sets

Permutation invariant neural networks are a promising tool for making predictions from sets. However, we show that existing permutation invariant architectures, Deep Sets and Set Transformer, can suffer from vanishing or exploding gradients when they are deep. Additionally, layer norm, the normalization of choice in Set Transformer, can hurt performance by removing information useful for prediction. To address these issues, we introduce the "clean path principle" for equivariant residual connections and develop set norm (sn), a normalization tailored for sets. With these, we build Deep Sets++ and Set Transformer++, models that reach high depths with better or comparable performance than their original counterparts on a diverse suite of tasks. We additionally introduce Flow-RBC, a new single-cell dataset and real-world application of permutation invariant prediction. We open-source our data and code here: https://github.com/rajesh-lab/deep_permutation_invariant.

Cardiovascular Risk Assessment Using Artificial Intelligence-Enabled Event Adjudication and Hematologic Predictors

BACKGROUND

Researchers routinely evaluate novel biomarkers for incorporation into clinical risk models, weighing tradeoffs between cost, availability, and ease of deployment. For risk assessment in population health initiatives, ideal inputs would be those already available for most patients. We hypothesized that common hematologic markers (eg, hematocrit), available in an outpatient complete blood count without differential, would be useful to develop risk models for cardiovascular events.

METHODS

We developed Cox proportional hazards models for predicting heart attack, ischemic stroke, heart failure hospitalization, revascularization, and all-cause mortality. For predictors, we used 10 hematologic indices (eg, hematocrit) from routine laboratory measurements, collected March 2016 to May 2017 along with demographic data and diagnostic codes. As outcomes, we used neural network-based automated event adjudication of 1 028 294 discharge summaries. We trained models on 23 238 patients from one hospital in Boston and evaluated them on 29 671 patients from a second one. We assessed calibration using Brier score and discrimination using Harrell's concordance index. In addition, to determine the utility of high-dimensional interactions, we compared our proportional hazards models to random survival forest models.

RESULTS

Event rates in our cohort ranged from 0.0067 to 0.075 per person-year. Models using only hematology indices had concordance index ranging from 0.60 to 0.80 on an external validation set and showed the best discrimination when predicting heart failure (0.80 [95% CI, 0.79-0.82]) and all-cause mortality (0.78 [0.77-0.80]). Compared with models trained only on demographic data and diagnostic codes, models that also used hematology indices had better discrimination and calibration. The concordance index of the resulting models ranged from 0.75 to 0.85 and the improvement in concordance index ranged up to 0.072. Random survival forests had minimal improvement over proportional hazards models.

CONCLUSIONS

We conclude that low-cost, ubiquitous inputs, if biologically informative, can provide population-level readouts of risk.

Feature tracking microfluidic analysis reveals differential roles of viscosity and friction in sickle cell blood

Characterization of blood flow rheology in hematological disorders is critical for understanding disease pathophysiology. Existing methods to measure blood rheological parameters are limited in their physiological relevance, and there is a need for new tools that focus on the microcirculation and extract properties at finer resolution than overall flow resistance. Herein, we present a method that combines microfluidic systems and powerful object-tracking computational technologies with mathematical modeling to separate the red blood cell flow profile into a bulk component and a wall component. We use this framework to evaluate differential contributions of effective viscosity and wall friction to the overall resistance in blood from patients with sickle cell disease (SCD) under a range of oxygen tensions. Our results demonstrate that blood from patients with SCD exhibits elevated frictional and viscous resistances at all physiologic oxygen tensions. Additionally, the viscous resistance increases more rapidly than the frictional resistance as oxygen tension decreases, which may confound analyses that extract only flow velocities or overall flow resistances. Furthermore, we evaluate the impact of transfusion treatments on the components of the resistance, revealing patient variability in blood properties that may improve our understanding of the heterogeneity of clinical responses to such treatments. Overall, our system provides a new method to analyze patient-specific blood properties and can be applied to a wide range of hematological and vascular disorders.

B lymphocyte-derived acetylcholine limits steady-state and emergency hematopoiesis

Autonomic nerves control organ function through the sympathetic and parasympathetic branches, which have opposite effects. In the bone marrow, sympathetic (adrenergic) nerves promote hematopoiesis; however, how parasympathetic (cholinergic) signals modulate hematopoiesis is unclear. Here, we show that B lymphocytes are an important source of acetylcholine, a neurotransmitter of the parasympathetic nervous system, which reduced hematopoiesis. Single-cell RNA sequencing identified nine clusters of cells that expressed the cholinergic α7 nicotinic receptor (Chrna7) in the bone marrow stem cell niche, including endothelial and mesenchymal stromal cells (MSCs). Deletion of B cell-derived acetylcholine resulted in the differential expression of various genes, including Cxcl12 in leptin receptor+ (LepR+) stromal cells. Pharmacologic inhibition of acetylcholine signaling increased the systemic supply of inflammatory myeloid cells in mice and humans with cardiovascular disease.

The effect of rigid cells on blood viscosity: linking rheology and sickle cell anemia

Sickle cell anemia (SCA) is a disease that affects red blood cells (RBCs). Healthy RBCs are highly deformable objects that under flow can penetrate blood capillaries smaller than their typical size. In SCA there is an impaired deformability of some cells, which are much stiffer and with a different shape than healthy cells, and thereby affect regular blood flow. It is known that blood from patients with SCA has a higher viscosity than normal blood. However, it is unclear how the rigidity of cells is related to the viscosity of blood, in part because SCA patients are often treated with transfusions of variable amounts of normal RBCs and only a fraction of cells will be stiff. Here, we report systematic experimental measurements of the viscosity of a suspension varying the fraction of rigid particles within a suspension of healthy cells. We also perform systematic numerical simulations of a similar mixed suspension of soft RBCs, rigid particles, and their hydrodynamic interactions. Our results show that there is a rheological signature within blood viscosity to clearly identify the fraction of rigidified cells among healthy deformable cells down to a 5% volume fraction of rigidified cells. Although aggregation of RBCs is known to affect blood rheology at low shear rates, and our simulations mimic this effect via an adhesion potential, we show that such adhesion, or aggregation, is unlikely to provide a physical rationalization for the viscosity increase observed in the experiments at moderate shear rates due to rigidified cells. Through numerical simulations, we also highlight that most of the viscosity increase of the suspension is due to the rigidity of the particles rather than their sickled or spherical shape. Our results are relevant to better characterize SCA, provide useful insights relevant to rheological consequences of blood transfusions, and, more generally, extend to the rheology of mixed suspensions having particles with different rigidities, as well as offering possibilities for developments in the field of soft material composites.

Ionophore-mediated swelling of erythrocytes as a therapeutic mechanism in sickle cell disease

Sickle cell disease (SCD) is characterized by sickle hemoglobin (HbS) which polymerizes under deoxygenated conditions to form a stiff, sickled erythrocyte. The dehydration of sickle erythrocytes increases intracellular HbS concentration and the propensity of erythrocyte sickling. Prevention of this mechanism may provide a target for potential SCD therapy investigation. Ionophores such as monensin can increase erythrocyte sodium permeability by facilitating its transmembrane transport, leading to osmotic swelling of the erythrocyte and decreased hemoglobin concentration. In this study, we treated 13 blood samples from patients with SCD with 10 nM of monensin ex vivo. We measured changes in cell volume and hemoglobin concentration in response to monensin treatment, and we perfused treated blood samples through a microfluidic device that permits quantification of blood flow under controlled hypoxia. Monensin treatment led to increases in cell volume and reductions in hemoglobin concentration in most blood samples, though the degree of response varied across samples. Monensin-treated samples also demonstrated reduced blood flow impairment under hypoxic conditions relative to untreated controls. Moreover, there was a significant correlation between the improvement in blood flow and the decrease in hemoglobin concentration. Thus, our results demonstrate that a reduction in intracellular HbS concentration by osmotic swelling improves blood flow under hypoxic conditions. Although the toxicity of monensin will likely prevent it from being a viable clinical treatment, these results suggest that osmotic swelling should be investigated further as a potential mechanism for SCD therapy.

White Blood Cell and Platelet Dynamics Define Human Inflammatory Recovery

Inflammation is the physiologic reaction to cellular and tissue damage caused by pathologic processes including trauma, infection, and ischemia 1 . Effective inflammatory responses integrate molecular and cellular functions to prevent further tissue damage, initiate repair, and restore homeostasis, while futile or dysfunctional responses allow escalating injury, delay recovery, and may hasten death 2 . Elevation of white blood cell count (WBC) and altered levels of other acute phase reactants are cardinal signs of inflammation, but the dynamics of these changes and their resolution are not established 3,4 . Patient responses appear to vary dramatically with no clearly defined signs of good prognosis, leaving physicians reliant on qualitative interpretations of laboratory trends 4,5 . We retrospectively, observationally studied the human acute inflammatory response to trauma, ischemia, and infection by tracking the longitudinal dynamics of cellular and serum markers in hospitalized patients. Unexpectedly, we identified a conserved pattern of recovery defined by co-regulation of WBC and platelet (PLT) populations. Across all inflammatory conditions studied, recovering patients followed a consistent WBC-PLT trajectory shape that is well-approximated by exponential WBC decay and delayed linear PLT growth. This recovery trajectory shape may represent a fundamental archetype of human physiologic response at the cellular population scale, and provides a generic approach for identifying high-risk patients: 32x relative risk of adverse outcomes for cardiac surgery patients, 9x relative risk of death for COVID-19, and 5x relative risk of death for myocardial infarction.

Diminished Reactive Hematopoiesis and Cardiac Inflammation in a Mouse Model of Recurrent Myocardial Infarction

BACKGROUND

Recurrent myocardial infarction (MI) is common in patients with coronary artery disease and is associated with high mortality. Long-term reprogramming of myeloid progenitors occurs in response to inflammatory stimuli and alters the organism's response to secondary inflammatory challenges.

OBJECTIVES

This study examined the effect of recurrent MI on bone marrow response and cardiac inflammation.

METHODS

The investigators developed a surgical mouse model in which 2 subsequent MIs affected different left ventricular regions in the same mouse. Recurrent MI was induced by ligating the left circumflex artery followed by the left anterior descending coronary artery branch. The study characterized the resulting ischemia by whole-heart fluorescent coronary angiography after optical organ clearing and by cardiac magnetic resonance imaging.

RESULTS

A first MI-induced bone marrow "memory" via a circulating signal, reducing hematopoietic maintenance factor expression in bone marrow macrophages. This dampened the organism's reaction to subsequent events. Despite a similar extent of injury according to troponin levels, recurrent MI caused reduced emergency hematopoiesis and less leukocytosis than a first MI. Consequently, fewer leukocytes migrated to the ischemic myocardium. The hematopoietic response to lipopolysaccharide was also mitigated after a previous MI. The increase of white blood count in 28 patients was lower after recurrent MI compared with their first MI.

CONCLUSIONS

The data suggested that hematopoietic and innate immune responses are shaped by a preceding MI.

Association of Red Blood Cell Distribution Width With Mortality Risk in Hospitalized Adults With SARS-CoV-2 Infection

Importance

Coronavirus disease 2019 (COVID-19) is an acute respiratory illness with a high rate of hospitalization and mortality. Biomarkers are urgently needed for patient risk stratification. Red blood cell distribution width (RDW), a component of complete blood counts that reflects cellular volume variation, has been shown to be associated with elevated risk for morbidity and mortality in a wide range of diseases.

Objective

To investigate whether an association between mortality risk and elevated RDW at hospital admission and during hospitalization exists in patients with COVID-19.

Design, Setting, and Participants

This cohort study included adults diagnosed with SARS-CoV-2 infection and admitted to 1 of 4 hospitals in the Boston, Massachusetts area (Massachusetts General Hospital, Brigham and Women's Hospital, North Shore Medical Center, and Newton-Wellesley Hospital) between March 4, 2020, and April 28, 2020.

Main Outcomes and Measures

The main outcome was patient survival during hospitalization. Measures included RDW at admission and during hospitalization, with an elevated RDW defined as greater than 14.5%. Relative risk (RR) of mortality was estimated by dividing the mortality of those with an elevated RDW by the mortality of those without an elevated RDW. Mortality hazard ratios (HRs) and 95% CIs were estimated using a Cox proportional hazards model.

Results

A total of 1641 patients were included in the study (mean [SD] age, 62[18] years; 886 men [54%]; 740 White individuals [45%] and 497 Hispanic individuals [30%]; 276 nonsurvivors [17%]). Elevated RDW (>14.5%) was associated with an increased mortality risk in patients of all ages. The RR for the entire cohort was 2.73, with a mortality rate of 11% in patients with normal RDW (1173) and 31% in those with an elevated RDW (468). The RR in patients younger than 50 years was 5.25 (normal RDW, 1% [n = 341]; elevated RDW, 8% [n = 65]); 2.90 in the 50- to 59-year age group (normal RDW, 8% [n = 256]; elevated RDW, 24% [n = 63]); 3.96 in the 60- to 69-year age group (normal RDW, 8% [n = 226]; elevated RDW, 30% [104]); 1.45 in the 70- to 79-year age group (normal RDW, 23% [n = 182]; elevated RDW, 33% [n = 113]); and 1.59 in those ≥80 years (normal RDW, 29% [n = 168]; elevated RDW, 46% [n = 123]). RDW was associated with mortality risk in Cox proportional hazards models adjusted for age, D-dimer (dimerized plasmin fragment D) level, absolute lymphocyte count, and common comorbidities such as diabetes and hypertension (hazard ratio of 1.09 per 0.5% RDW increase and 2.01 for an RDW >14.5% vs ≤14.5%; P < .001). Patients whose RDW increased during hospitalization had higher mortality compared with those whose RDW did not change; for those with normal RDW, mortality increased from 6% to 24%, and for those with an elevated RDW at admission, mortality increased from 22% to 40%.

Conclusions and Relevance

Elevated RDW at the time of hospital admission and an increase in RDW during hospitalization were associated with increased mortality risk for patients with COVID-19 who received treatment at 4 hospitals in a large academic medical center network.

Non-Parametric Combined Reference Regions and Prediction of Clinical Risk

BACKGROUND

Many clinical decisions depend on estimating patient risk of clinical outcomes by interpreting test results relative to reference intervals, but standard application of reference intervals suffers from two major limitations that reduce the accuracy of clinical decisions: (1) each test result is assessed separately relative to a univariate reference interval, ignoring the rich pathophysiologic information in multivariate relationships, and (2) reference intervals are intended to reflect a population's biological characteristics and are not calibrated for outcome prediction.

METHODS

We developed a combined reference region (CRR), derived CRRs for some pairs of complete blood count (CBC) indices (RBC, MCH, RDW, WBC, PLT), and assessed whether the CRR could enhance the univariate reference interval's prediction of a general clinical outcome, 5-year mortality risk (MR).

RESULTS

The CRR significantly improved MR estimation for 21/21 patient subsets defined by current univariate reference intervals. The CRR identified individuals with >2-fold increase in MR in many cases and uniformly improved the accuracy for all five pairs of tests considered. Overall, the 95% CRR identified individuals with a >7× increase in 5-year MR.

CONCLUSIONS

The CRR enhances the accuracy of the prediction of 5-year MR relative to current univariate reference intervals. The CRR generalizes to higher numbers of tests or biomarkers, as well as to clinical outcomes more specific than MR, and may provide a general way to use existing data to enhance the accuracy and precision of clinical decisions.

Unraveling Disease Pathophysiology with Mathematical Modeling

Modeling has enabled fundamental advances in our understanding of the mechanisms of health and disease for centuries, since at least the time of William Harvey almost 500 years ago. Recent technological advances in molecular methods, computation, and imaging generate optimism that mathematical modeling will enable the biomedical research community to accelerate its efforts in unraveling the molecular, cellular, tissue-, and organ-level processes that maintain health, predispose to disease, and determine response to treatment. In this review, we discuss some of the roles of mathematical modeling in the study of human physiology and pathophysiology and some challenges and opportunities in general and in two specific areas: in vivo modeling of pulmonary function and in vitro modeling of blood cell populations.

Longitudinal Changes in the Relationship Between Hemoglobin A1c and Glucose Tolerance Across Pregnancy and Postpartum

OBJECTIVE

To characterize the relationship between hemoglobin A1c (HbA1c) levels and glucose tolerance across pregnancy and postpartum.

DESIGN AND PARTICIPANTS

In a longitudinal study of pregnant women with gestational diabetes risk factors (N = 102), we performed oral glucose tolerance testing (OGTT) and HbA1c measurements at 10-15 weeks of gestation, 24-30 weeks of gestation (N = 73), and 6-24 weeks postpartum (N = 42). Complete blood counts were obtained from clinical records. We calculated HbA1c-estimated average glucose levels and compared them with mean OGTT glucose levels (average of fasting, 1- and 2-hour glucose levels). Linear mixed effects models were used to test for longitudinal changes in measurements.

RESULTS

Mean OGTT glucose increased between 10-15 and 24-30 weeks of gestation (β = 8.1 mg/dL, P = .001), while HbA1c decreased during the same time period (β = -0.13%, P < .001). At 10-15 weeks of gestation and postpartum the discrepancy between mean OGTT glucose and HbA1c-estimated average glucose was minimal (mean [standard deviation]: 1.2 [20.5] mg/dL and 0.16 [18.1] mg/dL). At 24-30 weeks of gestation, the discrepancy widened (13.2 [17.9] mg/dL, β = 12.7 mg/dL, P < .001, compared to 10-15 weeks of gestation, with mean OGTT glucose being higher than HbA1c-estimated average glucose). Lower hemoglobin at 24-30 weeks of gestation was associated with a greater discrepancy (β = 6.4 mg/dL per 1 g/dL lower hemoglobin, P = .03 in an age- and gestational age-adjusted linear regression model).

CONCLUSIONS

HbA1c accurately reflects glycemia in the 1st trimester, but underestimates glucose intolerance in the late 2nd trimester. Lower hemoglobin level is associated with greater underestimation. Accounting for gestational age and maternal hemoglobin may improve the clinical interpretation of HbA1c levels during pregnancy.

Data-driven physiologic thresholds for iron deficiency associated with hematologic decline

Iron-deficiency contributes to a ∼50% of anemia prevalence worldwide, but reference intervals for iron status tests are not optimized for anemia diagnosis. To address this limitation, we identified the serum ferritin (SF) thresholds associated with hematologic decline in iron-deficient patients, and the SF thresholds from which an SF increase was associated with hematologic improvement. Paired red blood cell and SF measurements were analysed from two adult cohorts at Massachusetts General Hospital (MGH), from 2008-2011 (N = 48 409), and 2016-2018 (N = 10 042). Inter-patient measurements in the first cohort were used to define optimal SF thresholds based on the physiologic relationship between SF and red cell measurements. Intra-patient measurements (1-26 weeks apart) in the second cohort were used to identify SF thresholds from which an SF increase was associated, with an increase in red cell measurements. The identified optimal SF thresholds varied with age, sex and red cell measure. Thresholds associated with a ∼5% decline in red cell index were typically in the range 10-25 ng/mL. Thresholds for younger women (18-45 year) were ∼5 ng/mL lower than for older women (60-95 years), and ∼10 ng/mL lower than for men. Thresholds from which a subsequent increase in SF was associated with a concomitant increase in red cell measure showed similar patterns: younger women had lower thresholds (∼15 ng/mL) than older women (∼25 ng/mL), or men (∼35 ng/mL). These results suggest that diagnostic accuracy may be improved by setting different SF thresholds for younger women, older women, and men. This study illustrates how clinical databases may provide physiologic evidence for improved diagnostic thresholds.

5-(Hydroxymethyl)furfural restores low-oxygen rheology of sickle trait blood in vitro

Sickle cell trait (SCT) is the benign heterozygous carrier state for the sickle variant of the HBB gene. Most of the ~300 million people with SCT worldwide will not experience any significant complications. However, accumulating evidence finds SCT associated with increased risk for the common conditions of chronic kidney disease and venous thromboembolism, and severe but rare renal medullary carcinoma and exercise-induced rhabdomyolysis. The mechanism is uncertain, but probably involves pathological rheology of SCT blood in regions of low oxygen tension, resulting from sickle haemoglobin polymerization in SCT red cells and leading to reduced blood flow and further tissue hypoxia and damage. Here, we used an in vitro microfluidic flow system to study the oxygen-dependent rheology of SCT blood and show that 5-(hydroxymethyl)furfural, a natural breakdown product of glucose and fructose-containing foods, such as fruit juices, can reduce the effects of hypoxia on SCT blood rheology in vitro, restoring near-normal flow velocities at very low oxygen. While opinions regarding the clinical significance of the risks associated with SCT are still evolving, these results suggest that a compound present in some food may provide a potential approach for managing risks that may be associated with SCT.

Single-cell modeling of routine clinical blood tests reveals transient dynamics of human response to blood loss

Low blood count is a fundamental disease state and is often an early sign of illnesses including infection, cancer, and malnutrition, but our understanding of the homeostatic response to blood loss is limited, in part by coarse interpretation of blood measurements. Many common clinical blood tests actually include thousands of single-cell measurements. We present an approach for modeling the unsteady-state population dynamics of the human response to controlled blood loss using these clinical measurements of single-red blood cell (RBC) volume and hemoglobin. We find that the response entails (1) increased production of new RBCs earlier than is currently detectable clinically and (2) a previously unrecognized decreased RBC turnover. Both component responses offset the loss of blood. The model provides a personalized dimensionless ratio that quantifies the balance between increased production and delayed clearance for each individual and may enable earlier detection of both blood loss and the response it elicits.

Rationale and Design for a GRADE Substudy of Continuous Glucose Monitoring

Background: The Glycemia Reduction Approaches in Diabetes: A Comparative Effectiveness (GRADE) study has enrolled a racially and ethnically diverse population with type 2 diabetes, performed extensive phenotyping, and randomly assigned the participants to one of four second-line diabetes medications. The continuous glucose monitoring (CGM) substudy has been added to determine whether there are racial/ethnic differences in the relationship between average glucose (AG) and hemoglobin A1c (HbA1c). CGM will also be used to compare time in target range, glucose variability, and the frequency and duration of hypoglycemia across study groups. Methods: The observational CGM substudy will enroll up to 1800 of the 5047 GRADE study participants from the four treatment groups, including as many as 450 participants from each of 4 racial/ethnic minority groups to be compared: Hispanic White, non-Hispanic White, non-Hispanic African American, and non-Hispanic Other. CGM will be performed for 2 weeks in proximity to a GRADE annual visit, during which an oral glucose tolerance test will be performed and HbA1c and glycated albumin measured. Indicators of interindividual variation in red blood cell turnover, based on specialized erythrocyte measurements, will also be measured to explore the potential causes of interindividual HbA1c variations. Conclusions: The GRADE CGM substudy will provide new insights into whether differences exist in the relationship between HbA1c and AG among different racial/ethnic groups and whether glycemic profiles differ among frequently used diabetes medications and their potential clinical implications. Understanding such differences is important for clinical care and adjustment of diabetes medications in patients of different races or ethnicities.

A microfluidic platform for simultaneous quantification of oxygen-dependent viscosity and shear thinning in sickle cell blood

The pathology of sickle cell disease begins with the polymerization of intracellular hemoglobin under low oxygen tension, which leads to increased blood effective viscosity and vaso-occlusion. However, it has remained unclear how single-cell changes propagate up to the scale of bulk blood effective viscosity. Here, we use a custom microfluidic system to investigate how the increase in the stiffness of individual cells leads to an increase in the shear stress required for the same fluid strain in a suspension of softer cells. We characterize both the shear-rate dependence and the oxygen-tension dependence of the effective viscosity of sickle cell blood, and we assess the effect of the addition of increasing fractions of normal cells whose material properties are independent of oxygen tension, a scenario relevant to the treatment of sickle patients with blood transfusion. For untransfused sickle cell blood, we find an overall increase in effective viscosity at all oxygen tensions and shear rates along with an attenuation in the degree of shear-thinning achieved at the lowest oxygen tensions. We also find that in some cases, even a small fraction of transfused blood cells restores the shape of the shear-thinning relationship, though not the overall baseline effective viscosity. These results suggest that untransfused sickle cell blood will show the most extreme relative rheologic impairment in regions of high shear and that introducing even small fractions of normal blood cells may help retain some shear-thinning capability though without addressing a baseline relative increase in effective viscosity independent of shear.

When HbA1c and Blood Glucose Do Not Match: How Much Is Determined by Race, by Genetics, by Differences in Mean Red Blood Cell Age?

Commentary placing genetic ancestry markers and racial difference in HbA1c in the context of more common variations in the HbA1c-average glucose relationship and their clinical implications.

Oxygen-dependent flow of sickle trait blood as an in vitro therapeutic benchmark for sickle cell disease treatments

Although homozygous sickle cell disease is often clinically severe, the corresponding heterozygous state, sickle cell trait, is almost completely benign despite the fact that there is only a modest difference in sickle hemoglobin levels between the two conditions. In both conditions, hypoxia can lead to polymerization of sickle hemoglobin, changes in red cell mechanical properties, and impaired blood flow. Here, we test the hypothesis that differences in the oxygen-dependent rheological properties in the two conditions might help explain the difference in clinical phenotypes. We use a microfluidic platform that permits quantification of blood rheology under defined oxygen conditions in physiologically sized microchannels and under physiologic shear rates. We find that, even with its lower sickle hemoglobin concentration, sickle trait blood apparent viscosity increases with decreasing oxygen tension and may stop flowing under completely anoxic conditions, though far less readily than the homozygous condition. Sickle cell trait blood flow becomes impaired at significantly lower oxygen tension than sickle cell disease. We also demonstrate how sickle cell trait can serve as a benchmark for sickle cell disease therapies. We characterize the rheological effects of exchange transfusion therapy by mixing sickle blood with nonsickle blood and quantifying the transfusion targets for sickle hemoglobin composition below which the rheological response resembles sickle trait. These studies quantify the differences in blood flow phenotypes of sickle cell disease and sickle cell trait, and they provide a potentially powerful new benchmark for evaluating putative therapies in vitro.

A microfluidic platform to study the effects of vascular architecture and oxygen gradients on sickle blood flow

Our goal was to develop a model of the microvasculature that would allow us to quantify changes in the rheology of sickle blood as it traverses the varying vessel sizes and oxygen tensions in the microcirculation. We designed and implemented a microfluidic model of the microcirculation that comprises a branching microvascular network and physiologic oxygen gradients. We used computational modeling to determine the parameters necessary to generate stable, linear gradients in our devices. Sickle blood from six unique patients was perfused through the microvascular network and subjected to varying oxygen gradients while we observed and quantified blood flow. We found that all sickle blood samples fully occluded the microvascular network when deoxygenated, and we observed that sickle blood could cause vaso-occlusions under physiologic oxygen gradients during the microvascular transit time. The number of occlusions observed under five unique oxygen gradients varied among the patient samples, but we generally observed that the number of occlusions decreased with increasing inlet oxygen tension. The model system we have developed is a valuable tool to address fundamental questions about where in the circulation sickle-cell vaso-occlusions are most likely to occur and to test new therapies.

Mechanistic modeling of hemoglobin glycation and red blood cell kinetics enables personalized diabetes monitoring

The amount of glycated hemoglobin (HbA1c) in diabetic patients' blood provides the best estimate of the average blood glucose concentration over the preceding 2 to 3 months. It is therefore essential for disease management and is the best predictor of disease complications. Nevertheless, substantial unexplained glucose-independent variation in HbA1c makes its reflection of average glucose inaccurate and limits the precision of medical care for diabetics. The true average glucose concentration of a nondiabetic and a poorly controlled diabetic may differ by less than 15 mg/dl, but patients with identical HbA1c values may have true average glucose concentrations that differ by more than 60 mg/dl. We combined a mechanistic mathematical model of hemoglobin glycation and red blood cell kinetics with large sets of within-patient glucose measurements to derive patient-specific estimates of nonglycemic determinants of HbA1c, including mean red blood cell age. We found that between-patient variation in derived mean red blood cell age explains all glucose-independent variation in HbA1c. We then used our model to personalize prospective estimates of average glucose and reduced errors by more than 50% in four independent groups of greater than 200 patients. The current standard of care provided average glucose estimates with errors >15 mg/dl for one in three patients. Our patient-specific method reduced this error rate to 1 in 10. Our personalized approach should improve medical care for diabetes using existing clinical measurements.

Deoxygenation Reduces Sickle Cell Blood Flow at Arterial Oxygen Tension

The majority of morbidity and mortality in sickle cell disease is caused by vaso-occlusion: circulatory obstruction leading to tissue ischemia and infarction. The consequences of vaso-occlusion are seen clinically throughout the vascular tree, from the relatively high-oxygen and high-velocity cerebral arteries to the relatively low-oxygen and low-velocity postcapillary venules. Prevailing models of vaso-occlusion propose mechanisms that are relevant only to regions of low oxygen and low velocity, leaving a wide gap in our understanding of the most important pathologic process in sickle cell disease. Progress toward understanding vaso-occlusion is further challenged by the complexity of the multiple processes thought to be involved, including, but not limited to 1) deoxygenation-dependent hemoglobin polymerization leading to impaired rheology, 2) endothelial and leukocyte activation, and 3) altered cellular adhesion. Here, we chose to focus exclusively on deoxygenation-dependent rheologic processes in an effort to quantify their contribution independent of the other processes that are likely involved in vivo. We take advantage of an experimental system that, to our knowledge, uniquely enables the study of pressure-driven blood flow in physiologic-sized tubes at physiologic hematocrit under controlled oxygenation conditions, while excluding the effects of endothelium, leukocyte activation, adhesion, inflammation, and coagulation. We find that deoxygenation-dependent rheologic processes are sufficient to increase apparent viscosity significantly, slowing blood flow velocity at arterial oxygen tension even without additional contributions from inflammation, adhesion, and endothelial and leukocyte activation. We quantify the changes in apparent viscosity and define a set of functional regimes of sickle cell blood flow personalized for each patient that may be important in further dissecting mechanisms of in vivo vaso-occlusion as well as in assessing risk of patient complications, response to transfusion, and the optimization of experimental therapies in development.

Prior cytopenia predicts worse clinical outcome in acute myeloid leukemia

The prognosis of acute myeloid leukemia (AML) is influenced by both disease-intrinsic and patient-related factors. In particular, AML following myelodysplastic syndrome (MDS) (AML with myelodysplasia-related changes, AML-MRC) has a poor prognosis. We hypothesized that patients with cytopenias prior to AML, but no known prior MDS, may share biologic features with AML-MRC. We evaluated 140 AML patients without prior MDS who had complete blood count (CBC) data available 6-36 months prior to their diagnosis. Cytopenia, defined as clinically unexplained thrombocytopenia or macrocytic anemia, was present in 29/140 (21%) patients. Compared to non-cytopenic patients, AML patients with prior cytopenia were older and more often met morphologic or cytogenetic criteria for AML-MRC. Prior cytopenia was associated with shorter survival in patients with intermediate-risk cytogenetics (median OS 4.2 versus 24.1 months, p<0.0001), but not in patients with adverse-risk cytogenetics (median OS 4.4 versus 6.0 months, p=0.57). Prior thrombocytopenia, but not macrocytic anemia, was significantly associated with shorter overall survival (p=0.01) independent of treatment approach, karyotype risk, and age on multivariable analysis. Our data suggest that AML patients with prior cytopenias have features similar to AML-MRC, and in particular support the use of prior unexplained thrombocytopenia as an independent marker of high-risk disease.

Modulation of red blood cell population dynamics is a fundamental homeostatic response to disease

Increased red blood cell (RBC) volume variation (RDW) has recently been shown to predict a wide range of mortality and morbidity: death due to cardiovascular disease, cancer, infection, renal disease, and more; complications in heart failure and coronary artery disease, advanced stage and worse prognosis in many cancers, poor outcomes in autoimmune disease, and many more. The mechanisms by which all of these diseases lead to increased RDW are unknown. Here we use a semi-mechanistic mathematical model of in vivo RBC population dynamics to dissect the factors controlling RDW and show that elevated RDW results largely from a slight reduction in the in vivo rate of RBC turnover. RBCs become smaller as they age, and a slight reduction in the rate of RBC turnover allows smaller cells to continue circulating, expanding the low-volume tail of the RBC population's volume distribution, and thereby increasing RDW. Our results show that mildly extended RBC lifespan is a previously unrecognized homeostatic adaptation common to a very wide range of pathologic states, likely compensating for subtle reductions in erythropoietic output. A mathematical model-based estimate of the clearance rate may provide a novel early-warning biomarker for a wide range of morbidity and mortality.

Age-related clonal hematopoiesis associated with adverse outcomes

BACKGROUND

The incidence of hematologic cancers increases with age. These cancers are associated with recurrent somatic mutations in specific genes. We hypothesized that such mutations would be detectable in the blood of some persons who are not known to have hematologic disorders.

METHODS

We analyzed whole-exome sequencing data from DNA in the peripheral-blood cells of 17,182 persons who were unselected for hematologic phenotypes. We looked for somatic mutations by identifying previously characterized single-nucleotide variants and small insertions or deletions in 160 genes that are recurrently mutated in hematologic cancers. The presence of mutations was analyzed for an association with hematologic phenotypes, survival, and cardiovascular events.

RESULTS

Detectable somatic mutations were rare in persons younger than 40 years of age but rose appreciably in frequency with age. Among persons 70 to 79 years of age, 80 to 89 years of age, and 90 to 108 years of age, these clonal mutations were observed in 9.5% (219 of 2300 persons), 11.7% (37 of 317), and 18.4% (19 of 103), respectively. The majority of the variants occurred in three genes: DNMT3A, TET2, and ASXL1. The presence of a somatic mutation was associated with an increase in the risk of hematologic cancer (hazard ratio, 11.1; 95% confidence interval [CI], 3.9 to 32.6), an increase in all-cause mortality (hazard ratio, 1.4; 95% CI, 1.1 to 1.8), and increases in the risks of incident coronary heart disease (hazard ratio, 2.0; 95% CI, 1.2 to 3.4) and ischemic stroke (hazard ratio, 2.6; 95% CI, 1.4 to 4.8).

CONCLUSIONS

Age-related clonal hematopoiesis is a common condition that is associated with increases in the risk of hematologic cancer and in all-cause mortality, with the latter possibly due to an increased risk of cardiovascular disease. (Funded by the National Institutes of Health and others.).

Red blood cell population dynamics

Hematology analyzers provide a static snapshot of the circulating population of red blood cells (RBCs). The RBC population is rapidly changing, with more than 2 million RBCs turning over every second in the typical healthy adult. The static snapshot provided by the complete blood count does not capture many of the dynamic aspects of this population, such as the rate of RBC maturation and the rate of RBC turnover. By integrating basic science with hematology analyzer measurements, it is possible to estimate the rates of these dynamic processes, yielding new insights into human physiology, with potential diagnostic application.

In vivo volume and hemoglobin dynamics of human red blood cells

Human red blood cells (RBCs) lose ∼30% of their volume and ∼20% of their hemoglobin (Hb) content during their ∼100-day lifespan in the bloodstream. These observations are well-documented, but the mechanisms for these volume and hemoglobin loss events are not clear. RBCs shed hemoglobin-containing vesicles during their life in the circulation, and this process is thought to dominate the changes in the RBC physical characteristics occurring during maturation. We combine theory with single-cell measurements to investigate the impact of vesiculation on the reduction in volume, Hb mass, and membrane. We show that vesicle shedding alone is sufficient to explain membrane losses but not volume or Hb losses. We use dry mass measurements of human RBCs to validate the models and to propose that additional unknown mechanisms control volume and Hb reduction and are responsible for ∼90% of the observed reduction. RBC population characteristics are used in the clinic to monitor and diagnose a wide range of conditions including malnutrition, inflammation, and cancer. Quantitative characterization of cellular maturation processes may help in the early detection of clinical conditions where maturation patterns are altered.

Developmental plasticity of red blood cell homeostasis

Most human physiologic set points like body temperature are tightly regulated and show little variation between healthy individuals. Red blood cell (RBC) characteristics such as hematocrit and mean cell volume are stable within individuals but can vary by 20% from one healthy person to the next. The mechanisms for the majority of this inter-individual variation are unknown and do not appear to involve common genetic variation. Here, we show that environmental conditions present during development, namely in utero iron availability, can exert long-term influence on a set point related to the RBC life cycle. In a controlled study of rhesus monkeys and a retrospective study of humans, we use a mathematical model of in vivo RBC population dynamics to show that in utero iron deficiency is associated with a lowered threshold for RBC clearance and turnover. This in utero effect is plastic, persisting at least 2 years after birth and after the cessation of iron deficiency. Our study reports a rare instance of developmental plasticity in the human hematologic system and also shows how mathematical modeling can be used to identify cellular mechanisms involved in the adaptive control of homeostatic set points.

Quantitative absorption cytometry for measuring red blood cell hemoglobin mass and volume

We present an optical system, called the quantitative absorption cytometer (QAC), to measure the volume and hemoglobin mass of red blood cells flowing through a microfluidic channel. In contrast to clinical hematology analyzers, where cells are sphered in order for both volume and hemoglobin to be measured accurately, the QAC measures cells in their normal physiological shape. Human red blood cells are suspended in a refractive index-matching absorbing buffer, driven through a microfluidic channel, and imaged using a transmission light microscope onto a color camera. A red and a blue LED illuminate cells and images at each color are used to independently retrieve cell volume and hemoglobin mass. This system shows good agreement with red blood cell indices retrieved by a clinical hematology analyzer and in fact measures a smaller coefficient of variation of hemoglobin concentration. In addition to cell indices, the QAC returns height and mass maps of each measured cell. These quantitative images are valuable for analyzing the detailed morphology of individual cells as well as statistical outliers found in the data. We also measured red blood cells in hypertonic and hypotonic buffers to quantify the correlation between volume and hemoglobin mass under osmotic stress. Because this method is invariant to cell shape, even extremely nonspherical cells in hypertonic buffers can be measured accurately.

The 2013 symposium on pathology data integration and clinical decision support and the current state of field

Background:

Pathologists and informaticians are becoming increasingly interested in electronic clinical decision support for pathology, laboratory medicine and clinical diagnosis. Improved decision support may optimize laboratory test selection, improve test result interpretation and permit the extraction of enhanced diagnostic information from existing laboratory data. Nonetheless, the field of pathology decision support is still developing. To facilitate the exchange of ideas and preliminary studies, we convened a symposium entitled: Pathology data integration and clinical decision support.

Methods:

The symposium was held at the Massachusetts General Hospital, on May 10, 2013. Participants were selected to represent diverse backgrounds and interests and were from nine different institutions in eight different states.

Results:

The day included 16 plenary talks and three panel discussions, together covering four broad areas. Summaries of each presentation are included in this manuscript.

Conclusions:

A number of recurrent themes emerged from the symposium. Among the most pervasive was the dichotomy between diagnostic data and diagnostic information, including the opportunities that laboratories may have to use electronic systems and algorithms to convert the data they generate into more useful information. Differences between human talents and computer abilities were described; well-designed symbioses between humans and computers may ultimately optimize diagnosis. Another key theme related to the unique needs and challenges in providing decision support for genomics and other emerging diagnostic modalities. Finally, many talks relayed how the barriers to bringing decision support toward reality are primarily personnel, political, infrastructural and administrative challenges rather than technological limitations.

Intracellular water exchange for measuring the dry mass, water mass and changes in chemical composition of living cells

We present a method for direct non-optical quantification of dry mass, dry density and water mass of single living cells in suspension. Dry mass and dry density are obtained simultaneously by measuring a cell's buoyant mass sequentially in an H2O-based fluid and a D2O-based fluid. Rapid exchange of intracellular H2O for D2O renders the cell's water content neutrally buoyant in both measurements, and thus the paired measurements yield the mass and density of the cell's dry material alone. Utilizing this same property of rapid water exchange, we also demonstrate the quantification of intracellular water mass. In a population of E. coli, we paired these measurements to estimate the percent dry weight by mass and volume. We then focused on cellular dry density - the average density of all cellular biomolecules, weighted by their relative abundances. Given that densities vary across biomolecule types (RNA, DNA, protein), we investigated whether we could detect changes in biomolecular composition in bacteria, fungi, and mammalian cells. In E. coli, and S. cerevisiae, dry density increases from stationary to exponential phase, consistent with previously known increases in the RNA/protein ratio from up-regulated ribosome production. For mammalian cells, changes in growth conditions cause substantial shifts in dry density, suggesting concurrent changes in the protein, nucleic acid and lipid content of the cell.

Optical measurement of biomechanical properties of individual erythrocytes from a sickle cell patient

Sickle cell disease (SCD) is characterized by the abnormal deformation of red blood cells (RBCs) in the deoxygenated condition, as their elongated shape leads to compromised circulation. The pathophysiology of SCD is influenced by both the biomechanical properties of RBCs and their hemodynamic properties in the microvasculature. A major challenge in the study of SCD involves accurate characterization of the biomechanical properties of individual RBCs with minimum sample perturbation. Here we report the biomechanical properties of individual RBCs from a SCD patient using a non-invasive laser interferometric technique. We optically measure the dynamic membrane fluctuations of RBCs. The measurements are analyzed with a previously validated membrane model to retrieve key mechanical properties of the cells: bending modulus; shear modulus; area expansion modulus; and cytoplasmic viscosity. We find that high cytoplasmic viscosity at ambient oxygen concentration is principally responsible for the significantly decreased dynamic membrane fluctuations in RBCs with SCD, and that the mechanical properties of the membrane cortex of irreversibly sickled cells (ISCs) are different from those of the other types of RBCs in SCD.

A biophysical indicator of vaso-occlusive risk in sickle cell disease

The search for predictive indicators of disease has largely focused on molecular markers. However, biophysical markers, which can integrate multiple pathways, may provide a more global picture of pathophysiology. Sickle cell disease affects millions of people worldwide and has been studied intensely at the molecular, cellular, tissue, and organismal level for a century, but there are still few, if any, markers quantifying the severity of this disease. Because the complications of sickle cell disease are largely due to vaso-occlusive events, we hypothesized that a physical metric characterizing the vaso-occlusive process could serve as an indicator of disease severity. Here, we use a microfluidic device to characterize the dynamics of "jamming," or vaso-occlusion, in physiologically relevant conditions, by measuring a biophysical parameter that quantifies the rate of change of the resistance to flow after a sudden deoxygenation event. Our studies show that this single biophysical parameter could be used to distinguish patients with poor outcomes from those with good outcomes, unlike existing laboratory tests. This biophysical indicator could therefore be used to guide the timing of clinical interventions, to monitor the progression of the disease, and to measure the efficacy of drugs, transfusion, and novel small molecules in an ex vivo setting.

Anisotropic light scattering of individual sickle red blood cells

We present the anisotropic light scattering of individual red blood cells (RBCs) from a patient with sickle cell disease (SCD). To measure light scattering spectra along two independent axes of elongated-shaped sickle RBCs with arbitrary orientation, we introduce the anisotropic Fourier transform light scattering (aFTLS) technique and measured both the static and dynamic anisotropic light scattering. We observed strong anisotropy in light scattering patterns of elongated-shaped sickle RBCs along its major axes using static aFTLS. Dynamic aFTLS analysis reveals the significantly altered biophysical properties in individual sickle RBCs. These results provide evidence that effective viscosity and elasticity of sickle RBCs are significantly different from those of the healthy RBCs.

Rapid monocyte kinetics in acute myocardial infarction are sustained by extramedullary monocytopoiesis

Monocytes (Mo) and macrophages (MΦ) are emerging therapeutic targets in malignant, cardiovascular, and autoimmune disorders. Targeting of Mo/MΦ and their effector functions without compromising innate immunity's critical defense mechanisms first requires addressing gaps in knowledge about the life cycle of these cells. Here we studied the source, tissue kinetics, and clearance of Mo/MΦ in murine myocardial infarction, a model of acute inflammation after ischemic injury. We found that a) Mo tissue residence time was surprisingly short (20 h); b) Mo recruitment rates were consistently high even days after initiation of inflammation; c) the sustained need of newly made Mo was fostered by extramedullary monocytopoiesis in the spleen; d) splenic monocytopoiesis was regulated by IL-1β; and e) the balance of cell recruitment and local death shifted during resolution of inflammation. Depending on the experimental approach, we measured a 24 h Mo/MΦ exit rate from infarct tissue between 5 and 13% of the tissue cell population. Exited cells were most numerous in the blood, liver, and spleen. Abrogation of extramedullary monocytopoiesis proved deleterious for infarct healing and accelerated the evolution of heart failure. We also detected rapid Mo kinetics in mice with stroke. These findings expand our knowledge of Mo/MΦ flux in acute inflammation and provide the groundwork for novel anti-inflammatory strategies for treating heart failure.

Propranolol and the risk of hospitalized myopathy: translating chemical genomics findings into population-level hypotheses

BACKGROUND

A recent large-scale, chemical screening study raised the hypothesis that propranolol may increase the risk of myopathy. We tested this hypothesis in a large population to assess whether (1) propranolol use is associated with an increased risk of myopathy and (2) the concurrent use of propranolol with a statin may further increase risk of myopathy.

METHODS

New users of propranolol and other beta-blockers (BBs) aged >/=65 were identified using data from Medicare and drug benefit programs in 2 states (1994-2005). The primary end point studied was hospitalization for myopathy or rhabdomyolysis. We used stratified Cox proportional hazards regression to estimate the multivariate-adjusted effect of propranolol compared to other BBs and controlled for demographic variables, risk factors for myopathy, other comorbidities, and health service use measures. We also assessed whether co-use of propranolol and statin further increases the risk, by including an interaction term for use of propranolol and statins.

RESULTS

We identified 9,304 initiators of propranolol and 130,070 initiators of other BBs and found 30 cases of hospitalized myopathy in 15,477 person-years (PYs) of propranolol use and 523 in 343,132 PYs of other BB use. Comparing propranolol with other BB users, the adjusted hazard ratio was 1.45 (95% CI 1.00-2.11) for myopathy and 1.48 (95% CI 0.82-2.67) for rhabdomyolysis. We could not detect interaction between propranolol and statins due to limited power. Similar results were observed when propranolol users were compared to other antihypertensive drug users.

CONCLUSIONS

Propranolol may be associated with a 45% increased risk of hospitalized myopathy in the elderly. Our study illustrates how results from in vitro chemical screens can be translated into hypotheses about drug toxicity at the population level.