The CALLY index first attracted attention in surgical oncology, where studies of patients undergoing gastrectomy for gastric cancer, hepatectomy for colorectal liver metastases and surgery for oral cavity and breast cancers showed that the measure carried prognostic significance for outcomes. Its appeal lies in its elegant simplicity. The index integrates three routinely measured blood components that capture distinct dimensions of a person’s internal physiology: C-reactive protein, an acute-phase protein produced by the liver that rises rapidly during systemic inflammation; albumin, the most abundant plasma protein, whose concentration reflects nutritional status, liver synthetic function and the degree of chronic inflammatory suppression; and the lymphocyte count, a readout of adaptive immune competence. By combining inflammation, nutrition and immunity into a single number, the CALLY index offers a panoramic snapshot of the body’s systemic state that no single biomarker can provide.

What has remained unclear, however, is whether the index merely correlates with disease or whether it plays some causal role in disease development. Observational associations are notoriously vulnerable to reverse causation and confounding. A patient with early neurodegeneration may have poor nutrition and chronic inflammation as consequences of the disease process rather than contributors to it. To cut through this ambiguity, the research team, led by investigators at Soochow University in Suzhou, China, turned to Mendelian randomization, a technique that exploits the random allocation of genetic variants at conception as a form of natural experiment. Because genetic variants inherited from parents are fixed at birth and generally unaffected by later disease processes, they can serve as proxies for a lifelong exposure, allowing researchers to test whether the exposure itself influences disease risk rather than the reverse.

The scale of the underlying data is central to the study’s power. The team first conducted a genome-wide association study of the CALLY index in 265,409 UK Biobank participants, scanning millions of genetic variants across the genome for statistical associations with the composite measure. This effort identified 154 genomic loci significantly associated with the index, providing a rich set of genetic instruments. The loci implicated genes involved in hepatic acute-phase responses, immune cell regulation and metabolic pathways, which is consistent with the three biological pillars the index is designed to capture. The breadth of the signal also confirmed that the CALLY index has a substantial and distributed genetic architecture, a prerequisite for reliable Mendelian randomization analysis.

With the genetic instruments in hand, the researchers performed a phenome-wide association study, or PheWAS, across 713 phenotypes in 113,747 UK Biobank participants. Rather than asking whether one biomarker predicts one disease, a PheWAS inverts the question and asks whether the genetically predicted biomarker is associated with any condition across the entire recorded spectrum of human illness. This agnostic screening approach is designed to surface unexpected connections that hypothesis-driven studies would never think to test. Out of the 713 phenotypes examined, 24 showed statistically significant associations with the genetically predicted CALLY index, a yield that is striking given the stringency of the multiple-testing corrections applied in such analyses.

The most consequential of these associations involved the neurodegenerative outcomes. Higher genetically predicted CALLY index values were linked to lower risks of Alzheimer’s disease, Alzheimer’s dementia and unspecified dementia, and the two-sample Mendelian randomization analysis confirmed these as causal protective effects rather than mere correlations. This finding dovetails with a growing body of evidence implicating systemic inflammation and immune dysfunction in Alzheimer’s pathogenesis. The blood-brain barrier, once viewed as an impenetrable fortress protecting the central nervous system, is now known to be permeable to inflammatory signals arising from systemic infection and chronic low-grade inflammation. Peripheral inflammatory activity can modulate microglial activation within the brain, influence amyloid-beta clearance and shape the trajectory of neurodegeneration. An index that integrates inflammatory burden, nutritional reserve and adaptive immune capacity may therefore capture, in a single measure, several converging pathways that influence the brain’s resilience against degenerative disease.

The metabolic findings reinforce this systems-level interpretation. The PheWAS and subsequent causal analyses linked higher CALLY index values to lower risks of lipid metabolism disorders, and pleiotropy analyses pointed to shared biological pathways in lipid metabolism and inflammation connecting the index to its disease outcomes. Chronic low-grade inflammation is a well-established feature of obesity, insulin resistance and dyslipidemia, and inflammatory cytokines released from adipose tissue can perturb hepatic lipid handling and accelerate atherosclerotic processes. Albumin, meanwhile, is a major carrier protein for fatty acids and other lipophilic molecules in the circulation, and its concentration is sensitive to both nutritional intake and inflammatory catabolism. The convergence of these signals suggests that the CALLY index functions as a barometer of the inflammatory-metabolic axis, a physiological axis whose dysfunction is increasingly recognized as a common root of multiple chronic diseases of aging.

Not every association in the study was reassuring. The Mendelian randomization analysis indicated a causal association between higher genetically predicted CALLY index and increased risk of sleep disorders, a finding that adds an intriguing and cautionary dimension to the biomarker’s profile. Sleep disturbance is itself tightly intertwined with inflammation and immune function; insufficient or fragmented sleep elevates inflammatory markers, while chronic inflammation disrupts the hypothalamic circuits that regulate circadian rhythm. The direction of the observed association, in which a more favorable inflammatory-nutritional-immune profile paradoxically accompanies greater sleep disorder risk, will require further investigation to disentangle. It may reflect shared genetic pathways, compensatory physiological responses or pleiotropic effects of the instrument variants that influence both the index and sleep biology through independent routes. The authors addressed this concern with a battery of sensitivity analyses, including linkage disequilibrium score regression and pleiotropy testing, designed to detect and correct for horizontal pleiotropy, the phenomenon in which genetic variants affect outcomes through pathways other than the exposure of interest.

The methodological rigor underpinning these conclusions deserves emphasis. Two-sample Mendelian randomization relies on three core assumptions: the genetic instruments must be strongly associated with the exposure, must not be confounded by population stratification or other environmental factors, and must influence the outcomes only through the exposure. Violations of the third assumption, known as pleiotropy, are the principal threat to validity, and the study deployed multiple complementary tests, including Egger regression-based approaches, to detect directional pleiotropic bias. The use of two independent data sources, with the UK Biobank supplying the exposure and outcome data and FinnGen providing replication material, further strengthens the case that the observed causal signals are not artifacts of a single cohort’s idiosyncrasies. The ethical framework of the underlying resource was also robust, with approval from the Northwest Multi-center Research Ethics Committee and informed consent from all participants.

The practical implications extend beyond the laboratory. Because the CALLY index is computed from three tests that are already part of standard clinical blood panels, it could be incorporated into risk prediction models at essentially no additional cost. If future prospective studies validate the causal relationships reported here, the index could help clinicians identify individuals whose inflammatory-nutritional-immune profile places them at elevated risk of dementia or metabolic disease decades before symptoms emerge, opening a window for preventive interventions targeting inflammation, nutrition and immune health. At the same time, the sleep disorder finding is a reminder that systemic physiology is a web of trade-offs rather than a simple gradient from good to bad. The study’s authors, supported by funding from the Natural Science Foundation of China, frame the CALLY index as a promising integrated biomarker for disease risk stratification and public health applications, and this phenome-wide genetic analysis provides the strongest evidence yet that the number scrawled on a routine lab report may quietly encode a person’s vulnerability to some of the most burdensome diseases of our time.

Subject of Research: Causal associations between the CRP–albumin–lymphocyte (CALLY) index and multisystem human diseases using phenome-wide Mendelian randomization

Article Title: CRP–albumin–lymphocyte index and multisystem diseases: a phenome-wide Mendelian randomization study

Article References: Niu, B., Xia, M.-H., Wu, J.-X., He, P., Bo, L., Wu, H.-Y., Lei, S.-F., & Deng, F.-Y. (2026). CRP–albumin–lymphocyte index and multisystem diseases: a phenome-wide Mendelian randomization study. Molecular Genetics and Genomics, 301(1), Article 199. https://doi.org/10.1007/s00438-026-02513-0

Image Credits: AI Generated

DOI: 10.1007/s00438-026-02513-0

Keywords: CALLY index, Mendelian randomization, PheWAS, GWAS, Alzheimer's disease, dementia, lipid metabolism, sleep disorders, inflammation, biomarkers, UK Biobank, FinnGen

The study, published as a short commentary in the Journal of Neurology, comes from a group led by Monica Gagliardi of the Neuroscience Research Center at Magna Graecia University in Catanzaro, in the southern Italian region of Calabria. The researchers measured two molecular features in whole blood from 58 patients with Parkinson’s disease, 35 patients with multiple system atrophy, and 62 healthy controls drawn from the same region. The first feature was the copy number of mitochondrial DNA, estimated by quantifying the mitochondrially encoded NADH dehydrogenase 1 gene, known as ND1, relative to the nuclear beta-actin gene. The second was telomere length, the protective caps at the ends of chromosomes that erode with each cell division and are widely regarded as a cellular clock of aging.

The technical approach relied on quantitative PCR, the workhorse method for amplifying and measuring specific DNA sequences. By comparing the abundance of a mitochondrial gene to a single-copy nuclear gene, the team obtained a relative estimate of how many mitochondrial genomes each blood sample carries per cell. Telomere length was measured with the same technique, comparing the amount of telomeric DNA to a reference gene. To guard against spurious results, the researchers applied multivariable linear regression adjusted for age and sex, two factors known to influence both mitochondrial DNA copy number and telomere length, and they demanded that findings survive correction for multiple comparisons using false discovery rate-adjusted Wald tests, a statistical safeguard that reduces the risk of false positives when many hypotheses are tested at once.

The results were striking. Mitochondrial DNA copy number, as indexed by the ND1 gene, was significantly reduced in both patient groups compared with healthy controls, with a false discovery rate-adjusted p-value of 1.63 times ten to the power of minus twenty-two for Parkinson’s disease and 2.56 times ten to the power of minus eight for multiple system atrophy. More importantly for the question of diagnosis, the marker also separated the two diseases from each other: Parkinson’s patients showed significantly lower ND1 copy number than multiple system atrophy patients, with an adjusted p-value of 8.43 times ten to the power of minus twenty-two. In a field where biomarkers that cleanly discriminate between overlapping neurodegenerative conditions are desperately scarce, a difference of that statistical magnitude in a peripheral blood test is remarkable.

Telomere length told a related but subtly different story. Both patient groups again showed significantly shorter telomeres than the controls, with adjusted p-values of 3.21 times ten to the power of minus sixteen for Parkinson’s disease and 1.44 times ten to the power of minus twelve for multiple system atrophy. But when the researchers compared the two patient groups directly against each other, the difference was far weaker, reaching only the threshold of statistical significance with an adjusted p-value of 5.47 times ten to the power of minus two. In other words, telomere shortening appears to be a shared feature of both alpha-synucleinopathies, a general signature of accelerated cellular aging, whereas mitochondrial DNA copy number carries disease-specific information that could, in principle, help clinicians tell the two conditions apart.

Why would mitochondrial DNA copy number be depleted in the blood of patients with these brain diseases? The answer likely lies in the intimate relationship between mitochondria and neurodegeneration. Mitochondria are the organelles that generate most of the chemical energy a cell needs, and they carry their own small circular genome, separate from the DNA in the nucleus. Cells adjust the number of mitochondrial genomes they harbor in response to energy demands and stress. Decades of research have implicated mitochondrial dysfunction in Parkinson’s disease, from studies showing impaired mitochondrial respiration in the blood cells of people with early and even prodromal Parkinson’s, to work demonstrating mitochondrial defects in the fibroblasts of patients with multiple system atrophy. Previous studies have also reported reduced mitochondrial DNA copy number as a biomarker of Parkinson’s disease, although the picture is complicated: some investigations, including one in individuals of African ancestry, found increased blood-derived mitochondrial DNA copy number, and recent work has even documented elevated mitochondrial DNA copy number in the cerebellum of people with Parkinson’s. The direction and meaning of the signal may depend on tissue, disease stage, ancestry, and the specific cell populations being counted.

That last caveat is an important one. Whole blood is a mixture of cell types, and different immune cells carry different amounts of mitochondrial DNA. Recent research has suggested that differences in the abundance of peripheral immune cell populations may link blood mitochondrial DNA copy number to Parkinson’s disease, meaning that what looks like a mitochondrial defect could partly reflect shifts in blood cell composition driven by inflammation. Neuroinflammation is increasingly recognized as a player in both Parkinson’s disease and multiple system atrophy, so the reduced copy number observed in this study could reflect either genuine mitochondrial depletion within cells or an altered mix of circulating leukocytes, or both. The authors themselves frame their findings as preliminary and call for longitudinal studies in larger, independent cohorts to validate the diagnostic and prognostic utility of the marker before it could enter clinical practice.

The telomere findings also fit into a rich and sometimes contradictory literature. Telomere dysfunction is known to trigger metabolic and mitochondrial compromise through a stress-response network involving the p53 protein, creating a mechanistic bridge between the two markers measured in this study. Mouse experiments have shown that telomere shortening accelerates synucleinopathy and impairs the response of microglia, the brain’s immune cells, in genetic models of Parkinson-like disease. Human studies of leukocyte telomere length in Parkinson’s disease have produced mixed results over the years, with some groups reporting shorter telomeres in patients, others finding no difference, and longitudinal work suggesting that longer telomeres at diagnosis may paradoxically predict faster progression to dementia in idiopathic parkinsonism. A recent prospective analysis of the UK Biobank linked leukocyte telomere length to the risk of neurodegenerative diseases more broadly, reinforcing the idea that biological aging markers capture something real about vulnerability to these conditions, even if the details remain contested.

What makes the new study noteworthy is not any single result but the head-to-head comparison of the two markers within the same cohort, using the same methods and the same statistical framework. By showing that mitochondrial DNA copy number separates Parkinson’s disease from multiple system atrophy while telomere length does not, the researchers provide a concrete argument for prioritizing mitochondrial DNA copy number as a non-invasive, blood-based biomarker of disease-specific mitochondrial alterations in alpha-synucleinopathies. If the finding holds up in larger and more diverse populations, it could eventually help solve one of clinical neurology’s persistent puzzles: distinguishing Parkinson’s disease from multiple system atrophy early in the disease course, when the distinction matters most for counseling patients, planning treatment, and selecting the right participants for clinical trials of disease-modifying therapies. For now, the measurement requires nothing more exotic than a blood draw and quantitative PCR, a combination that, if validated, would make this one of the most practical biomarker candidates to emerge from the intersection of mitochondrial biology and neurodegeneration research.

Subject of Research: Mitochondrial DNA copy number and telomere length as blood biomarkers in Parkinson's disease and multiple system atrophy

Article Title: Preliminary insights into whole-blood mitochondrial DNA copy number and telomere length in Parkinson’s disease and multiple system atrophy

Article References: Preliminary insights into whole-blood mitochondrial DNA copy number and telomere length in Parkinson’s disease and multiple system atrophy. (n.d.). https://doi.org/10.1007/s00415-026-14172-7

Image Credits: AI Generated

DOI: 10.1007/s00415-026-14172-7

Keywords: Parkinson's disease, multiple system atrophy, mitochondrial DNA copy number, telomere length, alpha-synucleinopathies, biomarkers, neurodegeneration, mitochondrial dysfunction, quantitative PCR, cellular aging, blood biomarker, Journal of Neurology