An ancestry-enriched PIEZO1 missense variant biases HbA1c-based diagnosis of prediabetes and type 2 diabetes in South Asians

An ancestry-enriched PIEZO1 missense variant common in South Asians lowers HbA1c levels independent of blood glucose, leading to the systematic underestimation of prediabetes and type 2 diabetes risk and a higher likelihood of missed diagnoses and complications when relying solely on HbA1c for screening.

The Gist

The Big Picture: A "False Negative" in the Blood Test

Imagine you are trying to measure how much sugar is in a bucket of water. You use a special dye that changes color based on the sugar level. This is how doctors currently check for diabetes using a test called HbA1c.

For most people, the color of the dye perfectly matches the sugar level. But this study discovered a genetic "glitch" in many people of South Asian ancestry that messes up the dye.

The Glitch:
There is a specific gene variant (a tiny change in our DNA) called PIEZO1. About 1 in 12 South Asians carries this variant.

• What it does: It acts like a "fast-forward" button for red blood cells. It makes them die and get replaced faster than usual.
• The Result: Because the red blood cells don't live long enough to get "stained" by sugar, the HbA1c test shows a lower color intensity (a lower score) than the person's actual sugar levels deserve.

The Analogy:
Imagine a factory where workers (red blood cells) stay on the job for 120 days. Sugar (glucose) stains their uniforms.

• Normal Worker: Stays 120 days, gets very stained. The boss (the doctor) sees a dark uniform and knows, "Wow, there's a lot of sugar in the factory!"
• The Glitch Worker: Leaves the factory after only 30 days. They haven't had time to get very stained, even though the factory is full of sugar. The boss looks at the clean uniform and thinks, "Oh, the sugar levels are fine," even though they are actually dangerous.

What the Researchers Found

The team looked at two groups of people:

1. South Indians (in India) to check the biology.
2. British Bangladeshis and Pakistanis (in the UK) to check the real-world health consequences.

Here is what they discovered:

1. The Test is Lying (But the Sugar is Real)
People with the gene variant had normal sugar levels in their blood, but their HbA1c test scores were artificially low. The test was "blind" to the real danger because the red blood cells were too young to show the stain.

2. The "Late Arrival" Problem
Because the test scores looked normal, doctors didn't diagnose these people with prediabetes or Type 2 diabetes when they should have.

• The Consequence: These patients missed out on early warning signs. They didn't get into prevention programs (like diet and exercise coaching) until it was too late.
• The Numbers: The researchers estimated that for every 100,000 South Asian adults, about 1,000 people missed a prediabetes diagnosis and 300 people missed a diabetes diagnosis over a 10-year period because of this genetic glitch.

3. The "Silent Damage"
Even though the test said "everything is fine," the sugar in their blood was actually high. This high sugar was silently damaging their bodies.

• The Eye Damage: People with this gene who did eventually get diagnosed with diabetes were actually more likely to develop diabetic eye disease (retinopathy). Why? Because the damage started years earlier, during the time when the test was lying and saying they were healthy.

The Economic Cost

The researchers also did the math on the cost to the healthcare system (specifically the UK NHS).

• Because these people weren't diagnosed early, they missed the chance to join free prevention programs that save money in the long run.
• The Bill: This "missed opportunity" costs the healthcare system roughly £1 million to £1.4 million for every 100,000 people over a decade. It's like ignoring a small leak in a roof; it seems cheap now, but the water damage later is expensive.

The Solution: Don't Rely on One Ruler

The main takeaway isn't that the HbA1c test is bad; it's that it's not perfect for everyone.

• Current Situation: Doctors use one ruler (HbA1c) to measure everyone.
• Proposed Change: For South Asian patients, doctors might need to use a "second ruler" (like a direct blood sugar test) or be extra careful if the HbA1c looks suspiciously low.

Summary in One Sentence

A common genetic trait in South Asians makes red blood cells live too short to show the true sugar levels in an HbA1c test, causing doctors to miss early warnings of diabetes and leading to more eye damage and higher healthcare costs.

The Bottom Line: We need to personalize medicine. Just because a test works for the majority doesn't mean it works for everyone, especially when our unique genetic "blueprints" change how the test reads.

Technical Summary

1. Problem Statement

Glycated haemoglobin (HbA1c) is the global standard for diagnosing and monitoring Type 2 Diabetes (T2D) and prediabetes. However, HbA1c levels are influenced not only by blood glucose but also by erythrocyte (red blood cell) biology, including red blood cell turnover and lifespan.

• The Gap: A specific missense variant in the PIEZO1 gene (rs563555492T), which is enriched in South Asian populations (carried by ~1 in 12 individuals), was previously associated with lower HbA1c levels in UK cohorts without corresponding changes in fasting glucose.
• The Uncertainty: It was unclear if this association held true in geographically distinct South Asian populations (outside the UK), whether the mechanism was purely erythrocytic (affecting red blood cells) or involved pancreatic insulin secretion, and what the clinical and economic consequences of delayed diagnosis were. Specifically, it was unknown if this variant led to missed diagnoses of prediabetes/T2D and increased risks of complications like diabetic eye disease.

2. Methodology

The study employed an integrated genetic and phenotypic analysis across two large, distinct South Asian cohorts:

A. Study Cohorts

1. Madras Diabetes Research Foundation (MDRF), South India:
   • Sample: 19,898 individuals (37.4% female).
   • Data: Linked genetic data and deep metabolic phenotyping (HbA1c, fasting/postprandial glucose, fasting/stimulated C-peptide).
   • Purpose: To replicate the genetic association and characterize the biological mechanism (glycemic vs. erythrocytic).
2. Genes & Health (G&H), UK:
   • Sample: 43,811 British Bangladeshis and British Pakistanis (54.4% female).
   • Data: Whole Exome Sequencing (WES) linked to longitudinal electronic health records (EHR) from primary care.
   • Purpose: To assess clinical outcomes (time to diagnosis of prediabetes, T2D, and diabetic eye disease) and model economic impacts.

B. Statistical & Analytical Approaches

• Genetic Association: Used additive genetic models (linear mixed models in Regenie) adjusted for age, sex, and principal components (PCs) to test associations with biomarkers in MDRF.
• Survival Analysis: In G&H, used flexible parametric survival models (RMS package) with restricted cubic splines to model time-to-event for:
  • Prediabetes and T2D diagnosis (follow-up started Jan 1, 2014, coinciding with HbA1c adoption in UK guidelines).
  • Diabetic eye disease (follow-up started at T2D diagnosis).
• Risk Modeling: Estimated 10-year absolute risks and attributable risks for a modeled population aged 40–50 years.
• Economic Modeling: Calculated the opportunity cost of missed prediabetes diagnoses by estimating foregone referrals to the NHS Diabetes Prevention Programme (NHS-DPP), using published QALY gains and cost savings data.

3. Key Contributions

• Replication: Confirmed the association between PIEZO1 rs563555492T and lower HbA1c in a geographically distinct South Indian cohort, proving the effect is not limited to the UK environment.
• Mechanistic Clarification: Demonstrated that the variant affects HbA1c via erythrocytic mechanisms (lower Mean Corpuscular Volume, higher bilirubin) rather than pancreatic dysfunction, as there was no association with fasting glucose, postprandial glucose, or C-peptide (insulin secretion marker).
• Clinical Impact Quantification: Provided the first quantitative estimates of "missed diagnoses" and subsequent complication risks attributable to this genetic variant in a real-world healthcare setting.
• Economic Valuation: Translated genetic bias into health-economic terms, estimating the cost of missed prevention opportunities for the NHS.

4. Key Results

A. Biomarker Associations (MDRF Cohort)

• HbA1c: The effect allele (T) was associated with significantly lower HbA1c ($\beta$ -0.15 SD units).
• Erythrocytes: Associated with lower Mean Corpuscular Volume (MCV) and raised total bilirubin (markers of hemolysis/erythrocyte turnover).
• Glycemia/Insulin: No significant association with fasting glucose, postprandial glucose, or C-peptide levels. This confirms the HbA1c reduction is non-glycemic.

B. Clinical Outcomes (G&H Cohort)

• Delayed Diagnosis: Carriers of the T allele had a significantly reduced hazard of being diagnosed with:
  • Prediabetes: HR 0.63 (95% CI 0.58–0.69).
  • T2D: HR 0.86 (95% CI 0.78–0.94).
  • Interpretation: The lower HbA1c masks the true glycemic risk, leading to later diagnosis.
• Complications: Among individuals who were diagnosed with T2D, carriers had an increased hazard of diabetic eye disease (HR 1.20, 95% CI 1.01–1.43). This suggests that the delay in diagnosis allowed hyperglycemia to persist untreated, increasing microvascular damage.
• Age at Diagnosis: Median age at diagnosis increased with the number of T alleles (GG: ~44.8y; GT: ~46.2y; TT: ~47.1y for prediabetes).

C. Population Impact & Economic Modeling

• Missed Diagnoses: In a modeled population of 100,000 adults (aged 40–50) over 10 years:
  • ~1,019 prediabetes diagnoses were missed.
  • ~303 T2D diagnoses were missed.
• Economic Burden: These missed diagnoses represent lost opportunities for prevention. The estimated economic burden (direct costs + monetized health loss) ranges from £970,000 to £1,390,000 per 100,000 individuals over 10 years.
• Cost-Effectiveness: Implementing alternative diagnostic strategies or targeted genetic screening would be cost-effective if the cost per person is below £9.70 – £13.90.

5. Significance and Implications

• Diagnostic Bias: The study provides robust evidence that relying solely on HbA1c for South Asian populations systematically underestimates glycemic risk for a substantial minority (approx. 8% of the population carrying the allele).
• Clinical Guidelines: Current guidelines may need to be adapted for South Asian populations. Clinicians should consider alternative biomarkers (e.g., fasting glucose, oral glucose tolerance tests) or genotype-informed HbA1c interpretation for patients with this variant to prevent delayed diagnosis.
• Health Equity: The findings highlight a genetic driver of health inequality, where a specific ancestry-enriched variant leads to worse outcomes (delayed treatment, higher complication rates) and increased healthcare costs.
• Mechanism: The confirmation of an erythrocytic mechanism (via PIEZO1) rather than a pancreatic one refines the understanding of how genetic variation influences diabetes biomarkers, distinguishing this from other variants that might affect insulin secretion.

In conclusion, the PIEZO1 rs563555492T variant acts as a "silent" genetic modifier that lowers HbA1c independent of glucose, creating a diagnostic blind spot that delays intervention and increases the risk of diabetic complications in South Asian populations.