Persistence of prolonged C-peptide production in type 1 diabetes as measured with an ultrasensitive C-peptide assay

Abstract

Objective: To examine persistence of C-peptide production by ultrasensitive assay years after onset of type 1 diabetes and factors associated with preserving β-cell function.

Research design and methods: Serum C-peptide levels, a marker of insulin production and surviving β-cells, were measured in human subjects (n = 182) by ultrasensitive assay, as was β-cell functioning. Twenty-two times more sensitive than standard assays, this assay's lower detection limit is 1.5 pmol/L. Disease duration, age at onset, age, sex, and autoantibody titers were analyzed by regression analysis to determine their relationship to C-peptide production. Another group of four patients was serially studied for up to 20 weeks to examine C-peptide levels and functioning.

Results: The ultrasensitive assay detected C-peptide in 10% of individuals 31-40 years after disease onset and with percentages higher at shorter duration. Levels as low as 2.8 ± 1.1 pmol/L responded to hyperglycemia with increased C-peptide production, indicating residual β-cell functioning. Several other analyses showed that β-cells, whose C-peptide production was formerly undetectable, were capable of functioning. Multivariate analysis found disease duration (β = -2.721; P = 0.005) and level of zinc transporter 8 autoantibodies (β = 0.127; P = 0.015) significantly associated with C-peptide production. Unexpectedly, onset at >40 years of age was associated with low C-peptide production, despite short disease duration.

Conclusions: The ultrasensitive assay revealed that C-peptide production persists for decades after disease onset and remains functionally responsive. These findings suggest that patients with advanced disease, whose β-cell function was thought to have long ceased, may benefit from interventions to preserve β-cell function or to prevent complications.

Figure 1

Sensitivity of standard (Cobas/Roche) versus ultrasensitive (Mercodia) C-peptide assays in a cohort of individuals with type 1 diabetes for whom weekly samples were taken for up to 20 weeks (n = 4 subjects, n = 54 blood samples). Each blood sample was tested by both assays. The shaded areas represent the areas below the detection limits of the assay. A: Standard assay failed to detect C-peptide in any of the selected serum sample. The shaded area across the lower portion of the panel displays the limit of detection of the Cobas assay (33.1 pmol/L), a limit defined as a maintained CV of 1.9%. B: Ultrasensitive assay detected C-peptide in the majority of the same samples. The dashed horizontal line across the entire lower portion of the panel displays the limit of detection (1.5 pmol/L). C and D: Samples taken from four patients studied for functional responses to glycemic level. Samples depicting hyperglycemia (glucose >150 mg/dL) and normoglycemia (glucose ≤150 mg/dL) revealed that C-peptide level was elevated with mild hyperglycemia, as opposed to normoglycemia, by ultrasensitive assay (D). The standard assay is not sensitive enough to reveal this functional response (C). (A high-quality color representation of this figure is available in the online issue.)

Figure 2

The relationships between C-peptide production and disease duration in subjects with type 1 diabetes (n = 182) by ultrasensitive assay. A: Stratified by six intervals of disease duration, in years, subjects with shorter disease duration tended to produce higher levels of C-peptide by the ultrasensitive assay for values up to 230 pmol/L or with the Mercodia regular assay for values >230 pmol/L, which persisted decades after disease onset. The shaded area across the lower portion of the panel displays the limit of detection of the ultrasensitive assay 1.5 pmol/L. B: Percentage of patients with detectable C-peptide above detection limits. (A high-quality color representation of this figure is available in the online issue.)

Figure 3

Investigating functionality of insulin-secreting β-cells based on blood glucose (A–D) or age at onset and lifetime duration of the C-peptide response (E and F) defines the time line of persistent C-peptide section (G). Hyperglycemic (n = 52) or normoglycemic (n = 52) samples were examined in the same subjects (n = 52), who gave samples on consecutive weeks and stratified into four ranges of C-peptide levels: 0–5 pmol/L (nonresponsive), 0–5 pmol/L (responsive), 5–100 pmol/L, and >100 pmol/L (A–D). Significantly higher C-peptide levels were produced in hyperglycemic samples, except around the lower levels of assay detection (range 0–5 pmol/L), where β-cells from some samples did not respond to hyperglycemia. Subjects divided into six groups based on age at onset produced varying levels of C-peptide, with the highest levels produced at ages 31–40 years and a sudden drop in the group >40 years old and studied for remaining C-peptide secretion (E and F). C-peptide production during and after type 1 diabetes diagnosis. Reduction of C-peptide levels starts sometime during the prediabetes stage and keeps going down with the progress of the disease. With the new ultrasensitive C-peptide assays, remaining β-cell function extends for years after loss of pancreas function and in some cases extends to 40 years after diagnosis (G).