Comparison of serum insulin levels in homozygous vs heterozygous transgenic tilapia expressing a humanized tilapia insulin gene as well as demonstration of lifelong stable human insulin expression

J.R. Wright Jr¹, O. Hrytsenko², B-Y. Xu³, R. Krause⁴, B. Pohajdak², R.V. Rajotte³, G.R. Rayat^3
1 University of Calgary, Pathology & Laboratory Medicine, Calgary, Canada; ² Dalhousie University, Halifax, Canada; ³ University of Alberta, Edmonton, Canada; ⁴ Calgary Laboratory Services, Calgary, Canada

Objective: Transgenic tilapia expressing a "humanized" tilapia insulin gene were produced for islet xenotransplantation (Transgenic Res. 13:313-323, 2004). For complex reasons, additional work was not done for ~5 years. Breeding has been re-established and young homozygous and heterozygous fish produced. We also have very old, massive non-fertile fish. Since transient transgene expression is common in attempts to produce over-expression or novel expression of proteins in large commercially important fish species, we tested if our very old fish still produce human insulin. We also compared human insulin and serum glucose levels in homozygous and heterozygous younger fish.

Methods: Fasting serum was obtained from: (Group-1) 69 6-8 year old F1 and F2 transgenic fish (zygosity unknown) with no living offspring, (Group-2) 26 2 year old heterozygous F2’s derived from a single mating of two heterozygous F1 fish, and (Group-3) 14 1-2 year old homozygous fish (7 were Group-2 siblings/7 were F3’s derived from mating two homozygous F2 siblings). Serum insulin and glucose levels were measured by Calgary Laboratory Services using the Abbott Axsym microparticle enzyme immunoassay and Roche Modular P hexokinase methods, respectively. Mean values were compared by unpaired T-test.

Results: Mean (+/- SD) serum insulin levels (pmol/L), serum glucose levels (mmol/L), and fish body weights (grams) were as follows: Group-1, 32.42±42.45 / 2.83±1.2 / 2372.2±779.5; Group-2, 58.86±50.3 / 4.29±1.33 / 603.35±164.5; Group-3, 141.24±78.9 / 4.04±1.09 / 625.28±180.63

Conclusions: First, in young transgenic fish, human insulin levels are significantly higher in the homozygous group (p=0.00025), but serum glucose levels are not different (p=0.55161). Second, human insulin expression is lifelong as the maximum lifespan for this species is 9 years (http://www.fishbase.org/summary/SpeciesSummary.php?id=2). Stable expression of insulin for >8 years far exceeds our expectations and supports the premise that transgenic tilapia islets, after further improvements, could become a viable donor source for clinical islet xenotransplantation.