Digenic Mutations in the FGF-21 Signaling Pathway Found in an Adolescent with Extreme Insulin Resistance

Fibroblast Growth fFactor-21 (FGF-21) is a secretory molecule that has multiple metabolic functions. However, its role in human insulin resistance remains unclear. Here we report a 12 year old female presenting to pediatric diabetes clinic with acanthosis nigricans, hirsutism, and acromegalic features. The subject was noted to have extremely elevated insulin (2446 uIU/mL) and elevated free testosterone (16 pg/mL). Due to her unique presentation, we hypothesized that she may have a genetic insulin resistance syndrome and whole exome sequencing was performed. 2 variants were identified in genes critical for FGF-21 signaling, Fibroblast Growth Factor Receptor 1 (FGFR1) and beta Klotho (KLB). These mutations were inherited in trans from each parent. FGFR1 and KLB are transmembrane co-factors that bind FGF-21, leading to activation of intracellular tyrosine kinases, and subsequent intracellular signaling. Fasting serum FGF-21 was elevated in the subject (391.3 pg/mL) compared to her father (104.3 pg/mL) and mother (225.2 pg/mL). A bicistronic expression plasmid was created recapitulating the above mutations found in this family. These plasmids were transiently transfected into L6 myoblasts, a cell line known to have low levels of FGF receptor expression. ERK phosphorylation (pERK), the major downstream signal of FGF-21, was measured after the cells were treated with FGF-21. pERK was strongly upregulated in the cells transfected with wild type FGFR1 and KLB (43,463 AU). However, mutations in either FGFR1 or KLB greatly attenuated pERK (13,601.5 and 15,526 AU respectively). The combination of mutations in both FGFR1 and KLB led to the lowest level of pERK (10,148.3 AU). We hypothesize that these digenic mutations in both FGFR1 and KLB are acting in a synergistic fashion leading to FGF-21 resistance. This helps explain the severe insulin resistance found in this subject and may represent a novel category of insulin resistance syndromes related to FGF-21.