Functionally significant insulin-like growth factor I receptor mutations in centenarians

Abstract

Rather than being a passive, haphazard process of wear and tear, lifespan can be modulated actively by components of the insulin/insulin-like growth factor I (IGFI) pathway in laboratory animals. Complete or partial loss-of-function mutations in genes encoding components of the insulin/IGFI pathway result in extension of life span in yeasts, worms, flies, and mice. This remarkable conservation throughout evolution suggests that altered signaling in this pathway may also influence human lifespan. On the other hand, evolutionary tradeoffs predict that the laboratory findings may not be relevant to human populations, because of the high fitness cost during early life. Here, we studied the biochemical, phenotypic, and genetic variations in a cohort of Ashkenazi Jewish centenarians, their offspring, and offspring-matched controls and demonstrated a gender-specific increase in serum IGFI associated with a smaller stature in female offspring of centenarians. Sequence analysis of the IGF1 and IGF1 receptor (IGF1R) genes of female centenarians showed overrepresentation of heterozygous mutations in the IGF1R gene among centenarians relative to controls that are associated with high serum IGFI levels and reduced activity of the IGFIR as measured in transformed lymphocytes. Thus, genetic alterations in the human IGF1R that result in altered IGF signaling pathway confer an increase in susceptibility to human longevity, suggesting a role of this pathway in modulation of human lifespan.

Fig. 1.

Phenotypes and chemotypes of the IGF system in the centenarian cohort. (A) Female offspring of centenarians (n = 105) have higher serum IGFI levels compared with age-matched female controls without a family history of unusual longevity (n = 67). IGFI levels in male offspring (n = 92) and male controls (n = 42) were identical. IGFI levels in serum were measured by ELISA (*, P < 0.01). Results are reported as mean ± SD. (B) Female offspring are shorter than controls as measured by maximal reported height (**, P < 0.001). Results are reported as mean ± SD. (C) Immortalized lymphocytes from the female centenarians carrying mutations (Carrier) in IGF1R (Ala-37–Thr, Arg-407–His, and Thr-470–Thr) show significant reductions in IGFIR levels compared with immortalized lymphocytes from female centenarians with no mutations (Noncarrier, n = 10) as measured by ELISA (⋀, P < 0.03). (D) IGF signaling is defective in the IGF1R mutation carriers (Carrier) of female centenarians as compared with female centenarians with no mutations (Noncarrier, n = 10) as measured by immunoblot analysis of the ratio of phosphorylated to total AKT in response to IGFI treatment in immortalized lymphocytes. (E) A representative immunoblot for total and phosphorylated AKT in immortalized lymphocytes from a centenarian carrying the Arg-407–His mutation and a control centenarian without the mutation.

Fig. 2.

2D gene scanning of human IGF1 and IGF1R genes. The entire coding regions and exon–intron junctions of the IGF1 and IGF1R genes were amplified by two-step PCR. Thirty short PCR fragments were distributed in 2D gels according to their size and melting temperature. (A) A 2D gene scanning pattern from a centenarian subject with the fragment identification number and a hetero-duplex band in exon 8.1 and exon 6 of the IGF1Rgene is shown. (B) The novel genetic variation in exon 6 was identified as 1355G>A (Arg-407–His) by nucleotide sequencing.

Fig. 3.

The position of mutated amino acids in the IGFIR. The locations of the mutated amino acids are indicated in the crystallographic structure encompassing the first three domains of the IGFIR (L1-CR-L2 fragment) (24). Color-coded residues are those when mutated reduce the binding affinity of IGFI for IGFIR by 2- to 10-fold (pale pink) or >20-fold (red) (23) or a compound heterozygote missense mutations (hot pink) from a patient with intrauterine and postnatal growth retardation (22). Arrows indicate the locations of the mutated amino acids identified in centenarians.