The severe short stature in two siblings with a heterozygous IGF1 mutation is not caused by a dominant negative effect of the putative truncated protein.
OBJECTIVE
While in previous studies heterozygosity for an Insulin-Like Growth Factor 1 (IGF1) defect only modestly decreased height and head circumference, we recently reported on two siblings with severe short stature with a maternally transmitted heterozygous duplication of 4 nucleotides, resulting in a frame shift and a premature termination codon in the IGF1 gene. In this paper we describe the structural and functional characteristics of the putative truncated IGF-I protein.


METHODS
Two children, their mother and maternal grandfather carried the mutation. In addition, two family members who were not affected were included in the study. Mutant (MT) IGF-I was synthesized in oxidized and reduced form using two methods. Neutral gel filtration studies were carried out with wild-type (WT) and synthetic MT IGF-I. Binding analysis of synthetic MT IGF-I to the IGF1R and insulin receptors were performed with EBNA-293 cells, stably transfected with the IGF-I receptor, and IM9 cells. L6 cells were used to examine the mitogenic potency and the potential antagonizing effect of synthetic MT IGF-I by [(3)H]-thymidine incorporation assays.


RESULTS
In the sera of both the carriers and non-carriers the proportion of (125)I-IGF-I that was associated with the 150 kDa complex was somewhat less (varying between ~37 and ~52%) than in normal pooled serum (~53-~63%) and, instead, slightly increased amounts of radioactivity were eluted in the 40-50 kDa fraction (consisting of binary IGF-IGFBP complexes) or remained unbound. Synthetic MT IGF-I did not bind to the IGF-I receptor, nor antagonize the growth-promoting effect of IGF-I. It did bind to IGFBPs, but was barely incorporated into 150 kDa complexes. Because in all cases WT IGF-I immunoreactivity was recovered in one peak, corresponding to the MW of WT IGF-I, i.e. ~7.6 kDa, an interaction of circulating truncated mutant peptide with WT IGF-I is very unlikely.


CONCLUSIONS
There is no evidence that the severe short stature associated with heterozygosity for this novel IGF1 mutation in children born from a mother with the same mutation is caused by a dominant negative effect of the truncated protein. We speculate that the growth failure is caused by a combination of partial IGF-I deficiency, placental IGF-I insufficiency, and other genetic factors.
Department of Pediatrics, Leiden University Medical Center, Leiden, The Netherlands. H.A.van_Duyvenvoorde@lumc.nl
H A
HA
van Duyvenvoorde
J
J
van Doorn
J
J
Koenig
L
L
Gauguin
W
W
Oostdijk
J D
JD
Wade
M
M
Karperien
C A L
CA
Ruivenkamp
M
M
Losekoot
P A
PA
van Setten
M J E
MJ
Walenkamp
C
C
Noordam
P
P
De Meyts
J M
JM
Wit
D000595:Amino Acid Sequence; D001483:Base Sequence; D001827:Body Height / Q000235:genetics; D002648:Child; D004392:Dwarfism / Q000235:genetics*; D005260:Female; D005799:Genes, Dominant; D006579:Heterozygote; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000737:chemistry / Q000235:genetics* / Q000502:physiology; D008297:Male; D008969:Molecular Sequence Data; D020125:Mutation, Missense / Q000502:physiology; D020033:Protein Isoforms / Q000235:genetics / Q000502:physiology; D035781:Siblings
D020033:Protein Isoforms; D007334:Insulin-Like Growth Factor I