Insulin gene mutations resulting in early-onset diabetes: marked differences in clinical presentation, metabolic status, and pathogenic effect through endoplasmic reticulum retention.
OBJECTIVE
Heterozygous mutations in the human preproinsulin (INS) gene are a cause of nonsyndromic neonatal or early-infancy diabetes. Here, we sought to identify INS mutations associated with maturity-onset diabetes of the young (MODY) or nonautoimmune diabetes in mid-adult life, and to explore the molecular mechanisms involved.


METHODS
The INS gene was sequenced in 16 French probands with unexplained MODY, 95 patients with nonautoimmune early-onset diabetes (diagnosed at <35 years) and 292 normoglycemic control subjects of French origin. Three identified insulin mutants were generated by site-directed mutagenesis of cDNA encoding a preproinsulin-green fluorescent protein (GFP) (C-peptide) chimera. Intracellular targeting was assessed in clonal beta-cells by immunocytochemistry and proinsulin secretion, by radioimmunoassay. Spliced XBP1 and C/EBP homologous protein were quantitated by real-time PCR.


RESULTS
A novel coding mutation, L30M, potentially affecting insulin multimerization, was identified in five diabetic individuals (diabetes onset 17-36 years) in a single family. L30M preproinsulin-GFP fluorescence largely associated with the endoplasmic reticulum (ER) in MIN6 beta-cells, and ER exit was inhibited by approximately 50%. Two additional mutants, R55C (at the B/C junction) and R6H (in the signal peptide), were normally targeted to secretory granules, but nonetheless caused substantial ER stress.


CONCLUSIONS
We describe three INS mutations cosegregating with early-onset diabetes whose clinical presentation is compatible with MODY. These led to the production of (pre)proinsulin molecules with markedly different trafficking properties and effects on ER stress, demonstrating a range of molecular defects in the beta-cell.
Meur
Gargi
G
Section of Cell Biology, Division of Medicine, Imperial College London, London, UK.
Simon
Albane
A
Harun
Nasret
N
Virally
Marie
M
Dechaume
Aurélie
A
Bonnefond
Amélie
A
Fetita
Sabrina
S
Tarasov
Andrei I
AI
Guillausseau
Pierre-Jean
PJ
Boesgaard
Trine Welløv
TW
Pedersen
Oluf
O
Hansen
Torben
T
Polak
Michel
M
Gautier
Jean-François
JF
Froguel
Philippe
P
Rutter
Guy A
GA
Vaxillaire
Martine
M
D000293:Adolescent; D000328:Adult; D017668:Age of Onset; D003924:Diabetes Mellitus, Type 2 / Q000235:genetics* / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005192:Family Health; D005260:Female; D005602:France; D049452:Green Fluorescent Proteins / Q000235:genetics; D006579:Heterozygote; D006801:Humans; D050417:Insulin-Secreting Cells / Q000502:physiology; D008297:Male; D016297:Mutagenesis, Site-Directed; D010375:Pedigree; D017354:Point Mutation; D011384:Proinsulin / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D021381:Protein Transport / Q000502:physiology; D012333:RNA, Messenger / Q000378:metabolism; D013312:Stress, Physiological / Q000502:physiology; D055815:Young Adult
D012333:RNA, Messenger; D049452:Green Fluorescent Proteins; D011384:Proinsulin