Rare variants in single-minded 1 (SIM1) are associated with severe obesity
Shwetha Ramachandrappa, Anne Raimondo, Anna M.G. Cali, Julia M. Keogh, Elana Henning, Sadia Saeed, Amanda Thompson, Sumedha Garg, Elena G. Bochukova, Soren Brage, Victoria Trowse, Eleanor Wheeler, Adrienne E. Sullivan, Mehul Dattani, Peter E. Clayton, Vippan Datta, John B. Bruning, Nick J. Wareham, Stephen O’Rahilly, Daniel J. Peet, Ines Barroso, Murray L. Whitelaw, I. Sadaf Farooqi
Shwetha Ramachandrappa, Anne Raimondo, Anna M.G. Cali, Julia M. Keogh, Elana Henning, Sadia Saeed, Amanda Thompson, Sumedha Garg, Elena G. Bochukova, Soren Brage, Victoria Trowse, Eleanor Wheeler, Adrienne E. Sullivan, Mehul Dattani, Peter E. Clayton, Vippan Datta, John B. Bruning, Nick J. Wareham, Stephen O’Rahilly, Daniel J. Peet, Ines Barroso, Murray L. Whitelaw, I. Sadaf Farooqi
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Research Article

Rare variants in single-minded 1 (SIM1) are associated with severe obesity

Abstract

Single-minded 1 (SIM1) is a basic helix-loop-helix transcription factor involved in the development and function of the paraventricular nucleus of the hypothalamus. Obesity has been reported in Sim1 haploinsufficient mice and in a patient with a balanced translocation disrupting SIM1. We sequenced the coding region of SIM1 in 2,100 patients with severe, early onset obesity and in 1,680 controls. Thirteen different heterozygous variants in SIM1 were identified in 28 unrelated severely obese patients. Nine of the 13 variants significantly reduced the ability of SIM1 to activate a SIM1-responsive reporter gene when studied in stably transfected cells coexpressing the heterodimeric partners of SIM1 (ARNT or ARNT2). SIM1 variants with reduced activity cosegregated with obesity in extended family studies with variable penetrance. We studied the phenotype of patients carrying variants that exhibited reduced activity in vitro. Variant carriers exhibited increased ad libitum food intake at a test meal, normal basal metabolic rate, and evidence of autonomic dysfunction. Eleven of the 13 probands had evidence of a neurobehavioral phenotype. The phenotypic similarities between patients with SIM1 deficiency and melanocortin 4 receptor (MC4R) deficiency suggest that some of the effects of SIM1 deficiency on energy homeostasis are mediated by altered melanocortin signaling.

Authors

Shwetha Ramachandrappa, Anne Raimondo, Anna M.G. Cali, Julia M. Keogh, Elana Henning, Sadia Saeed, Amanda Thompson, Sumedha Garg, Elena G. Bochukova, Soren Brage, Victoria Trowse, Eleanor Wheeler, Adrienne E. Sullivan, Mehul Dattani, Peter E. Clayton, Vippan Datta, John B. Bruning, Nick J. Wareham, Stephen O’Rahilly, Daniel J. Peet, Ines Barroso, Murray L. Whitelaw, I. Sadaf Farooqi

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Figure 3

Variants in SIM1 significantly reduce transcriptional activation.

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Variants in SIM1 significantly reduce transcriptional activation. (A) We...
(A) Western blot analysis of Myc-tagged SIM1 expression in whole-cell extracts from doxycycline-induced 293 Flp-In T-Rex cells. (B and C) Two independently derived WT SIM1 293 Flp-In T-Rex stable cell lines, 13 different SIM1 mutant 293 Flp-In T-Rex stable cell lines, and 1 empty vector cell line were transiently transfected with SIM1 luciferase reporter plasmid, Renilla control plasmid, and either ARNT (B) or ARNT2 (C) expression plasmid. Cells were then stimulated with doxycycline and analyzed for luciferase activity. Bars represent the mean Firefly/Renilla value expressed relative to WT, which has been normalized to 100%. Results shown are the average of at least 3 experiments performed in triplicate + SEM, expressed relative to the 2 WT lines, which have been combined into a single average value and normalized to 100%. Log values generated for each mutant were compared with WT. *P < 0.05; **P < 0.01; ***P < 0.001. (D) Homology model of the SIM1 ARNT2 PAS-A heterodimer. SIM1 is shown in yellow and ARNT2 is shown in green, with amino acids that contain point variants colored red. (E and F) Ribbon diagrams of the SIM1 ARNT2 PAS-A heterodimer, with amino acids that contain point variants depicted as sticks colored pink.