Both underweight and obesity have been associated with increased mortality1,2. Underweight, defined as body mass index (BMI) ≤ 18,5 kg/m2 in adults 3 and ≤ −2 standard deviations (SD) in children4,5, is the main sign of a series of heterogeneous clinical conditions such as failure to thrive (FTT) 6–8, feeding and eating disorder and/or anorexia nervosa9,10. In contrast to obesity, few genetic variants underlying these clinical conditions have been reported 11, 12. We previously demonstrated that hemizygosity of a ~600 kb region on the short arm of chromosome 16 (chr16:29.5–30.1Mb), causes a highly-penetrant form of obesity often associated with hyperphagia and intellectual disabilities13. Here we show that the corresponding reciprocal duplication is associated with underweight. We identified 138 (132 novel cases) duplication carriers (108 unrelated carriers) from over 95,000 individuals clinically-referred for developmental or intellectual disabilities (DD/ID), psychiatric disorders or recruited from population-based cohorts. These carriers show significantly reduced postnatal weight (mean Z-score −0.6; p=4.4×10−4) and BMI (mean Z-score −0.5; p=2.0×10−3). In particular, half of the boys younger than 5 years are underweight with a probable diagnosis of FTT, while adult duplication carriers have an 8.7-fold (p=5.9×10−11; CI_95=[4.5–16.6]) increased risk of being clinically underweight. We observe a significant trend towards increased severity in males, as well as a depletion of male carriers among non-medically ascertained cases. These features are associated with an unusually high frequency of selective and restrictive feeding behaviours and a significant reduction in head circumference (mean Z-score −0.9; p=7.8×10−6). Each of the observed phenotypes is the converse of one reported in carriers of deletions at this locus, correlating with changes in transcript levels for genes mapping within the duplication but not within flanking regions. The reciprocal impact of these 16p11.2 copy number variants suggests that severe obesity and being underweight can have mirror etiologies, possibly through contrasting effects on eating behaviour.

Three genes encoding for mitochondrial complex II proteins are linked to hereditary paraganglioma. We have recently shown that an inactivation of the SDHD gene is associated with a complete loss of mitochondrial complex II activity and a stimulation of the angiogenic pathway (Gimenez-Roqueplo, A. P., J. Favier, P. Rustin, J. J. Mourad, P. F. Plouin, P. Corvol, A. Rötig, and X. Jeunemaitre, 2001, Am J Hum Genet 69:1186-1197). Here, we relate the case of a malignant sporadic pheochromocytoma induced by a germline missense mutation of the SDHB gene. Within the tumor, a loss of heterozygosity at chromosome 1pter led to a null SDHB allele and to a complete loss of complex II enzymatic activity. In situ hybridization and immunohistochemistry experiments showed a high expression of hypoxic-angiogenic responsive genes, similar to that previously observed in inherited-SDHD tumors. This observation highlights the role of the complex II mitochondrial genes in the oxygen-sensing pathway and in the regulation of angiogenesis of neural crest-derived tumors.