Underweight and obese phenotypes can both pose health risks. But whereas obesity has been associated with a number of genetic variants, little is known about the genetic basis of underweight. A large-scale screen of data from 28 cytogenetic centres in Europe and North America now shows that being underweight is frequently associated with duplication of a short region on chromosome 16. Deletion of this same chromosomal region has previously been associated with obesity. The observed associated phenotypes are opposites, or mirrors, of those reported in carriers of deletions at this locus, and correlate with changes in transcript levels for genes within the duplication but not within the adjacent regions. The suggestion is that severe obesity and being underweight could have mirror etiologies, possibly through contrasting effects on energy balance. Both obesity and being underweight have been associated with increased mortality1,2. Underweight, defined as a body mass index (BMI) ≤ 18.5 kg per m2 in adults and ≤ −2 standard deviations from the mean in children, is the main sign of a series of heterogeneous clinical conditions including failure to thrive3,4,5, feeding and eating disorder and/or anorexia nervosa6,7. In contrast to obesity, few genetic variants underlying these clinical conditions have been reported8,9. We previously showed that hemizygosity of a ∼600-kilobase (kb) region on the short arm of chromosome 16 causes a highly penetrant form of obesity that is often associated with hyperphagia and intellectual disabilities10. Here we show that the corresponding reciprocal duplication is associated with being underweight. We identified 138 duplication carriers (including 132 novel cases and 108 unrelated carriers) from individuals clinically referred for developmental or intellectual disabilities (DD/ID) or psychiatric disorders, or recruited from population-based cohorts. These carriers show significantly reduced postnatal weight and BMI. Half of the boys younger than five years are underweight with a probable diagnosis of failure to thrive, whereas adult duplication carriers have an 8.3-fold increased risk of being clinically underweight. We observe a 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 eating behaviours and a significant reduction in head circumference. Each of the observed phenotypes is the converse of one reported in carriers of deletions at this locus. The phenotypes correlate with changes in transcript levels for genes mapping within the duplication but not in flanking regions. The reciprocal impact of these 16p11.2 copy-number variants indicates that severe obesity and being underweight could have mirror aetiologies, possibly through contrasting effects on energy balance.

Background The recurrent ∼600 kb 16p11.2 BP4-BP5 deletion is among the most frequent known genetic aetiologies of autism spectrum disorder (ASD) and related neurodevelopmental disorders. Objective To define the medical, neuropsychological, and behavioural phenotypes in carriers of this deletion. Methods We collected clinical data on 285 deletion carriers and performed detailed evaluations on 72 carriers and 68 intrafamilial non-carrier controls. Results When compared to intrafamilial controls, full scale intelligence quotient (FSIQ) is two standard deviations lower in carriers, and there is no difference between carriers referred for neurodevelopmental disorders and carriers identified through cascade family testing. Verbal IQ (mean 74) is lower than non-verbal IQ (mean 83) and a majority of carriers require speech therapy. Over 80% of individuals exhibit psychiatric disorders including ASD, which is present in 15% of the paediatric carriers. Increase in head circumference (HC) during infancy is similar to the HC and brain growth patterns observed in idiopathic ASD. Obesity, a major comorbidity present in 50% of the carriers by the age of 7 years, does not correlate with FSIQ or any behavioural trait. Seizures are present in 24% of carriers and occur independently of other symptoms. Malformations are infrequently found, confirming only a few of the previously reported associations. Conclusions The 16p11.2 deletion impacts in a quantitative and independent manner FSIQ, behaviour and body mass index, possibly through direct influences on neural circuitry. Although non-specific, these features are clinically significant and reproducible. Lastly, this study demonstrates the necessity of studying large patient cohorts ascertained through multiple methods to characterise the clinical consequences of rare variants involved in common diseases.

Chromosomal rearrangements occur constitutionally in the general population and somatically in the majority of cancers. Detection of balanced rearrangements, such as reciprocal translocations and inversions, is troublesome, which is particularly detrimental in oncology where rearrangements play diagnostic and prognostic roles. Here we describe the use of Hi-C as a tool for detection of both balanced and unbalanced chromosomal rearrangements in primary human tumour samples, with the potential to define chromosome breakpoints to bp resolution. In addition, we show copy number profiles can also be obtained from the same data, all at a significantly lower cost than standard sequencing approaches.

This study identifies multiple copies of the AML1 gene on a duplicated chromosome 21, dup(21), as a recurrent abnormality in acute lymphoblastic leukemia (ALL). Clusters of AML1 signals were visible at interphase by fluorescence in situ hybridization (FISH). In metaphase, they appeared tandemly duplicated on marker chromosomes of five distinct morphological types: large or small acrocentrics, metacentrics, submetacentrics or rings. The markers comprised only chromosome 21 material. Karyotypes were near-diploid and, besides dup(21), no other established chromosomal changes were observed. A total of 20 patients, 1.5 and <0.5% among consecutive series of childhood and adult ALL respectively, showed this phenomenon. Their median age was 9 years, white cell counts were low and all had a pre-B/common immunophenotype. Although this series is not the first report of this abnormality, it is the largest, permitting a detailed description of the variety of morphological forms that duplicated chromosome 21 can assume.