Early-onset Obesity: Unrecognized First Evidence for GNAS Mutations and Methylation Changes

Annette Grüters-Kieslich; Monica Reyes; Amita Sharma; Cem Demirci; Terry J. DeClue; Erwin Lankes; Dov Tiosano; Dirk Schnabel; Harald Jüppner

Disclosures

J Clin Endocrinol Metab. 2017;102(8):2670-2677. 

In This Article

Discussion

Early-onset obesity had brought several infants to medical attention, long before the diagnosis of PHP1A or PHP1B was considered. In fact, excessive weight gain preceded other clinical or laboratory signs of these related diseases, in some patients with PHP1B by more than one decade. Only for two patients (patients 1B and 2B) was the diagnosis of PHP1B considered because of clinically symptomatic hypocalcemia with elevated PTH serum levels at age 11.5 and 14.0 years, respectively. In both cases, no AHO features other than obesity were present. PHP1A was initially considered for siblings 4B and 5B. However, instead of a mutation involving GNAS exons 1 to 13, the frequent 3-kb STX16 deletion was identified, establishing the diagnosis of PHP1B. Likewise, STX16 deletions were found in patients 1B, 3B, and 7B, and patients 2B and 6B showed broad GNAS methylation changes. For the two latter patients, the underlying genetic defect could not be determined; however, paternal uniparental isodisomy involving a large portion of chromosome 20q was excluded through analysis of six microsatellite markers (data not shown).[32,35]

The five patients with a diagnosis of PHP1A had been referred primarily because of obesity, but brachydactyly and subcutaneous calcifications were initially not noted. Also, their mothers showed no obvious evidence for pseudopseudohypoparathyroidism. BWS or Prader-Willi syndrome was suspected in one patient; however, both disorders were ruled out through genetic analyses (data not shown). Two PHP1A patients had older siblings affected by the same disease, and both sibpairs had developed obesity in the first year of life despite intensive dietary counseling. Our findings have demonstrated that PHP1A and, surprisingly, PHP1B can be indistinguishable entities with regard to early-onset obesity. This shared metabolic phenotype is most likely caused by silenced paternal Gsα expression in those parts of the central nervous system that contribute to the regulation of food intake and energy expenditure, combined with globally absent or diminished maternal Gsα expression due to genetic or epigenetic GNAS changes. Consequently, as yet undefined tissues or cells are likely to show in our patients a major reduction of Gsα expression and thus profound cyclic adenosine monophosphate deficiency, leading to abnormal weight regulation.

All parents were concerned that the obesity of their childrenwas related to their impressive increase in appetite. It is, therefore, conceivable that resistance at the hypothalamic Gsα-coupled MC4R and/or melanocortin 3 receptor contributes significantly to the excessiveweight gain in patients with PHP1A[23,24] and patients with PHP1B, as suggested by a recently reported case.[43] It remains to be determined, however, whether additional abnormalities such as abnormal Gq/G11-dependent signaling at the MC4R contribute to obesity in PHP1A or PHP1B patients, as suggested for rodents.[17]

Most patients in whom PHP1B and PHP1A were diagnosed later had an elevated TSH level well before an increased PTH level was documented. However, the fT4/TT4 levels were only slightly reduced or low-normal, making it unlikely that subclinical hypothyroidism was responsible for the development of early-onset obesity. The association between increased TSH levels and obesity, which was encountered in six of our seven patients with PHP1B, confirms that impairedGsα-dependent signaling is not always restricted to the proximal renal tubules.

Unlike PHP1A, PHP1B due to maternal STX16 deletions is associated with increased weights and lengths at birth.[44] These genetic mutations cause loss-ofmethylation at GNAS exon A/B alone, which is sufficient to impair maternal Gsα expression.[45] Biallelic expression of A/B or another GNAS-derived transcript could be responsible for these observations during fetal growth and similar mechanisms could affect postnatal energy metabolism and longitudinal growth. Moreover, generalized imprinting disorders can lead to overlapping epigenetic findings, as described for a patient with clinical and laboratory features of BWS and PHP1B,[42] making it difficult to distinguish these disorders by the clinical symptoms alone. It is therefore conceivable that our two PHP1B patients with broad GNAS methylation changes (patients 2B and 6B) could harbor additional epigenetic changes elsewhere, including methylation changes at the variably methylated region located at the intron 2–exon 3 border of POMC that influence body weight regulation.[46] However, five of the seven patients with PHP1B revealed the well-established, disease-causing deletion in STX16, making it unlikely that additional genetic mutations contributed to abnormal regulation of appetite and body weight homeostasis in these patients.

Patients with an onset of excessive obesity within the first year of life should be regarded as having a high risk of monogenic disorders, including mutations involving GNAS exons 1 to 13 and, as shown in the present study, epigenetic changes at this complex locus. Establishing the diagnosis of PHP1A and PHP1B in patients with earlyonset obesity is important for genetic counseling and parental support. Furthermore, both disorders could potentially provide additional treatment options, such as phosphodiesterase inhibitors to enhance the actions of limited amounts of cyclic adenosine monophosphate derived from the paternal allele or novel MC4R agonists to overcome the partial resistance at the melanocortin receptors in the paraventricular nucleus.[2] The number of children with PHP1B followed up longitudinally after birth has been limited owing to the rarity of the disease. However, as shown in the present study, the prevalence of genetic and epigenetic GNAS changes as the cause of obesity might have been underestimated, because TSH elevations and early-onset obesity failed to prompt further diagnostic evaluation of our PHP1B patients. Furthermore, reduced paternal Gsα expression can occur in additional tissues or cell types, thus leading to other, potentially unrecognized features in the presence of maternal GNAS defects. Larger patient cohorts need to be investigated to determine whether abnormalities other than early-onset obesity and impaired TSH signaling can be identified in PHP1B patients before the development of PTH-resistant hypocalcemia, and whether features such as obesity might be the only evidence of impaired Gsαdependent signaling.

Measurement of PTH and TSH should be considered for all children with substantial early-onset obesity in the first year of life. However, the level of either hormone may not be elevated during infancy or throughout life[18,19,47] and thus will not necessarily be helpful in establishing the diagnosis for all PHP1B cases. Furthermore, DNAsequencing approaches are usually not designed to detect methylation variants and might not detect small deletions or genomic rearrangements. A targeted search for genetic and epigenetic GNAS variants will therefore be necessary to reliably exclude abnormalities in this complex genetic locus, in particular if unexplained obesity is present during infancy, but also later in life. Laboratory testing should be performed to confirm the diagnosis.

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