Regulation of gene transcription is the most common way to control gene expression. Enhancers are DNA sequences that promote transcription. Although they act on genes on the same DNA molecule, they can be located thousands of base pairs away from the site of the gene being regulated. Promoters, in contrast, are DNA sequences that define where transcription of a gene begins and in what direction it proceeds, and they are typically located 25–35 base pairs upstream of where transcription begins. Enhancer-promoter interactions are critical to gene expression. Research in recent years has revealed that the genome is highly organized at a local level into genome units, called topologically associating domains (TADs). These units are insulated by boundaries that facilitate certain gene promoter-enhancer interactions, while decreasing interactions outside the TAD.
This project investigated the over-expression of the GPR101 gene in the pituitary tumours of children with X-linked acrogigantism (X-LAG), believed to be caused by the creation of a new chromatin domain (neo-TAD) where new enhancer-promoter interactions take place. X-LAG is an extremely rare and severe pediatric form of pituitary gigantism that has been described in 40 patients worldwide. It mirrors exactly the disease suffered by the tallest humans ever recorded. Excessive body growth begins during infancy and is caused by enhanced production of growth hormone (GH) from the pituitary gland. This pathology is associated with a genetic defect: duplication of the GPR101 gene. GPR101 specifies for a receptor localized in the outer layer of cells.
Understanding what determines the overexpression of GPR101 will be fundamental in a new therapeutic perspective for patients affected by gigantism. This represents the first step towards the future development of treatments that specifically inhibit the enhancer sequences that interact with GPR101: turning off GPR101’s high expression will reduce the excessive secretion of GH and the associated debilitating symptoms.
The objective of this proposal was to identify the molecular mechanisms underlying GPR101 overexpression in the pituitary tumors of children with GPR101 duplications causing X-LAG. This was tested by:
1. determining the chromatin structure around the GPR101 gene in healthy subjects and patients with X-LAG;
2. characterizing the GPR101 promoter and identifying and functionally characterizing novel pituitary-specific enhancers.
My results identified X-LAG as a new TADopathy; to my knowledge this is the first TADopathy to be described in endocrinology. Xq26.3 duplications disrupt the local chromatin architecture by forming a neo-TAD and this rewiring of GPR101-enhancer interaction causes the marked over-expression of GPR101 in X-LAG pituitary tumors, which in turn drives tumoral GH hypersecretion and gigantism in affected children.
