ESPE Yearbook of Paediatric Endocrinology

Am J Hum Genet. 2022;109(4):553-570. PMID: 35202564. doi: 10.1016/j.ajhg.2022.02.002.

Brief Summary: The authors present elegant data showing that X-LAG is a TADopathy of the endocrine system and that the rewiring of GPR101 -enhancer interactions most likely causes the upregulation of GPR101 expression in X-LAG-related pituitary tumors.

The X-LAG microduplication Xq26.3 contains the genes VGLL1, CD40LG, ARHGEF6, RBMX, and GPR101. GPR101 encodes a class A rhodopsin-like orphan G-protein coupled receptor. Pituitary tumors overexpress GPR101, but the exact mechanism leading to this has been unclear. Here, the authors suggest this is due to rewiring of regulatory elements and formation of a new topologically associated domain (TAD). TADs are most likely a new concept to us pediatric endocrinologists. Generally, temporospatial expression of genes is mediated by cis-regulatory elements (CREs), a.k.a. promoters and enhancers, and by transcription factors. To simplify, the genome is divided into discrete regulatory units, named TADs, which take part in regulating the specificity of enhancer-promoter interactions. TADs are separated from each other by TAD borders, which are enriched by DNA-binding factor CTCF. It has been shown previously that the disruption of TADs through duplications, deletions, or insertions can rewire long-range regulatory architecture and result in pathogenic phenotypes. Examples of such diseases are Quebec platelet disorder and limb malformations. (1, 2)

The authors used Hi-C and 4C-seq methods in 6 patients with X-LAG and showed that GPR101 lies within a TAD, separate from centromeric genes within the X-LAG locus. The microduplications in the Xq26.3 chromosomal region led to disruption of the TAD structure, demonstrated by ectopic interactions that crossed the TAD border. The rearrangement, caused by the duplications, forms a neo-TAD that exposes the GPR101 promoter to interact with nearby cis-regulatory elements (CREs), i.e., enhancer adoption within the new regulatory unit. Subsequently, the authors demonstrated in vitro that one of the identified pituitary CREs within the neo-TAD significantly enhanced reporter gene expression.

In summary, rewiring GPR101 -enhancer interaction within the new regulatory unit is likely to cause the high levels of aberrant expression of GPR101 in pituitary tumors seen in X-LAG. Therefore, X-LAG is actually a TADopathy.

References: 1. Disruptions of topological chromatin domains cause pathogenic rewiring of gene-enhancer interactions. Lupiáñez DG, Kraft K, Heinrich V, Krawitz P, Brancati F, Klopocki E, Horn D, Kayserili H, Opitz JM, Laxova R, Santos-Simarro F, Gilbert-Dussardier B, Wittler L, Borschiwer M, Haas SA, Osterwalder M, Franke M, Timmermann B, Hecht J, Spielmann M, Visel A, Mundlos S. Cell. 2015 May 21;161(5):1012–1025. 2. Enhancer-gene rewiring in the pathogenesis of Quebec platelet disorder. Liang M, Soomro A, Tasneem S, Abatti LE, Alizada A, Yuan X, Uusküla-Reimand L, Antounians L, Alvi SA, Paterson AD, Rivard GÉ, Scott IC, Mitchell JA, Hayward CPM, Wilson MD. Blood. 2020 Dec 3;136(23):2679–2690.