Truncating RAX Mutations: Anophthalmia, Hypopituitarism, Diabetes Insipidus, and Cleft Palate in Mice and Men

Abstract

ORCiD numbers: 0000-0001-7955-2534 (C. Brachet).Context: The transcription factor RAX is a paired-type homeoprotein that plays a critical role in eye and forebrain development of vertebrate species. RAX knockout mice have anophthalmia, cleft palate, and an abnormal hypothalamus and display perinatal lethality. In humans, homozygous or compound heterozygous RAX mutations have been reported to cause bilateral microphthalmia or anophthalmia without consistent associated features. Congenital hypopituitarism can be associated with various eye or craniofacial anomalies; however, the co-occurrence of congenital hypopituitarism, anophthalmia, cleft palate, and diabetes insipidus has been very rare. Results:We report the case of a child with anophthalmia, congenital hypopituitarism, diabetes insipidus, and bilateral cleft lip and palate who had a homozygous frameshift truncating mutation c.266delC (p.Pro89Argfs*114) in exon 1 of the RAX gene. Rax knockout mice show loss of ventral forebrain structures, pituitary, and basosphenoid bone and palate and a misplaced anterior pituitary gland along the roof of the oral cavity.Conclusions: Our patient's phenotype was more severe than that reported in other patients. Although most of the previously reported patients with RAX mutations showed either a missense or some less severe mutation in at least one of their RAX alleles, our patient was homozygous for truncating mutations that would yield a severe, null protein phenotype. The severity of the genetic defect, the precise match between the knockout mouse and the patient's endocrine phenotypes, and the prominent roles of RAX in eye and pituitary development and diencephalic patterning suggest that the RAX null mutations could fully account for the observed phenotype. (J Clin Endocrinol Metab

Conserved roles of Rax/rx3 genes in hypothalamus and pituitary development

“…The patient also displayed diabetes insipidus, a possible sign of arginine vasopressin deficiency. Thus, the absence of a visible pituitary and panhypopituitarism with greatly reduced, albeit still detectable, pituitary hormones in the patient are compatible with the phenotypes observed in the mouse (Zhang et al, 2000;Medina-Martínez et al, 2009;Orquera et al 2016;Brachet et al, 2019). Rax-null mice analysed at birth lack the basosphenoid bone and palate, a phenotype that is also related to the midline defects in the patient (Brachet et al, 2019) and might be the result of the lack of colonisation and accumulation of mesenchyme cells between the ventral hypothalamus and the oral cavity that is observed in Rax mutants at early embryonic stages (Zhang et al, 2000;Orquera et al, 2016).…”

“…Since, in mouse and zebrafish, dorso-anterior progenitors require Rax/rx3 (see above) this raises the possibility that Rax indirectly governs Rathke's pouch induction via its ability to direct dorso-anterior hypothalamic progenitor development. However, although Rax mutant embryos lack Shh expression in the dorso-anterior hypothalamus, the fact that Rathke's pouch is still present and develops pituitary cell types in these mutants (Orquera et al, 2016;Brachet et al, 2019) indicates that this reduction in Shh expression happens after the temporal window of adenohypophysis induction by Shh. Indeed, RaxKO@E8.0 embryos are still expressing Shh in the developing forebrain at E9.0 (Orquera et al, 2016), when Rathke's pouch induction has already begun.…”

“…Human patients carrying homozygous or compound heterozygous mutations in RAX (a Rax1 paralogue, Table 1) can develop anophthalmia or microphthalmia (Voronina et al, 2004;Lequeux et al, 2008;Abouzeid et al, 2012;Chassaing et al, 2014;Brachet et al, 2019). In addition, homozygous point mutations in RAX have been found in a patient with coloboma and retinoschisis (Huang et al, 2017), while heterozygous mutations in RAX have been found in two patients with unilateral microphthalmia (González-Rodríguez et al, 2010) and one patient with unilateral coloboma (London et al, 2009).…”

“…Patients with homozygous or compound heterozygous mutations in RAX usually have at least one allele with a mutation that is predicted to be less severe for protein function, such as missense mutations or truncations that preserve part of, or the whole, homeodomain (see Table 2 in Brachet et al, 2019). Surprisingly, the patient described by Brachet et al (2019) is homozygous for a truncating mutation in exon 1 at proline 89 (p.Pro89Argfs*114), predicted to give rise to a protein that lacks the homeodomain and thus be a null or severely hypomorphic mutation.…”

“…Soon after these findings, loss-of-function studies in Xenopus and in teleost fish showed that Rax genes have a conserved role in eye development in vertebrates (Winkler et al, 2000;Loosli et al, 2003;Kennedy et al, 2004;Rojas-Muñoz et al, 2005;Andreazzoli et al, 2003;Bailey et al, 2004). Human patients with anophthalmia, microphthalmia and other eye defects carry mutations in Rax genes, indicating the relevance of these genes to human pathology (Voronina et al, 2004;Wang et al, 2004;Van de Sompele et al, 2018;Brachet et al, 2019).…”