The Etiology of Moebius Syndrome-Making the Case for Animal Models

Abstract

Moebius syndrome (MBS) is a rare disease consisting of uni-/bilateral palsy of CN VI and VII without impairment of vertical eye movements. Its uncommon nature means that the etiology is still uncertain. It is thought to be caused by vascular lesions leading to infarction in the nuclei of cranial nerves VI and VII on the posterior aspect of the pons. However, several genes have also been discussed as possibly causative. We performed a literature search in the PUBMED database and on the Science Direct platform with terms related to the pathology and to each etiology individually. Included were original papers and review articles published in peer-reviewed international journals and reference books and databases on the subjects discussed. We excluded articles not published in English, conference communications, dissertations, monographs, and other non-peer-reviewed forms of publication. The total number of publications thus included was 62. This review discusses the functions of the three most related genes found in recent research (PLXND1, REV3L, TUBB3) and the results of animal studies focusing on their mutations. We note that the PLXND1 and REV3L mutations have been most associated with MBS and that the current studies on their function suggest histological lesions similar to the target disease, albeit without clear phenotypic expression. We ascertain that TUBB3 mutations are mostly related to CEFOM3, which is a differential diagnosis for MBS. Regarding the vascular etiology, we review the types of lesions involved and discuss their timing in relation to embryologic stages. We also highlight the main investigation methods available. A multitude of the factors discussed might be causative of MBS, and we thus consider it necessary to attempt the development of an animal model for the disease. To this end, we propose the development of transgenic mice models containing the single nucleotide mutations documented in human patients, and we discuss the use of the chick embryo model for the vascular etiology.

Keywords: Moebius sequence; Moebius syndrome; PLXND1; REV3L; TUBB3; animal model; mice.

Figure 1

PLXND1 function: Image (A) details the role of the PLXND1 gene. Located on chromosome 3q21, it codes a transmembrane protein (Plexin D1) involved in semaphorin signaling and found on endothelial neovascular cells. Interaction with semaphorin 3C (Sema 3C) signals from developing neurons halts incursions of new blood vessels in the path of the developing axon. A lack of Plexin D1 leads to aberrant vasculogenesis and a disruption of normal axonal growth. Image (B) details the results of mice experiments into Plexin D1 function. Homozygous loss-of-function (LOF) mutations cause cardiac and large vessel abnormalities, leading to rapid postnatal lethality. Vertebral splits have also been documented. Heterozygous LOF mutations lead to abnormal axonal pathfinding, especially involving the cranial nerves.

Figure 2

REV3L function: Image (A) details the role of the REV3L gene. Located on chromosome 6q21, it codes the catalytic (ζ) subunit of the DNA polymerase (pol ζ). This polymerase is active in repairing single- or double-strand breaks and in performing translesion synthesis repair of damaged DNA. Image (B) details the results of mice experiments into REV3L function. Homozygous loss of function (LOF) leads to intrauterine lethality at 12.5 dpc. Heterozygous LOF mutations lead to mice presenting normal phenotypes and preserved fertility. However, these mice present altered immune responses due to affected immunoglobulin activity, brain atrophy and hindbrain hypoplasia, and signs of neural apoptosis with decreased concentrations of neurons within the facial (CN VII) nucleus.

Figure 3

TUBB3 function: Image (A) details the role of the TUBB3 gene. Located on chromosome 16q24.3, it codes the protein β-III tubulin. Together with other members of the tubulin family, these proteins form microtubules. The process of microtubule formation and dissolution is continuous within cells. β-III tubulin appears in mice embryos starting from E9.5 and is neuron-specific, not being present in glial cells. Image (B) details the results of mice experiments into TUBB3 function. Homozygous deletion leads to normal phenotypes, with the other tubulins having compensatory expression. However, inactivation of the gene during the embryologic period (E14.5) leads to a breakdown of cortical neural migration. Homozygous R262C mutation (a mutation identified as disease-inducing for this gene) leads to lethality immediately after birth. These mice present multiple histological abnormalities such as hypoplasia of the corpus callosum, anterior commissure, and basal ganglia. They also have impaired migration of the cranial nerves, mostly affecting CN III, IV, and V. The heterozygous form of the R262C mutation produces only mild callosal hypoplasia.

Figure 4

Vascular development and pathogenic theories in mice: Image (A) is a coronal section at the level of the pons in adult mice. The nuclei of the facial nerve (nCN VII) and of the abducens nerve (nCN VI) are located near the floor of the ventricle. The area is located posteriorly, distant from the main arteries, leading to the formation of a less densely vascularized area called the “watershed area”. Image (B) is a coronal section at the level of the pons between 7.5–8.5 dpc. We can observe the incipient development of an anterior vascular plexus. Image (C) is a coronal section at the level of the pons at 9.5 dpc. The vascular plexus has surrounded the entirety of the neural tube. Image (D) is a coronal section at the level of the pons at 10.5 dpc. The plexus is organized into blood vessels surrounding the neural tubes with ramifications forming and incipient penetration of the neural tube. Image (E) is a coronal section at the level of the pons at 12.5 dpc. The arteries have continued ramifying and have entered the neural tissue, leading to the formation of a periventricular plexus.

Figure 5

Flowchart detailing selection methodology for the review.