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Human Disorders of Cranial Motor Neuron Development and Wiring
Summary: Elizabeth Engle is studying how errors in the development and wiring of cranial motor neurons result in human disorders of eye and facial movement. She is also investigating the genetic contributions to common forms of strabismus, such as esotropia and exotropia.
We use clinical data, genetics, animal models, and cellular and biochemical approaches to study congenital strabismus and human defects in cranial motor neuron and nerve development.
Congenital Cranial Dysinnervation Disorders
The common final pathway for normal ocular motility depends on correct development of oculomotor, trochlear, and abducens cranial motor neurons and their precise innervation of six extraocular muscles (Figure 1a). We have found that complex congenital eye-movement disorders can serve as sensitive indicators for errors in the development of these lower motor neurons and their processes—and that they result from mutations in genes essential to specific steps in the development of ocular cranial motor neurons and their axons (Figure 1b–h). These include mutations in genes essential to motor neuron development and to axon navigation and targeting. Thus, these disorders provide a paradigm for studying brainstem motor neuron development, and motor neuron development in general.
We begin this work by identifying and phenotyping patients with congenital eye-movement disorders, in collaboration with clinicians worldwide. This has led to the definition of a new category of human malformation syndromes, the congenital cranial dysinnervation disorders (CCDDs). Following identification of new disease genes, we have refined and expanded the CCDD phenotypes, revealing co-inheritance of additional developmental defects for some CCDDs, and providing insight into the etiologies of more-prevalent pediatric disorders. These include scoliosis, co-inherited with ROBO3 mutations; autism, conotruncal heart defects, and internal carotid artery anomalies co-inherited with HOXA1 mutations; and limb anomalies resulting from SALL4 mutations. While continuing the genetic arm of our studies, we have also created animal models for select CCDDs, and we are using cell biological and biochemical approaches to investigate their pathogenesis.
CCDDs Resulting from Errors in Motor Neuron Development
The human HOXA1-related syndromes: aberrant hindbrain segmentation resulting in absent abducens motor neurons (Figure 1d). Working with our collaborators, we found that two overlapping syndromes result from recessive mutations in HOXA1: the Bosley-Salih-Alorainy syndrome (BSAS) identified in Middle Easterners, and the previously described Athabascan brainstem dysgenesis syndrome (ABDS) in Native Americans. Affected individuals have horizontal gaze palsy and most have congenital sensorineural deafness secondary to bilateral absence of the cochlea, semicircular canals, and vestibule, and variable malformations of the internal carotid arteries. In addition, some BSAS patients have autism spectrum disorder, while some ABDS patients have central hypoventilation, mental retardation, facial weakness, vocal cord paralysis, and conotruncal heart defects. We mapped these disorders and identified three HOXA1 founder mutations, each predicted to result in loss of HOXA1 function. The human phenotype overlaps with the Hoxa1–/– mouse models reported in the early 1990s by Mario Capecchi (HHMI, University of Utah) and Pierre Chambon (Institute of Genetics and Molecular and Cellular Biology in Strasbourg, France). Vascular patterning defects of the internal carotid arteries and cardiac outflow tracts have not been reported in Hoxa1–/– mice. It is also intriguing that individuals with loss of function of HOXA1 can be autistic or mentally retarded, given that forebrain and cerebellar defects have not been reported in Hoxa1–/– mice and Hoxa1 expression has not been detected above the brainstem. We believe that this is the only recognized Mendelian disorder resulting from mutations in a human HOX gene critical for development of the central nervous system, and the only known viable homozygous truncating mutations in any human HOX gene.
Congenital fibrosis of the extraocular muscles type 2 (CFEOM2): early failure of oculomotor and trochlear motor neuron development (Figure 1e). Individuals born with CFEOM2, an autosomal-recessive syndrome, have bilateral ptosis, with their eyes primarily fixed in an exotropic position. This phenotype suggested that only the abducens-innervated lateral rectus, innervated by the abducens nerve, is functioning normally. Indeed, our orbital magnetic resonance imaging of patients revealed absence of the oculomotor and, likely, trochlear nerves. We found that CFEOM2 results from homozygous loss-of-function mutations in PHOX2A. The homeodomain transcription factor Phox2a was found by others to be expressed in proliferating oculomotor and trochlear motor neuron precursors, and Phox2a–/– mice lack oculomotor and trochlear nuclei. These mice also lack the locus coeruleus and parasympathetic and sensory ganglia of the head, phenotypes that we do not detect in CFEOM2 patients.
CCDDs Resulting from Errors in Axon Navigation and Targeting
Horizontal gaze palsy with progressive scoliosis (HGPPS): aberrant axon targeting of abducens motor neurons in the hindbrain (Figure 1b). Individuals with HGPPS, an autosomal-recessive syndrome, are born with absent horizontal eye movements and develop severe progressive scoliosis, starting in infancy or childhood. In collaboration with Joanna Jen's laboratory (University of California, Los Angeles), we examined the neuroanatomic basis of HGPPS and identified ROBO3 as the mutated gene.
Unexpectedly, electrophysiological studies revealed that the descending corticospinal and ascending dorsal somatosensory tracts whose axons normally cross the midline in the medulla to reach their contralateral targets in the spinal cord and thalamus, respectively, do not cross the midline in HGPPS patients. We found that affected individuals harbor recessive loss-of-function mutations in ROBO3. ROBO3 shares homology with roundabout genes important in axon guidance in developing Drosophila and zebrafish, and has highest homology to mouse Rig1. Marc Tessier-Lavigne (Genentech) and his colleagues removed Robo3 function in mouse and found complete failure of spinal commissural axons and hindbrain precerebellar axons and neurons to cross the midline. In humans, the ocular motor axons and extraocular muscle appear normal by magnetic resonance imaging, suggesting that the horizontal gaze palsy may result from errors in the input onto the abducens motor neurons by axons and neurons normally destined to cross the midline. Remarkably, HGPPS patients have few symptoms attributable to the lack of corticospinal and dorsal column–medial lemniscus tract crossing. It seems likely that these uncrossed axons succeed in finding and innervating their correct targets, albeit on the incorrect side of the body.
Congenital fibrosis of the extraocular muscles type 1 (CFEOM1): aberrant axonal targeting of the extraocular muscles (Figure 1f). Individuals with CFEOM1, an autosomal-dominant disorder, are born with bilateral ptosis and with the primary position of both eyes fixed downward. Our postmortem neuropathologic examination of an affected individual revealed absence of the superior division of the oculomotor nerve and the corresponding motor neurons in the midbrain oculomotor nucleus, and marked abnormalities of the levator palpebrae superioris and superior rectus muscles, which are normally innervated by this branch and elevate the eyelid and the globe, respectively. We mapped CFEOM1 and identified heterozygous mutations in the developmental kinesin, KIF21A. Thus, KIF21A becomes the first kinesin implicated in a human neurodevelopmental disorder. Kinesins are molecular motors that interact with and transport cargo along microtubules, and our findings, in agreement with those of Lawrence Goldstein (HHMI, University of California, San Diego), demonstrate that Kif21a is expressed primarily in the central nervous system of mouse and is engaged in anterograde axonal transport. We have identified KIF21A mutations in more than 70 probands with CFEOM1 and, remarkably, have identified only 11 unique missense mutations. These repetitive de novo mutations alter only six amino acid residues. Five of the altered amino acid residues are located within a single coiled-coil region of the KIF21A stalk, and the sixth is at the end of the motor domain adjacent to the stalk, suggesting that CFEOM1 may result from altered function, rather than haploinsufficiency of KIF21A. To understand the role of KIF21A in health and disease, we have generated knockin and knockout mouse models and are identifying its cargos.
Additional CCDDs. We continue to identify and genetically map new ocular CCDD syndromes and are searching for the genes mutated at the DURS2, FEOM3, and PTOS1 loci (Figure 1c, 1g, and 1h, respectively). We are also beginning to study potential lower cranial nerve disorders, such as facial weakness, tongue hypoplasia/paresis, palatal weakness, dysphagia, and some hypoventilation/respiratory disorders.
Neurogenetic Basis of Common Strabismus
Normal eye movement and fused binocular vision depend on the correct development of a complex neuronal network of sensory, motor, and integrative pathways. Common strabismus affects up to 5 percent of the population, and both esotropia and exotropia have genetic susceptibility and are more likely to result from cortical rather than brainstem defects. We have begun a large-scale collaborative study of the common forms of strabismus to identify strabismus and strabismus susceptibility genes.
This work is also supported by the National Eye Institute.
Last updated: September 9, 2008
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