HOXA1 mutations are not a common cause of Duane anomaly

Bosley-Salih-Alorainy syndrome in patients from India

Bi-allelic HOXA1 pathogenic variants clinically manifest as two distinct syndromes, Bosley-Salih-Alorainy syndrome (BSAS) and Athabascan brainstem dysgenesis syndrome, mainly reported in two different populations from Saudi Arabia and southwest North America, respectively. Here we report two siblings of Indian origin with BSAS phenotype caused by a novel homozygous exon 2 HOXA1 pathogenic variants.

Congenital cranial dysinnervation disorders

The European Neuromuscular Centre (ENMC) derived the term Congenital Cranial Dysinnervation Disorders in 2002 at an international workshop for a group of congenital neuromuscular diseases. CCDDs are congenital, non-progressive ophthalmoplegia with restriction of globe movement in one or more fields of gaze. This group of sporadic and familial strabismus syndromes was initially referred to as the 'congenital fibrosis syndromes' because it was assumed that the primary pathologic process starts in the muscles of eye motility. Over the last few decades, evidence has accumulated to support that the primary pathologic process of these disorders is neuropathic rather than myopathic. This is believed that for normal development of extra ocular muscles and for preservation of muscle fiber anatomy, normal intra-uterine development of the innervation to these muscles is essential. Congenital dysinnervation to these EOMs can lead to abnormal muscle structure depending upon the stage and the extent of such innervational defects. Over last few years new genes responsible for CCDD have been identified, permitting a better understanding of associated phenotypes, which can further lead to better classification of these disorders. Introduction of high-resolution MRI has led to detailed study of cranial nerves courses and muscles supplied by them. Thus, due to better understanding of pathophysiology and genetics of CCDDs, various treatment modalities can be developed to ensure good ocular alignment and better quality of life for patients suffering from the same.

The genetics of nonsyndromic bilateral Duane retraction syndrome

PURPOSE

To assess the importance of monogenic mutations and chromosomal copy number variants (CNVs) in the occurrence of nonsyndromic bilateral Duane retraction syndrome (bilateral nsDRS).

METHODS

The medical records of 12 patients with bilateral nsDRS were reviewed. Genes associated with DRS and associated congenital cranial dysinnervation disorders (SALL4, CHN1, HOXA1, TUBB3, and KIF21A) were sequenced in the standard fashion in each patient. Array comparative genomic hybridization (array CGH) was performed using Affymetrix Cytogenetics Whole-Genome 2.7M array, and the results were analyzed using Affymetrix Chromosome Analysis Suite v1.2. CNVs were assessed as unlikely to be pathologic if they were also present in the Database of Genomic Variants (DGV) or our local database of array CGH results in 150 normal individuals of Middle Eastern ethnicity.

RESULTS

No patient had a sequence mutation in SALL4, CHN1, HOXA1, TUBB3, or KIF21A. These 12 patients each had 36-42 chromosomal deletions and/or duplications (mean with standard deviation, 26.25 ± 6.77), but all of these CNVs were present either in the DGV or in our local database of normal individuals of similar ethnicity and, therefore, are considered nonpathogenic.

CONCLUSIONS

The results reported here suggest that bilateral nsDRS is not usually associated with mutations in these genes or with chromosomal CNVs. Current evidence suggests other factors such as epigenetic and/or teratogenic abnormalities may be a potential cause of bilateral nsDRS.

The genetic basis of incomitant strabismus: consolidation of the current knowledge of the genetic foundations of disease

In recent years, our understanding of the genetic foundations of incomitant strabismus has grown significantly. Much new understanding has been gleaned since the concept of congenital cranial dysinnervation disorders (CCDDs) was introduced in 2002, and the genetic basis of CCDDs continues to be elucidated. In this review, we aim to provide an update of the genetic and clinical presentation of these disorders. Disorders reviewed include Duane syndrome (DS), HOXA1 and HOXB1 syndromes, Moebius syndrome, congenital fibrosis of the extraocular muscles (CFEOM), and horizontal gaze palsy with progressive scoliosis (HGPPS).

Recent progress in understanding congenital cranial dysinnervation disorders

BACKGROUND

In 2002, the new term congenital cranial dysinnervation disorder (CCDD) was proposed as a substitute for the traditional concept of congenital fibrosis of the extraocular muscles (CFEOM) based on mounting genetic, neuropathologic, and imaging evidence, suggesting that many, if not all, of these disorders result from a primary neurologic maldevelopment rather than from a muscle abnormality. This report provides an update 8 years after that original report.

EVIDENCE ACQUISITION

Review of pertinent articles published from January 2003 until June 2010 describing CCDD variants identified under PubMed MeSH terms congenital fibrosis of the extraocular muscles, congenital cranial dysinnervation disorders, individual phenotypes included under the term CCDD, and congenital ocular motility disorders.

RESULTS

At present, a total of 7 disease genes and 10 phenotypes fall under the CCDD umbrella. A number of additional loci and phenotypes still await gene elucidation, with the anticipation that more syndromes and genes will be identified in the future. Identification of genes and their function, along with advances in neuroimaging, have expanded our understanding of the mechanisms underlying several anomalous eye movement patterns.

CONCLUSIONS

Current evidence still supports the concept that the CCDDs are primarily due to neurogenic disturbances of brainstem or cranial nerve development. Several CCDDs are now known to have nonophthalmologic associations involving neurologic, neuroanatomic, cerebrovascular, cardiovascular, and skeletal abnormalities.

Imaging findings in congenital cranial dysinnervation disorders

In 2002, the term congenital cranial dysinnervation disorders (CCDDs) was proposed to group heterogeneous syndromes with congenital abnormalities of ocular muscle and facial innervations. The concept of neurogenic etiology has been supported by discovery of genes that are essential to the normal development of brainstem, cranial nerves, and their axonal connections. The CCDDs include Duane retraction syndrome, congenital fibrosis of the extraocular muscles, Möbius syndrome, horizontal gaze palsy with progressive scoliosis, the human homeobox-related disorders, pontine cap tegmental dysplasia, and an expanding list. The purpose of this review was to update the imaging features, as well as clinical and genetic information, regarding cases of CCDDs.

Potential linkage of different phenotypic forms of childhood strabismus to a recessive susceptibility locus (16p13.12-p12.3)

PURPOSE

To perform linkage analysis on an inbred family with members who exhibit different phenotypic forms of childhood strabismus.

METHODS

Prospective clinical examination and linkage analysis.

RESULTS

three of the ten siblings and their cousin each had a different phenotypic form of childhood strabismus: infantile esotropia with convergence excess, esotropia associated with anisometropic amblyopia, unilateral esotropic Duane syndrome, and monocular elevation deficiency. Linkage analysis for the four strabismic individuals, an unaffected sibling, and the unaffected parents identified a single disease locus on chromosome 16p13.12-p12.3 (Ensembl cytogenetic band) with a 2.5 maximum logarithm of odds score. The region is 6 MB in size and comprises 80 genes.

DISCUSSION

Linkage analysis in this unique family suggests that childhood strabismus can be recessive and that different phenotypic forms of childhood strabismus can share the same underlying genotype.

Atypical Silver-Russell phenotype resulting from maternal uniparental disomy of chromosome 7

We report on a patient with atypical Silver-Russell phenotype comprising severe growth retardation, unusual facies, bilateral Duane anomaly and infantile hypercalcemia caused by maternal uniparental iso/heterodisomy (mUPD) of chromosome 7. The development of myoclonus in this patient lends further support to the hypothesis that abnormal imprinting of the SGCE gene is responsible for some cases of myoclonus-dystonia syndrome. This case highlights the utility of SNP microarray technology as an accessible tool for the diagnosis of mUPD7 in atypical cases. We propose that depending on the balance of iso- and heterodisomic segments in a particular patient, mUPD7 may result in a range of phenotypes not confined to classic Silver-Russell syndrome.

HOXA1 mutations are not a common cause of Möbius syndrome

The HOXA1-related syndromes result from autosomal-recessive truncating mutations in the homeobox transcription factor, HOXA1. Limited horizontal gaze and sensorineural deafness are the most common features; affected individuals can also have facial weakness, mental retardation, autism, motor disabilities, central hypoventilation, carotid artery, and/or conotruncal heart defects. Möbius syndrome is also phenotypically heterogeneous, with minimal diagnostic criteria of nonprogressive facial weakness and impaired ocular abduction; mental retardation, autism, motor disabilities, additional eye movements restrictions, hearing loss, hypoventilation, and craniofacial, lingual, and limb abnormalities also occur. We asked, given the phenotypic overlap between these syndromes and the variable expressivity of both disorders, whether individuals with Möbius syndrome might harbor mutations in HOXA1. Our results suggest that HOXA1 mutations are not a common cause of sporadic Möbius syndrome in the general population.

Two pedigrees segregating Duane's retraction syndrome as a dominant trait map to the DURS2 genetic locus

PURPOSE

The genetic bases of Duane's retraction syndrome (DRS) were investigated to determine its molecular etiologies. In prior studies, the transcription factors SALL4 and HOXA1 were identified as the genes mutated in DRS with radial anomalies, and in DRS with deafness, vascular anomalies, and cognitive deficits, respectively. Less is known, however, about the genetic etiology of DRS when it occurs in isolation, and only one genetic locus for isolated DRS, the DURS2 locus on chromosome 2, has been mapped to date. Toward the goal of identifying the DURS2 gene, two pedigrees have been ascertained that segregate DRS as a dominant trait.

METHODS

Members of two large dominant DRS pedigrees were enrolled in an ongoing study of the genetic basis of the congenital cranial dysinnervation disorders, and linkage analysis was conducted to determine whether their DRS phenotype maps to the DURS2 locus.

RESULTS

By haplotype analysis, the DRS phenotype in each family cosegregates with markers spanning the DURS2 region. Linkage analysis reveals maximum lod scores >2, establishing that the DRS phenotype in these two pedigrees maps to the DURS2 locus.

CONCLUSIONS

These two pedigrees double the published pedigrees known to map to the DURS2 locus and can thus contribute toward the search for the DURS2 gene. The affected members represent a genetically defined population of DURS2-linked DRS individuals, and hence studies of their clinical and structural features can enhance understanding of the DURS2 phenotype, as described in the companion paper.