A number sign (#) is used with this entry because of evidence that both Athabaskan brainstem dysgenesis syndrome (ABDS, observed in 2 Native American tribes) and the Bosley-Salih-Alorainy syndrome, or BSAS (observed in Saudi Arabian and Turkish families), are caused by homozygous mutations in the HOXA1 gene (142955) on chromosome 7p15.

Homozygous loss-of-function mutations in the HOXA1 gene result in disorders with variable phenotypic expressivity that span a spectrum. Two related, but somewhat distinctive, phenotypes have been described in different populations: the Athabaskan brainstem dysgenesis syndrome (ABDS) in Native Americans, and Bosley-Salih-Alorainy syndrome (BSAS) in individuals from the Middle East, including Turkey and Saudi Arabia. Features common to both disorders include Duane retraction syndrome with variable gaze palsies, sensorineural deafness associated with inner ear abnormalities, and delayed motor development. More variable features, observed in both disorders, include conotruncal cardiac malformations, cerebral vascular malformations, and impaired intellectual development with autism. Unique to ABDS are central hypoventilation, often resulting in early death, facial weakness, and more severe cognitive deficits. These features are thought to be due to a more severe malformation of the hindbrain in ABDS compared to BSAS (summary by Tischfield et al., 2005).

Athabaskan Brainstem Dysgenesis

Several Native American tribes, including the Navajo, are of Athabaskan descent. It is thought that the Athabaskans were relatively recent immigrants to North America, coming across the Bering Strait approximately 4,000 years ago (Erickson, 1999). An argument for their center of dispersal being close to the Bering Strait comes from the observation that the languages have diversified most between the tribes in the Northwest Territories and Alaska, i.e., there has been a greater period of time for the language to diversify in this 'original homesite' in North America. Friedman et al. (1996, 1997) described the cases of 7 Athabaskan children with congenital horizontal gaze palsy, deafness, and central hypoventilation. All of the children had global developmental delay and 3 had seizures. Convergence and vertical eye movements remained intact. Brainstem auditory evoked response testing in 6 and cold-caloric testing in 5 of the children showed no response, consistent with lack of cranial nerve VIII function. Facial movements were sparse, but cranial nerve VII function was intact. Three of the children had congenital cardiac defects, which were outflow tract anomalies in 2. Lack of cranial nerve VII palsy and the presence of central hypoventilation distinguished the disorder from Moebius syndrome (see 157900 and 601471).

Erickson (1999) reviewed this disorder along with other Southwestern U.S. Athabaskan (Navajo and Apache) genetic diseases. They stated that 7 of the affected children were Navajo Indians, 2 were Apache, and 1 child was of Apache and Pima heritage. There was a pair of affected sibs. In another family, a sib's death was attributed to sudden infant death syndrome (SIDS), suggesting that the death was probably caused by the central hypoventilation of Athabaskan brainstem dysgenesis. Although most of the cases were initially diagnosed as a 'variant' Moebius syndrome, it is clear that the Athabaskan cases do not fit that diagnosis. The lateral gaze palsy in affected patients is not due to dysfunction of the sixth cranial nerve, but rather a lack of conjugate horizontal gaze. On attempted lateral gaze, these patients could not move the ipsilateral eye laterally, but neither could they move the contralateral eye medially (which they should be able to do with a strictly sixth nerve palsy). Patients could move their eyes medially during convergence. None of the patients was noted to have the characteristic pattern of facial palsy that is most typical of Moebius syndrome. All of the patients had been deaf, and deafness is very rare in Moebius syndrome. Cardiac outflow tract anomalies were observed in 60% of patients, suggesting that this is not solely a neurologic disorder. Vocal cord weakness was present in 20% of the patients.

Holve et al. (2003) reported the cases of 10 affected children of whom 8 were Navajo and 2 Apache. Consistent features included horizontal gaze palsy, sensorineural deafness, central hypoventilation, and developmental delay. Some patients had swallowing dysfunction, vocal cord paralysis, facial paresis, seizures, and cardiac outflow tract anomalies.

Bosley-Salih-Alorainy Syndrome

Tischfield et al. (2005) observed an autosomal recessive syndrome in 5 consanguineous families, 4 Saudi Arabian and 1 Turkish, that they called Bosley-Salih-Alorainy syndrome (BSAS). All 9 affected individuals had bilateral Duane syndrome (126800), a congenital horizontal eye movement disorder. Eight affected individuals had profound sensorineural deafness, and 3 had external ear defects. Seven affected individuals had delayed motor milestones. Two individuals with BSAS from different Saudi Arabian families were cognitively and behaviorally impaired and met DSM-IV criteria for autism spectrum disorder (see 209850). In each family the parents did not show features of BSAS. In thin magnetic resonance sections through the caudal pons from 1 affected individual, exiting abducens cranial nerves could not be identified. Otherwise, the cerebrum, cerebellum, and brainstem appeared normal. The inner ear was imaged in 7 of 8 individuals with deafness; bilateral absence of the cochlea, semicircular canals, and vestibule (common cavity deformity) was found in 5 of them, and cochlear aplasia in 2. The ninth individual had normal hearing and inner ear anatomy. Computed tomography imaging of the skull base was performed in 3 individuals with BSAS. One had bilateral absence and 2 had left-sided absence of the carotid canal, the foramen through which the internal carotid artery (ICA) normally enters the skull. In 4 individuals in whom magnetic resonance angiography (MRA) of both the head and neck were performed and in 3 individuals who underwent MRA of the head only, variable ICA malformations, ranging from unilateral hypoplasia to bilateral agenesis, were found.

Bosley et al. (2007) reviewed the clinical features of 9 Saudi patients with genetically confirmed BSAS. Eight patients had previously been reported by Tischfield et al. (2005); the ninth patient was newly reported. All 9 individuals had bilateral Duane retraction syndrome type 3, but the extent of abduction and adduction varied between eyes and individuals. Eight patients were deaf with the common cavity deformity of the inner ear, while 1 patient had normal hearing and skull base development. Six had delayed motor milestones, and 2 had autism. Six patients who were adequately studied showed cerebrovascular malformations ranging from unilateral internal carotid artery hypoplasia to bilateral agenesis. Brain imaging was normal, although most patients had malformations involving the inner ear and petrous bone, as well as a common cavity deformity affecting the cochlear region.

Phenotypic Overlap

Tischfield et al. (2005) noted that the BSAS phenotype and the phenotype of the Hoxa1 knockout mouse overlap that of Athabaskan brainstem dysgenesis syndrome. Similar to individuals with BSAS, 10 reported children with ABDS had horizontal gaze restriction, sensorineural deafness, and delayed motor development (Holve et al., 2003). Review of MRA available from 3 children with ABDS showed left ICA agenesis, right ICA hypoplasia, and anterior cerebral artery aplasia, each in a different child. In contrast to the 9 individuals with BSAS, however, all 10 children with ABDS also had central hypoventilation and mental retardation, and some of them had facial weakness, vocal cord paralysis, and conotruncal heart defects, including tetralogy of Fallot and double aortic arch. Central hypoventilation was not observed in any individuals with BSAS; this feature can be fatal without medical intervention (Holve et al., 2003). Similarly, Hoxa1 -/- mice have respiratory failure with perinatal death (del Toro et al., 2001). Thus, the ABDS phenotype converges more closely with the phenotypic spectrum reported in the knockout mice and suggests that hindbrain maldevelopment rostral to rhombomere 5 may be more severe in individuals with ABDS than in those with BSAS. Phenotypic differences between BSAS and ABDS may result in part from genetic modifiers in these isolated human populations.

Bosley et al. (2008) reported 3 unrelated Native American patients with ABDS and 6 patients from 3 consanguineous Saudi Arabian families with BSAS. All probands were identified because of severe horizontal gaze restriction and deafness, and all but 1 had developmental delay. However, 2 affected sibs did not have did not have horizontal gaze restriction or deafness. Three BSAS patients and 2 ABDS patients had congenital heart disease, whereas 2 ABDS patients had only mild cognitive changes similar to BSAS. These clinical features blurred the distinction between BSAS and ABDS and broadened the clinical spectrum of homozygous HOXA1 mutations. Bosley et al. (2008) postulated that isolated bilateral Duane retraction syndrome or isolated congenital heart disease could potentially be the sole manifestation of homozygous HOXA1 mutations, and that isolated deafness, autism, or cerebrovascular abnormalities could also potentially occur.

The transmission pattern of ABDS and BSAS in the families reported by Bosley et al. (2008) was consistent with autosomal recessive inheritance with incomplete penetrance or variable expressivity of certain features.

Using SNP-based linkage analysis of the largest family with BSAS, Tischfield et al. (2005) identified a single, fully informative 8.5-Mb region on chromosome 7p15.3-p14.3 in which only the affected children were homozygous. Further analysis with additional microsatellite markers confirmed coinheritance of the haplotype with disease status in all 5 pedigrees with BSAS and identified a homozygous subregion of approximately 300 kb on 7p15.2 that was haploidentical in affected Saudi Arabian individuals. The maximum combined 2-point lod score was 7.7.

Because the HOXA cluster falls in the BSAS haploidentical region, and because of similarities between the BSAS phenotype and the pathology of the Hoxa1 -/- mouse,Tischfield et al. (2005) analyzed the HOXA1 gene for mutations causing BSAS. Affected individuals in the 4 Saudi Arabian families were homozygous for a guanine insertion, 175-176insG, putatively resulting in a reading frameshift and the introduction of a premature stop codon (142955.0001). The Turkish individual with BSAS had a homozygous 84C-G transversion resulting in the substitution of a stop codon for a tyrosine residue (Y28X; 142955.0002). Both mutations were heterozygous in parents of affected individuals. The mutations were predicted to affect the synthesis of all 3 human HOXA1 transcripts and to result in loss of HOXA1 function.

Tischfield et al. (2005) analyzed genomic DNA from 5 of the reported individuals with ABDS and 4 of their phenotypically normal parents. All 5 affected individuals were homozygous across the HOXA1 locus and carried a homozygous R26X mutation in the HOXA1 gene (142955.0003).

Bosley et al. (2008) identified a homozygous R26X mutation in 3 Native American patients with ABDS. Two of the patients had congenital heart defects: a ventricular septal defect that closed spontaneously and total anomalous pulmonary venous return, respectively.

In 6 affected members from 3 Saudi Arabian families with BSAS, Bosley et al. (2008) identified homozygous mutations in the HOXA1 gene (142955.0001; 142955.0004). Five patients had conotruncal or septal heart defects.