Joubert's syndrome: new cases and review of clinicopathologic correlation

Genetic Etiologies, Diagnosis, and Management of Neonatal Cystic Kidney Disease

Fetal kidney development is a complex and carefully orchestrated process. The proper formation of kidney tissue involves many transcription factors and signaling pathways. Pathogenic variants in the genes that encodethese factors and proteins can result in neonatal cystic kidney disease. Advancements in genomic sequencing have allowed us to identify many of these variants and better understand the genetic underpinnings for an increasing number of presentations of childhood kidney disorders. This review discusses the genes essential in kidney development, particularly those involved in the structure and function of primary cilia, and implications of gene identification for prognostication and management of cystic kidney disorders.

Disconnections in Infantile-Onset Saccade Initiation Delay: A Hypothesis

Infantile-onset saccade initiation delay (ISID), commonly known as congenital ocular motor apraxia, is characterized by difficulty in triggering horizontal volitional saccades. It typically presents with head thrusts in infancy and is often associated with developmental delay. Patients with ISID are reported to have abnormalities in various brain regions including the corpus callosum, brainstem, and cerebellum. We propose that ISID is caused by the disruption or disconnection of axons linking analogous brain regions involved in processing saccades across the two sides of the brain or bilateral damage to these regions.

A novel case of natural killer cell deficiency associated with Joubert syndrome

Joubert syndrome (JS) is a rare, complex autosomal recessive inherited disorder mostly characterized by partial or complete agenesis of the cerebellar vermis. There is a wide clinical and genetic heterogeneity in the syndrome. The main clinical features of JS are hypotonia, ataxia, developmental delay, oculomotor apraxia, breathing abnormalities and peculiar neuroimaging findings. A lot of additional features have been reported. Here, we first reported a case of the syndrome with natural killer (NK) cell deficiency. NK cell deficiency in JS might be not an incidental phenomenon. NK cell deficiency might be associated with JS when there are additional features such as recurrent infections and tumors. NK cell deficiency may be part of the clinical spectrum of JS. Reduced cellular immunity in association with NK cell deficiency may be a feature in a subset of JS patients, especially if there is a history of recurrent infections, tumors and autoimmune disorders.

Joubert syndrome: brain and spinal cord malformations in genotyped cases and implications for neurodevelopmental functions of primary cilia

Joubert syndrome (JS) is an autosomal recessive ciliopathy characterized by hypotonia, ataxia, abnormal eye movements, and intellectual disability. The brain is malformed, with severe vermian hypoplasia, fourth ventriculomegaly, and "molar tooth" appearance of the cerebral and superior cerebellar peduncles visible as consistent features on neuroimaging. Neuropathological studies, though few, suggest that several other brain and spinal cord structures, such as the dorsal cervicomedullary junction, may also be affected in at least some patients. Genetically, JS is heterogeneous, with mutations in 13 genes accounting for approximately 50% of patients. Here, we compare neuropathologic findings in five subjects with JS, including four with defined mutations in OFD1 (2 siblings), RPGRIP1L, or TCTN2. Characteristic findings in all JS genotypes included vermian hypoplasia, fragmented dentate and spinal trigeminal nuclei, hypoplastic pontine and inferior olivary nuclei, and nondecussation of corticospinal tracts. Other common findings, seen in multiple genotypes but not all subjects, were dorsal cervicomedullary heterotopia, nondecussation of superior cerebellar peduncles, enlarged arcuate nuclei, hypoplastic reticular formation, hypoplastic medial lemnisci, and dorsal spinal cord disorganization. Thus, while JS exhibits significant neuropathologic as well as genetic heterogeneity, no genotype-phenotype correlations are apparent as yet. Our findings suggest that primary cilia are important for neural patterning, progenitor proliferation, cell migration, and axon guidance in the developing human brain and spinal cord.

Self-reported sleep and breathing disturbances in Joubert syndrome

Joubert syndrome is a rare autosomal recessive disease characterized by malformations of the cerebellar vermis, hypotonia, developmental delay, and respiratory variability. Because little is known about sleep and ventilatory dysregulation in this patient population, a questionnaire was distributed at the Joubert Syndrome and Related Disorders Foundation Conference. This questionnaire addressed respiratory and sleep abnormalities, and included the Pediatric Sleep Questionnaire. Parents or proxies completed questionnaires for patients unable to do so themselves because of young age or neurologic problems. Twenty surveys were collected. The median age was 8.3 years, and 45% were female. Seven patients (35%) reported existing episodic tachypnea, four (20%) reported apnea, and three (15%) reported both. Snoring was reported by 10 patients (50%), of whom four snored nightly and five had coexisting daytime tachypnea. Six of 14 (43%) Pediatric Sleep Questionnaire responders had scores suggestive of sleep-related breathing disorder. These results suggest that episodic tachypnea, apnea, snoring, and Pediatric Sleep Questionnaire scores suggestive of sleep-related breathing disorder are common in Joubert syndrome. Early detection and improved understanding of sleep and breathing abnormalities may contribute to improved outcomes for patients with Joubert syndrome.

Pontine tegmental cap dysplasia: the neurotologic perspective

OBJECTIVES

Pontine tegmental cap dysplasia (PTCD) is a rare congenital malformation. Clinical and imaging findings in 3 patients and the authors' experience with bilateral cochlear implantation in 1 patient are described.

STUDY DESIGN

Retrospective review.

SETTING

Two tertiary medical centers.

SUBJECTS AND METHODS

Three patients were evaluated by an otolaryngologist and underwent magnetic resonance imaging (MRI) of the temporal bones and brain. High-resolution computed tomography (CT) scanning of the temporal bones was performed in 2 patients. Imaging findings of the brain, the presence and course of resolvable cranial nerves, the membranous labyrinth, and internal auditory canals were reviewed. Clinical data were reviewed.

RESULTS

All patients demonstrated typical brain characteristics of PTCD. Mild, bilateral cochlear dysplasia was noted in 2, and all had a normal vestibular labyrinth. The cochleovestibular nerves were universally absent bilaterally. The facial nerves were subjectively deficient bilaterally in 1 patient, unilaterally in the second patient, and normal in the third. An accessory canal for the seventh cranial nerve, referred to as a duplicated internal auditory canal, was present in all patients. Auditory brainstem response testing revealed profound bilateral sensorineural hearing loss in all of the patients; none suffered facial weakness. A single patient underwent bilateral cochlear implantation with only minimal response.

CONCLUSION

The authors report 3 cases of PTCD with emphasis on imaging of the seventh and eighth cranial nerves and clinical neurotologic findings. All patients manifested duplicated internal auditory canals, a previously unreported finding in PTCD. Bilateral profound sensorineural hearing loss is due to absence of the cochleovestibular nerve. Prognosis for cochlear implantation is poor.

Ataxia, hyperpnoea and mental retardation: was it the molar tooth?

Joubert's syndrome is a rare autosomal recessive disease, which is under-diagnosed, associated with other brain and body malformations, and carries a poor prognosis. We describe a 6-year-old boy who presented with non-progressive instability of stance and gait, mental retardation and a new onset of generalised seizures with the typical brain imaging findings of Joubert's syndrome. We believe this is the first diagnosed case of Joubert's syndrome in Iraq.

Clinical and molecular features of Joubert syndrome and related disorders

Joubert syndrome (JBTS; OMIM 213300) is a rare, autosomal recessive disorder characterized by a specific congenital malformation of the hindbrain and a broad spectrum of other phenotypic findings that is now known to be caused by defects in the structure and/or function of the primary cilium. The complex hindbrain malformation that is characteristic of JBTS can be identified on axial magnetic resonance imaging and is known as the molar tooth sign (MTS); other diagnostic criteria include intellectual disability, hypotonia, and often, abnormal respiratory pattern and/or abnormal eye movements. In addition, a broad spectrum of other anomalies characterize Joubert syndrome and related disorders (JSRD), and may include retinal dystrophy, ocular coloboma, oral frenulae and tongue tumors, polydactyly, cystic renal disease (including cystic dysplasia or juvenile nephronophthisis), and congenital hepatic fibrosis. The clinical course can be variable, but most children with this condition survive infancy to reach adulthood. At least eight genes cause JSRD, with some genotype-phenotype correlations emerging, including the association between mutations in the MKS3 gene and hepatic fibrosis characteristic of the JSRD subtype known as COACH syndrome. Several of the causative genes for JSRD are implicated in other ciliary disorders, such as juvenile nephronophthisis and Meckel syndrome, illustrating the close association between these conditions and their overlapping clinical features that reflect a shared etiology involving the primary cilium.

Joubert syndrome: insights into brain development, cilium biology, and complex disease

Joubert syndrome (JS) is a primarily autosomal recessive condition characterized by hypotonia, ataxia, abnormal eye movements, and intellectual disability with a distinctive mid-hindbrain malformation (the "molar tooth sign"). Variable features include retinal dystrophy, cystic kidney disease, liver fibrosis and polydactyly. Recently, substantial progress has been made in our understanding of the genetic basis of JS, including identification of seven causal genes (NPHP1, AHI1, CEP290, RPGRIP1L, TMEM67/MKS3, ARL13B and CC2D2A). Despite this progress, the known genes account for <50% of cases and few strong genotype-phenotype correlations exist in JS; however, genetic testing can be prioritized based on clinical features. While all seven JS genes have been implicated in the function of the primary cilium/basal body organelle (PC/BB), little is known about how the PC/BB is required for brain, kidney, retina and liver development/function, nor how disruption of PC/BB function leads to diseases of these organs. Recent work on the function of the PC/BB indicates that the organelle is required for multiple signaling pathways including sonic hedgehog, WNT and platelet derived growth factor. Due to shared clinical features and underlying molecular pathophysiology, JS is included in the rapidly expanding group of disorders called ciliopathies. The ciliopathies are emerging as models for more complex diseases, where sequence variants in multiple genes contribute to the phenotype expressed in any given patient.

Manifestações renais na síndrome de Joubert

OBJETIVO:Descrever o caso clínico de paciente com Síndrome de Joubert associada a alterações renais. DESCRIÇÃO DO CASO: Paciente de dois meses de idade admitida com quadro hipotonia e hiperpneia. Ao exame físico, observaram-se, além da respiração irregular, movimentos oculares anormais e hipertensão arterial; não se evidenciaram alterações na ausculta cardíaca e pulmonar. Durante investigação clínico-laboratorial inicial, as causas cardíacas e pulmonares foram descartadas. Aventaram-se hipóteses diagnósticas de patologias neurológicas com doença renal. Os exames laboratoriais mostraram presença de alcalose respiratória, acidose metabólica e hipercalemia, com função renal normal. A ressonância magnética evidenciou alterações neurológicas compatíveis com "sinal do dente molar", quadro frequentemente associada à Sindrome de Joubert. Levando-se em consideração a associação dessa síndrome com alterações renais, a investigação nefrológica demonstrou imagens císticas em parênquima renal. COMENTÁRIOS: Patologias cardíacas e pulmonares estão frequentemente associadas a manifestações clínicas como taquipneia e distúrbios metabólicos. Entretanto, pode ser necessária uma investigação neurológica porque diversas doenças que acometem o sistema nervoso central apresentam tais alterações. A associação entre alterações renais e malformações de sistema nervoso central é frequente em diversos processos sindrômicos, justificando-se a sua investigação. A Síndrome de Joubert e as desordens a ela relacionadas caracterizam-se por aplasia do vermix cerebelar, ataxia, movimentos oculares anormais, respiração irregular e retardo do desenvolvimento neuropsicomotor. As alterações renais mais comuns são os cistos renais e a nefronoftise, que pode progredir para doença renal terminal.

Eye movement abnormalities in Joubert syndrome

PURPOSE

Joubert syndrome is a genetic disorder characterized by hypoplasia of the midline cerebellum and deficiency of crossed connections between neural structures in the brain stem that control eye movements. The goal of the study was to quantify the eye movement abnormalities that occur in Joubert syndrome.

METHODS

Eye movements were recorded in response to stationary stimuli and stimuli designed to elicit smooth pursuit, saccades, optokinetic nystagmus (OKN), vestibulo-ocular reflex (VOR), and vergence using video-oculography or Skalar search coils in 8 patients with Joubert syndrome. All patients underwent high-resolution magnetic resonance imaging (MRI).

RESULTS

All patients had the highly characteristic molar tooth sign on brain MRI. Six patients had conjugate pendular (n = 4) or see-saw nystagmus (n = 2); gaze holding was stable in four patients. Smooth-pursuit gains were 0.28 to 1.19, 0.11 to 0.68, and 0.33 to 0.73 at peak stimulus velocities of 10, 20, and 30 deg/s in six patients; smooth pursuit could not be elicited in four patients. Saccade gains in five patients ranged from 0.35 to 0.91 and velocities ranged from 60.9 to 259.5 deg/s. Targeted saccades could not be elicited in five patients. Horizontal OKN gain was uniformly reduced across gratings drifted at velocities of 15, 30, and 45 deg/s. VOR gain was 0.8 or higher and phase appropriate in three of seven subjects; VOR gain was 0.3 or less and phase was indeterminate in four subjects.

CONCLUSIONS

The abnormalities in gaze-holding and eye movements are consistent with the distributed abnormalities of midline cerebellum and brain stem regions associated with Joubert syndrome.

Joubert syndrome (and related disorders) (OMIM 213300)

Joubert syndrome (JS) and related disorders are characterized by the 'molar tooth sign' (cerebellar vermis hypoplasia and brainstem anomalies) on MRI, hypotonia, developmental delay, ataxia, irregular breathing pattern and abnormal eye movements. Combinations of additional features such as polydactyly, ocular coloboma, retinal dystrophy, renal disease, hepatic fibrosis, encephalocele, and other brain malformations define clinical sub-types. Recent identification of the NPHP1, AHI1, and CEP290 genes has started to reveal the molecular basis of JS, which may implicate the primary cilium in these disorders. Additional genes remain to be identified.

Diagnostic approach to cerebellar disease in children

We reviewed the clinical records of 51 extensively investigated pediatric patients with structural abnormalities of the cerebellum as revealed by magnetic resonance imaging (MRI). Ten had hypoplasia of the vermis, 21 had hypoplasia of the vermis and cerebellar hemispheres, 2 had pontocerebellar hypoplasia, and 18 had progressive cerebellar atrophy. A clear diagnosis was reached in 37 (72.5%). Initial characterization of the cerebellar alterations by MRI separated hypoplastic from atrophic cases and confirmed MRI as an essential preliminary means for distinguishing malformations from metabolic-degenerative conditions. However, the diagnostic possibilities are so numerous that it is not feasible to propose a standardized diagnostic protocol for pediatric patients with an altered cerebellum. Subsequent investigations should be suggested by the neuroradiologic and clinical peculiarities of each case.

Spectrum of malformations of the hindbrain (cerebellum, pons, and medulla) in a cohort of children with high rate of parental consanguinity

We review 25 patients with a spectrum of hindbrain (cerebellum, pons, and medulla) malformations from a cohort of children with high parental consanguinity rate. Twenty-three of the 25 patients were born to consanguineous parents. The patients were classified in four groups. Eleven patients of 6 families had malformation of the hindbrain and midbrain with molar tooth sign (10 patients of 5 families with typical Joubert syndrome), 5 patients showed severe supratentorial anomalies in addition to the hindbrain malformations, 5 patients had pontocerebellar or cerebellar hypoplasia with anterior horn cell disease in the spinal cord (spinal muscular atrophy), and 4 patients showed malformations affecting predominantly the hindbrain without substantial involvement of other systems. A locus for Joubert syndrome was previously identified on chromosome 9q34.3 in two families, and a second locus on chromosome 11p12-q13.3 in another family. A third Joubert syndrome locus has been mapped at 6q23 and a mutation in the AHI1 gene at this site has been found recently in a further family from this cohort. Delineation of homogeneous subgroups of patients with hindbrain malformations and molecular genetic analysis of these groups may lead to identification of further loci, genes and mutations responsible for the malformations.

Distinguishing the four genetic causes of Jouberts syndrome-related disorders

Jouberts syndrome-related disorders are a group of recessively inherited conditions showing cerebellar vermis hypoplasia and the molar tooth sign of the midbrain-hindbrain junction. Recent analyses have suggested at least three loci, JBTS1 (9q34.3), -2 (11p11.2-q12.3), and -3 (6q23), but the phenotypic spectrum associated with each locus has not been delineated. In addition, deletions of the NPHP1 gene, usually responsible for isolated juvenile nephronophthisis, are occasionally encountered among Jouberts syndrome-related disorder patients. Here, we describe four novel families showing evidence of linkage to two of these loci, provide a 3.6Mb refinement of the JBTS2 locus, and perform a detailed comparison of all linked families identified so far, to define the clinical and radiographical hallmarks for each genetic condition. We find that JBTS1 and -3 primarily show features restricted to the central nervous system, with JBTS1 showing largely pure cerebellar and midbrain-hindbrain junction involvement, and JBTS3 displaying cerebellar, midbrain-hindbrain junction, and cerebral cortical features, most notably polymicrogyria. Conversely, JBTS2 is associated with multiorgan involvement of kidney, retina, and liver, in addition to the central nervous system features, and results in extreme phenotypic variability. This provides a useful framework for genetic testing strategies and prediction of which patients are most likely to experience development of systemic complications.

Burkitt lymphoma in a child with Joubert syndrome

Joubert syndrome is a rare disorder, characterized by hypoplasia, or aplasia of the cerebellar vermis, hypotonia, ataxia, and psychomotor retardation. The molecular basis underlying the disease is still unknown. There are various syndromes, which are associated with malignancies. Previously known associations between Joubert syndrome and tumors, are benign soft tissue tumors of the tongue and laryngeal lymphangioma. This report describes a 17-year-old boy known with Joubert syndrome, who was diagnosed with Burkitt lymphoma. The boy received chemotherapy, which successfully induced complete remission.

Anesthetic management in Joubert syndrome

We review the anesthetic implications of Joubert syndrome and report that spinal anesthesia under intravenous propofol sedation proved satisfactory for repair of an inguinal hernia in a spontaneously ventilating infant with this syndrome. We caution that anatomical peculiarities may complicate performance of a caudal epidural block and suggest consideration of the use of caffeine to ameliorate the apneic episodes seen in early infancy in this syndrome.

Joubert syndrome: review and report of seven new cases

Joubert syndrome (JS) is an autosomal-recessive disorder, characterized by hypotonia, ataxia, global developmental delay and molar tooth sign on magnetic resonance imaging. A variety of other abnormalities have been described in children with JS, including abnormal breathing, abnormal eye movements, a characteristic facial appearance, delayed language, hypersensitivity to noise, autism, ocular and oculomotor abnormalities, meningoencephaloceles, microcephaly, low-set ears, polydactyly, retinal dysplasia, kidney abnormalities (renal cysts), soft tissue tumor of the tongue, liver disease and duodenal atresia. Even within siblings the phenotype may vary, making it difficult to establish the exact clinical diagnostic boundaries of JS. We review the clinical characteristics of seven cases that fulfill the criteria of JS.

Specific clinical and brain MRI features in mentally retarded patients with mutations in theOligophrenin-1 gene

Oligophrenin-1 (OPHN-1) gene disruption is known as responsible for so called "non-specific" X-linked mental retardation (MR) Billuart et al. [1998: Nature 392:923-926]. In order to search for a possible specific clinical and radiological profile for mutation in the OPHN-1 gene, clinical and 3D brain MRI studies were performed in the two families with a known mutation in OPHN-1 reported so far: a 19-year-old female with an X;12 balanced translocation encompassing OPHN-1, and four affected males of family MRX60 sharing a frameshift mutation in OPHN-1. Clinical data shared by affected individuals were neonatal hypotonia with motor delay but no obvious ataxia, marked strabismus, early onset complex partial seizures, and moderate to severe MR. Brain MRIs performed in three individuals exhibited a specific vermian dysgenesis including an incomplete sulcation of anterior and posterior vermis with the most prominent defect in lobules VI and VII. In addition, a non-specific cerebral cortico-subcortical atrophy was also observed. These clinical and radiological features suggest a distinct clinico-radiological syndrome. These preliminary data need to be confirmed in other families and will be helpful for further targeted mutation screening of the OPHN-1 gene in male patients with similar clinico-radiological features. In addition, OPHN-1 inactivation should be considered as a relevant model of developmental vermis disorganization, leading to a better understanding of the possible role of the cerebellum in MR.

Description, nomenclature, and mapping of a novel cerebello-renal syndrome with the molar tooth malformation

Cerebello-oculo-renal syndromes (CORSs) and Joubert syndrome (JS) are clinically and genetically heterogeneous autosomal recessive syndromes that share a complex neuroradiological malformation resembling a molar tooth on brain axial images, a condition referred to as "molar tooth on imaging" (MTI) or the "molar tooth sign." The current literature on these syndromes is complex, with overlapping and incomplete phenotypes that complicate the selection of clinically homogeneous cases for genetic purposes. So far, only one locus (JBTS1 on 9q34) has been mapped, in two families with JS. Here, we describe a large consanguineous family with JS and nephronophthisis, representing a novel cerebello-renal phenotype. We have mapped this condition to the pericentromeric region of chromosome 11 and have named the locus "CORS2." The acronym "CORS" is proposed for all loci associated with JS, CORSs, and related phenotypes sharing the MTI, because this neuroradiological sign seems to be the unifying feature of these clinically heterogeneous syndromes.

Linkage analysis in families with Joubert syndrome plus oculo-renal involvement identifies the CORS2 locus on chromosome 11p12-q13.3

Joubert syndrome (JS) is an autosomal recessive developmental brain condition characterized by hypoplasia/dysplasia of the cerebellar vermis and by ataxia, hypotonia, oculomotor apraxia, and neonatal breathing dysregulation. A form of JS that includes retinal dysplasia and cystic dysplastic kidneys has been differentiated from other forms of JS, called either "JS type B" or "cerebello-oculo-renal syndrome" (CORS), but the genetic basis of this condition is unknown. Here, we describe three consanguineous families that display CORS. Linkage analysis defines a novel locus on chromosome 11p12-q13.3, with a maximum two-point LOD score of Z=5.2 at the marker D11S1915. Therefore, the cerebello-oculo-renal form of JS is a distinct genetic entity from the Joubert syndrome 1 (JBTS1) locus described elsewhere, in which there is minimal involvement of retina or kidney. We suggest the term "CORS2" for this new locus.

Genetics of brain development and malformation syndromes

The identification of the specific genes responsible for several childhood neurologic disorders has provided a framework with which to understand key development stages in human brain development. Common genetic disorders of brain development include septo-optic dysplasia, schizencephaly, holoprosencephaly, periventricular heterotopia, lissencephaly, and Joubert syndrome. For each of these disorders, a critical step in brain development is interrupted. The identification of the responsible genes is providing scientists a window into the key modulators of brain development, and providing clinicians the opportunity to offer genetic testing to individual patients and their families.

Autosomal recessive cerebellar ataxia with oculomotor apraxia (ataxia-telangiectasia-like syndrome) is linked to chromosome 9q34

Ataxia with oculomotor apraxia (ataxia-telangiectasia-like syndrome [AOA]; MIM 208920) is an autosomal recessive disorder characterized by ataxia, oculomotor apraxia, and choreoathetosis. These neurological features resemble those of ataxia-telangiectasia (AT), but in AOA there are none of the extraneurological features of AT, such as immunodeficiency, neoplasia, chromosomal instability, or sensitivity to ionizing radiation. It is unclear whether these patients have a true disorder of chromosomal instability or a primary neurodegenerative syndrome, and it has not been possible to identify the defective gene in AOA, since the families have been too small for linkage analysis. We have identified a new family with AOA, and we show that the patients have no evidence of chromosomal instability or sensitivity to ionizing radiation, suggesting that AOA in this family is a true primary cerebellar ataxia. We have localized the disease gene, by linkage analysis and homozygosity mapping, to a 15.9-cM interval on chromosome 9q34. This work will ultimately allow the disease gene to be identified and its relevance to other types of autosomal recessive cerebellar ataxias to be determined.

Cerebellar vermis hypoplasia - non progressive congenital ataxia: clinical and radiological findings in a pair of siblings

We describe the clinical and radiological findings of a pair of siblings with cerebellar vermis hypoplasia and compare them with the literature. Both of them present pregnancies and deliveries uneventful and both presented some grade of hypotonia, ataxia, ocular motor abnormalities and mild motor delay and slurred speech. These siblings meet many of the criteria described in non-progressive congenital ataxia in which can occur familial cases with cerebellar atrophy, including vermis hypoplasia. As differential diagnosis we compare them with related syndromes and with Joubert's syndrome which main radiological finding on MRI is vermis hypoplasia associated with "molar tooth" appearance. The correct answer for these cases will only be possible by molecular genetics.

Homozygosity mapping in families with Joubert syndrome identifies a locus on chromosome 9q34.3 and evidence for genetic heterogeneity

Joubert syndrome is a rare developmental defect of the cerebellar vermis, with autosomal recessive inheritance. The phenotype is highly variable and may include episodic hyperpnea, abnormal eye movements, hypotonia, ataxia, developmental delay, and mental retardation. Even within sibships the phenotype may vary, making it difficult to establish the exact clinical diagnostic boundaries of Joubert syndrome. To genetically localize the gene region, we have performed a whole-genome scan in two consanguineous families of Arabian/Iranian origins, with multiple affected probands. In one family, we detected linkage to the telomeric region of chromosome 9q, close to the marker D9S158, with a multipoint LOD score of Z=+3.7. The second family did not show linkage to this region, giving a first indication of genetic heterogeneity underlying Joubert syndrome. These findings were supported by subsequent analysis of two smaller families-one compatible with linkage to 9q; the other, unlinked. We conclude that Joubert syndrome is clinically and genetically heterogeneous and that one locus maps to chromosome 9q.