On this page
Joubert syndrome (AHI1)
This condition primarily affects brain development, leading to a range of neurological, developmental, and sometimes organ-related challenges. It typically becomes apparent in infancy or early childhood.
Available at Jeen Health
Clinical tests that include this
Overview
Joubert syndrome is a rare genetic disorder that primarily impacts the development of the brain, specifically the cerebellum and brainstem. These brain regions are crucial for co-ordinating movement, balance, and other essential functions. A hallmark feature, often visible on brain imaging, is the 'molar tooth sign' - a distinctive appearance of the midbrain and cerebellum [PMID:31505500]. Joubert syndrome is classified as a ciliopathy, a group of disorders caused by defects in cellular structures called cilia, which play vital roles in cell signalling and development.
The condition can present with a wide spectrum of signs and symptoms, varying significantly between individuals. While some features are common, the severity and combination of problems can differ, making each person's experience unique. It is a condition that generally becomes noticeable in infancy or early childhood as developmental milestones are missed or specific physical features become apparent.
Symptoms & clinical features
Individuals with Joubert syndrome often experience a range of neurological and developmental symptoms. Key features frequently include hypotonia (reduced muscle tone) in infancy, which can make feeding and movement difficult. Developmental delay is common, affecting motor skills like sitting and walking, as well as speech and language development [PMID:31505500].
Characteristic eye movement abnormalities, such as oculomotor apraxia (difficulty moving the eyes to follow objects voluntarily), are frequently observed. Breathing pattern irregularities, including episodes of rapid breathing (tachypnoea) followed by periods of no breathing (apnoea), can occur in infants, particularly during sleep. Other potential symptoms can include kidney problems, liver fibrosis, retinal dystrophy affecting vision, and polydactyly (extra fingers or toes) [PMID:24075139].
Video: Genetics 101
Affected organs
The primary organs affected in Joubert syndrome are those of the central nervous system, particularly the brain. The cerebellum, which controls balance and co-ordination, and the brainstem, which regulates vital functions like breathing and heart rate, are malformed. This leads to the characteristic 'molar tooth sign' seen on brain imaging.
Beyond the brain, other organ systems can also be affected. These may include the kidneys, which can develop cysts or scarring (fibrosis), and the liver, which can also show signs of fibrosis. The eyes can be affected by retinal dystrophy, potentially leading to vision impairment. In some cases, extra fingers or toes (polydactyly) may be present.
Risks & severity
The severity of Joubert syndrome can vary considerably among affected individuals. Some may experience significant developmental challenges and medical complications, while others have milder symptoms. Early onset of breathing difficulties in infancy can be a serious concern for some [PMID:31505500].
Developmental delay is a common outcome, and the degree can range from mild to severe, impacting motor skills, speech, and cognitive development. The specific combination and severity of symptoms, including kidney or liver involvement, can influence the overall health and prognosis. Regular monitoring for potential complications is an important part of managing the condition.
Genetic causes
Joubert syndrome is caused by pathogenic variants in genes that are important for the structure and function of primary cilia. For the form of Joubert syndrome associated with AHI1, the causative gene is AHI1 (Abelson helper integration site 1). The AHI1 gene provides instructions for making a protein that is found in the primary cilium, a small, hair-like projection on the surface of most cells.
Cilia act like antennae for cells, playing a critical role in signalling pathways that are essential for normal development, especially of the brain and other organs. When the AHI1 protein is not functioning correctly due to a pathogenic variant, it disrupts these signalling processes. This disruption leads to the characteristic developmental abnormalities seen in Joubert syndrome, particularly affecting the cerebellum and brainstem.
- AHI1 Abelson helper integration site 1
Inheritance pattern
Joubert syndrome associated with AHI1 follows an autosomal recessive inheritance pattern. This means that an individual must inherit two pathogenic variants, one from each parent, to develop the condition.
Parents who each carry one pathogenic variant in the AHI1 gene are known as carriers. Carriers typically do not show symptoms of Joubert syndrome themselves because they have one working copy of the gene. With each pregnancy, if both parents are carriers, there is a 25% (1 in 4) chance that their child will inherit two pathogenic variants and develop the condition, a 50% (2 in 4) chance their child will be a carrier like them, and a 25% (1 in 4) chance their child will inherit two working copies of the gene and not be affected or a carrier.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
A diagnosis of Joubert syndrome is usually suspected based on clinical features, such as developmental delay, hypotonia, and characteristic eye movements. Brain imaging, typically Magnetic Resonance Imaging (MRI), is crucial for confirming the diagnosis by identifying the 'molar tooth sign' and other specific cerebellar and brainstem abnormalities.
Confirmation of Joubert syndrome involves genetic testing. This testing can identify pathogenic variants in relevant genes, such as AHI1. Referrals for genetic testing are typically made by a paediatrician or clinical geneticist through the NHS Genomic Medicine Service.
Management & lifestyle
Management of Joubert syndrome focuses on addressing the individual's specific symptoms and needs, often involving a multidisciplinary team of specialists. This team may include paediatricians, neurologists, physiotherapists, occupational therapists, speech and language therapists, and ophthalmologists.
Physiotherapy can help improve muscle tone and motor skills, while occupational therapy assists with daily living activities. Speech and language therapy can support communication development. Regular monitoring for potential complications, such as kidney disease, liver problems, and vision changes, is important. This may involve periodic blood tests, urine tests, and eye examinations. Educational support and early intervention programmes are also vital for development. Management strategies are always tailored to the individual and do not follow a one-size-fits-all approach.
UK care pathway
In the UK, individuals suspected of having Joubert syndrome would typically be referred by their GP or paediatrician to a clinical genetics service. This service can provide expert assessment, genetic counselling, and arrange for appropriate genetic testing through the NHS Genomic Medicine Service.
Genetic counsellors play a key role in explaining the condition, its inheritance pattern, and what a genetic diagnosis might mean for the individual and their family. They can also discuss reproductive options and provide support and information. Ongoing care usually involves a team of specialists at regional NHS hospitals, co-ordinated to address the diverse needs of the patient.
Frequently asked questions
What is the 'molar tooth sign'?
The 'molar tooth sign' is a distinctive appearance of the cerebellum and brainstem that can be seen on a brain MRI scan. It is a key diagnostic feature of Joubert syndrome, resembling a molar tooth due to the abnormal development of certain brain structures.
How common is Joubert syndrome?
Joubert syndrome is considered a rare genetic condition. Exact prevalence figures are not well established.
Will my child definitely have all the symptoms of Joubert syndrome?
No, the range and severity of symptoms in Joubert syndrome can vary significantly from person to person, even within the same family. Some individuals may have mild symptoms, while others experience more significant challenges affecting multiple organ systems.
Can Joubert syndrome be inherited if no one else in my family has it?
Yes, Joubert syndrome is typically inherited in an autosomal recessive pattern. This means both parents can be carriers of a pathogenic gene variant without showing any symptoms themselves. If each parent passes on their variant, their child can be affected by the condition.
What kind of support is available for families affected by Joubert syndrome?
Families in the UK can access support through clinical genetics services, patient support groups, and multidisciplinary healthcare teams. These teams provide medical management, therapeutic interventions, and guidance on educational and social support networks.