Joubert Syndrome: Why Get Genetic Testing After MRI?

When an MRI shows the “molar tooth sign,” it gives you a definitive answer that your child has Joubert syndrome. This brings clarity, but it often marks the beginning of a new set of questions. Parents frequently wonder why their child needs expensive and lengthy genetic testing if the diagnosis is already confirmed. The answer comes down to predicting the future: while the MRI confirms the condition, genetic testing aims to tell your medical team which specific gene is responsible. Identifying the exact gene allows doctors to anticipate which other organs—specifically the kidneys, liver, and eyes—might be affected and to create a tailored, lifelong screening plan for your child.

The Molar Tooth Sign vs. The Underlying Cause

Joubert syndrome is a “ciliopathy,” meaning it affects the cilia—tiny, hair-like structures on the surfaces of cells throughout the body. Because cilia are found in many different organs, Joubert syndrome can involve much more than just the brain. To date, researchers have identified more than 40 different genes that can cause Joubert syndrome ^[1].

While the molar tooth sign on an MRI is the hallmark of Joubert syndrome ^[2], it only shows the structural changes in the brain. Next-generation sequencing (NGS) and whole-exome sequencing—typically performed using a blood draw or cheek swab—are highly effective tools used to find the specific genetic mutation causing those changes ^[1]. Because so many genes are involved, a causative gene is found in a significant proportion of cases, though it is still possible for results to occasionally be inconclusive if the specific mutation hasn’t been discovered yet ^[1]. Knowing the exact genetic cause is crucial for predicting your child’s specific health risks ^[3].

Because this testing can be expensive, many families find it helpful to work closely with a genetic counselor. They can help navigate insurance pre-authorizations, discuss whether both parents need to be tested, and explore if your child qualifies for sponsored or no-cost genetic testing programs.

Predicting Organ Involvement

Different genes are associated with different risks to specific organs. Identifying the exact mutation helps your care team know where to look and what to monitor. This concept is called genotype-phenotype correlation, which simply means linking a person’s specific genes (genotype) to their physical traits and medical conditions (phenotype) ^[3].

• Kidney Health: Up to one-third of individuals with Joubert syndrome develop kidney disease ^[4]. Mutations in genes like CEP290, TMEM67, and AHI1 are strongly linked to a higher risk of progressive kidney issues ^[4][3]. However, mutations in other genes, such as C5orf42 and KIAA0586, are associated with a much lower likelihood of kidney problems ^[4].
• Liver Health: Liver disease, particularly a condition called hepatic fibrosis (scarring of the liver), is a major concern for some children with Joubert syndrome ^[5]. Biallelic mutations (mutations in both copies of a gene) in the TMEM67 gene are strongly associated with liver involvement and a specific variant of Joubert syndrome known as COACH syndrome ^[6][7][8].
• Eye Health: Vision problems, including severe retinal dystrophy (degeneration of the retina leading to vision loss, often first noticed as involuntary eye movements or difficulty seeing in low light) and colobomas (missing pieces of tissue in structures that form the eye), are common ^[9]. CEP290 mutations are frequently tied to early-onset vision loss ^[10][11], while TMEM67 mutations carry an increased risk for colobomas ^[4].

Tailoring a Lifelong Care Plan

Every child diagnosed with Joubert syndrome requires comprehensive baseline screening for their kidneys, liver, and eyes. However, a precise molecular (genetic) diagnosis allows your medical team to personalize the frequency and intensity of a long-term multidisciplinary management plan ^[12]. If a gene associated with high kidney risk is identified, your child will receive focused, long-term monitoring of their kidney function ^[3]. If the gene is tied to liver issues, doctors will closely monitor liver enzymes and check for signs of portal hypertension (high blood pressure in the vein that carries blood to the liver) ^[4][13].

Furthermore, having a genetic diagnosis provides vital information for your family’s future. It helps you understand the recurrence risk—typically, Joubert syndrome follows an autosomal recessive inheritance pattern (though rare X-linked forms exist), meaning there is often a 25% chance of the condition occurring in future pregnancies ^[14]. Knowing the exact gene allows for targeted genetic counseling and family planning options ^[15].