Overview 

Timothy syndrome (TS) is a genetic condition that causes defects in the heart and several other parts of the body. While a child is unlikely to obtain this condition due to its rarity, those who do can face life-threatening symptoms. Timothy syndrome’s defining factor is the presence of long QT syndrome. This is when the cardiac muscles take longer to reset between heartbeats due to abnormalities in the heart’s electrical conduction system. As a result, the heartbeat is off-rhythm, which risks sudden death. When it comes to genetic diseases, it is always crucial to understand the genetic basis. This allows specialists to target the root cause and regulate the expressed symptoms. In the case of TS, the gene controlling the calcium channels in cardiac cells, the CACNA1C gene, is affected.¹ In this article, we shall explore its role and the consequences of mutation on calcium channels. 

Clinical features of timothy syndrome

Often, when understanding a genetic disease, we move from external to internal. The first step is to identify the symptoms, which gives us external points to focus on. From there, a common genetic factor can be found. In TS, symptoms of the heart include:

• Prolonged QT which is the delay in resetting the heartbeat due to electrical abnormalities. This causes the heartbeat to become out of rhythm
• Abnormal heart structure - thickening of the cardiac walls, which can prevent the heart from pumping blood
• The lower chambers of the heart, the ventricles, are beating too fast (ventricular tachycardia)²

These result in cardiac arrest or sudden death. Additionally, physical characteristics present at birth are: 

• Fusion of the finger/ toes - webbed appearance (syndactyly)
• Small, misaligned teeth 
• Small upper jaw 
• Flat nasal bridge 
• Low-set ears 
• Thin upper lip²

There are specific areas of the brain targeted in TS. These areas control the function of body systems. As a result, the following body system symptoms are seen: 

• Development delays and impaired cognitive functions 
• Frequent infections occur when the immune system is weakened 
• Seizures (epilepsy or photosensitive epilepsy) 
• Autism spectrum disorder- the child may face difficulty in communication and socialisation
• Low blood sugar
• Low body temperature²

With all these symptoms, TS can be split into two categories: type 1 and type 2. TS1 exhibits the typical characteristics of TS. The prolonged QT is the defining trait of TS1 caused by the calcium channel defects from the CACNA1C mutation.³ In contrast, TS2 is a variant of Timothy syndrome. Symptoms like syndactyly are not observed; however, heart defects and prolonged QT are. In TS2, the prolonged QT is more severe, and hip dysplasia also occurs. Hip dysplasia is when the hip joint is not properly formed, a unique aspect not seen in TS1. Like TS1, TS2 has a mutation in the CACNA1C gene; however, there is an additional mutation of the Gly406Arg variant.⁴  

Genetic basis of timothy syndrome

Now we can look at the root cause of these symptoms, which is found on the genetic level. TS is a dominant genetic disorder. This means that only one copy is needed from one parent for the condition to be expressed. If the abnormal gene is present, the child will inherit TS. 

TS also occurs through “de novo” mutation, which is when there is a genetic change in a person for the first time. “De novo” mutations are the most common cause of TS. Moreover, it is possible that the parents may have no manifestation of TS, but have a child diagnosed with TS. While this occurs in only 10% of cases, it would occur if any of the parents carry the mutation. 

As previously discussed, the main gene in play is the CACNA1C. This gene is key in regulating the formation of calcium channels in the cardiac cells. The calcium channel is composed of several long sequences of small molecules called amino acids, and changes in amino acids in this sequence are called a mutation. The CACNA1C gene is coded within this amino acid sequence. Both TS1 and TS2 have the CACNA1C gene mutation. This is a gain-of-function mutation that occurs at several points across the gene.⁶ 

At G406R, glycine at position 406 is replaced with arginine. The change in the amino acid causes a delay in the channel closing. As a result, excess calcium ions flow into the cells. In TS2,  further mutations at G402S and GL06R are found. The G402S is associated with decreased channel activation. As a result, there is a prolonged imbalance of charges across membranes. The disruption of ion flow causes the prolonged QT.⁵

Role of calcium channel mutations 

The L-type calcium channels are found in cardiac muscle cells. The channels allow for the movement of calcium ions in cells. This triggers muscle contractions, which are crucial for the pumping of blood. Furthermore, the channels are involved in several signalling pathways of the heart:

• Gene transcription 
• Cellular growth processes 
• Bioenergetics 
• Tissue remodelling 
• Immune response activation 

These pathways communicate through electrical activity, with calcium as the central ion. Other areas of the heart are also regulated by calcium ions, such as pacemaker activity and heart contractions. Calcium is essential in maintaining the positive potential of the membrane. The ion channel is voltage-gated and moves calcium ions into the cell. As ion has a 2+ charge, which creates a difference in potential, this triggers the movement of Ca²⁺ out of the cell and causes contractions. Hence, the movement of calcium ions needs to be balanced to keep regular heartbeats. Mutations in the calcium ion channels, therefore, disrupt atrioventricular conduction and ventricular muscle cells.⁵ 

Diagnosis and genetic testing 

Diagnosis of TS can be done before birth or during early childhood. During ultrasound scans, the doctor looks for irregularities in the heart rate. This can manifest as bradycardia, a slow heart rate. The typical physical characteristics can be seen in prenatal scans, such as syndactyly and distinct facial features. During infancy, an ECG can be done. This examines the child’s heart rhythm by measuring the electrical activity. Here, doctors can identify prolonged QT, the hallmark of TS. If these cardiac concerns are present, the child’s neurological development should be tested.²

Alongside this, genetic testing is carried out, and the CACNA1C gene is analysed if TS is suspected. A blood sample is taken to assess the gene, as any mutations of the CACNA1C gene are crucial in confirming the diagnosis of TS. Mutations in the G406R variants are also key. Parents who have a child with TS may use prenatal genetic testing in future pregnancies to identify affected foetuses.¹ 

Therapeutic approaches and future therapies

The treatments for TS usually use a combination of therapeutic approaches. However, the disease has a high mortality rate and few reach adulthood; therefore, most therapies are targeted towards children. Orally administered drugs like beta blockers are usually prescribed, which prevent any sudden increases in heart rate. However, the most effective treatment is an implantable cardioverter defibrillator (ICD). This device can recognise any irregular heart rhythms and administer an electric shock to regulate the heart rate. Pacemakers are used to prevent excessive slowing of the heart rate. 

Antibiotics are given for any respiratory infections; however, the medicine must not impact or prolong QT. Surgical procedures can correct syndactyly, and the heart rate should be monitored closely when using anaesthesia. Frequent blood glucose monitoring and cardiac assessments must be carried out. The child must adhere to a strict lifestyle to ensure no additional triggering of prolonged QT, although this will impact the quality of life of the child. 

Future therapies rely on targeting the calcium channels. Calcium channel blockers, such as verapamil, are currently being trialled; while they may theoretically be effective, there is currently no real evidence for this class of drug effectively reducing life-threatening arrhythmias. 

Another strategy is sodium channel inhibition. This route has shown a promising reduction in prolonged QT. Drugs in this class, such as mexiletine, are effective; in addition,  ranolazine may also be used, which is an antianginal drug targeting multiple ion channels. It was proven effective due to its impact on the sodium channels. Finally, roscovitine is an anti-cancer drug which inhibits cyclin-dependent kinases (CDKs), enzymes which regulate the cell cycle. In the study, roscovitine showed the ability to correct cellular abnormalities found at the genetic origins of TS. Despite such promise, further testing must be carried out to confirm this property.¹

Personalised approaches can be taken using gene therapies. Antisense oligonucleotide (ASO) is a correction therapy used to correct any genetic defects at the RNA level, which allows for the restoration of normal cellular function by decreasing the involvement of variants. In TS, it restores the calcium channel functions by suppressing the CACNA1C exon 8A expression. This shows promise for the future of TS treatment.⁷

Summary 

Timothy syndrome is a rare genetic condition which entails severe cardiac, neurological and developmental abnormalities. It is caused by mutations in the CACNA1C gene, which is responsible for the regulation of the L-type calcium channels found in cardiac cells. This mutation has a gain-of-function effect and causes an excess influx of calcium ions.

There are several consequences to TS:

• Cardiac dysfunction - irregular heartbeats, which are life-threatening 
• Neurological defects - autism and other cognitive disabilities 
• Developmental abnormalities - syndactyly and distinctive facial features

The diagnosis uses genetic testing to make a definitive conclusion. This is crucial for early detection of TS. Drugs targeting specific channels and gene therapies are used as interventions for TS, and Further understanding of the molecular basis of TS will allow for greater outcomes from treatments.