Introduction

The body is constantly under threat of infections (e.g., fungal infection). Most people aren’t too concerned about these because their immune system can easily handle them. But what if your body doesn’t fight back against these infections the way it should? What if certain immune system conditions that cause this lack of immune response result in minor infections becoming life-threatening? 

One such condition is CARD9 (Caspase-recruitment domain 9) protein deficiency, found mainly in certain immune cells (myeloid) like macrophages and neutrophils. These cells are vital in the immune response as they help the body recognise and respond to pathogens.

When the CARD9 gene is mutated, this ability of immune cells is weakened. This is a rare but significant immune disorder that impairs the body’s natural ability to fight off pathogens and is a real and ongoing struggle for many people.¹ 

Today, scientists are starting to better understand this rare condition and develop treatments that go beyond traditional medicine. New approaches focus on promising immune-based therapies and experimental strategies aimed at strengthening the body’s defence system from within. These developments are not just exciting but a ray of hope and potentially life-saving for those living with this condition. 

Understanding CARD9 deficiency

Molecular function of CARD9 in immune signalling

CARD9 plays a key role in the body's innate immune response, especially against fungi. It acts as a signaling bridge that activates white blood cells (WBCs) (neutrophils and monocytes) when fungal components are detected. When someone is born with mutations in the CARD9 gene, this signalling bridge is broken. Consequently, their body might fail to recognise fungal threats or struggle to recruit the appropriate immune cells.

The condition is inherited in an autosomal recessive way, meaning the person with this deficiency has inherited a disease-causing gene variant from each parent. If an individual receives one normal and one disease-causing gene variant, the person will be a carrier for the disease but usually will not show symptoms. The risk for two carrier parents to both pass the disease gene variant and have an affected child is 25% with each pregnancy.^{2 3,4}

Consequences of CARD9 mutations

This malfunction can lead to:

• Decreased production of inflammatory cytokines ( chemical messengers of our body) 
• Impaired signalling and functioning of Neutrophils⁴
• Increased susceptibility to fungal infections⁴ 

Signs & symptoms

The signs and symptoms of CARD9 deficiency show a variety of conditions which can be life threatening as discussed below:^3,4

• Fungal disease: either invasive candidiasis or candidiasis of the eye, brain, skin and bones
• Deep or chronic invasive dermatophytosis (ringworm): a long-lasting deep skin infection caused by a ringworm fungi
• Black mold disease: a toxic reaction caused by inhaling mold spores and mycotoxins, which causes headache, brain fog, sneezing & coughing
• Meningitis: inflammation of the protective membranes around the brain and spinal cord, called meninges

Diagnostic approaches

Diagnosing CARD9 deficiency requires attention to abnormal patterns of infection. It is particularly important to look out for severe or recurrent unexplained fungal infections that don’t respond to standard treatment. This is especially the case for children and adults with a family history of CARD9 deficiency. To confirm the diagnosis, doctors can use the following techniques.⁴

• Advance tests like genetic sequencing of either the whole-exome or whole-genome to identify CARD9 mutations⁴
• Blood tests to measure immune cell counts and functions e.g., neutrophils⁵ 
• Cultures and biopsies to identify the specific fungal pathogens involved⁵

Timely diagnosis and genetic counselling for future pregnancies are recommended. These can help guide more effective therapies and prevent complications.⁴ 

Current medical therapies

Antifungal treatments

Standard antifungal drugs (like Azoles and Echinocandins) help manage infections like serious candida and nail or scalp fungal infections. However, if it affects the brain or eyes, surgery should be performed along with antifungal medicines.⁴

People with CARD9 deficiency often need long-term therapy or repeated courses until a good clinical response has been monitored. This is followed by maintenance dosing (using lower doses) for a certain period of time to preserve health.⁴

Immune boosters

In some people with CARD9 deficiency, regular antifungal medicines may not be enough to fully resolve or control infections. 

Physicians have tried using a treatment called GM-CSF (Granulocyte-Macrophage Colony Stimulating Factor), which helps boost the body’s immunity. This may help these patients recover better when used alongside regular antifungal treatment as an adjuvant therapy. In fact, some patients with repeated brain infections from candida showed improvement with this therapy, making it a promising treatment option. 

However, physicians need to be cautious when using GM-CSF as it may worsen inflammation or cause other side effects. That is why it's not a standard treatment yet. However, research is underway to utilise GM-CSF as a potential treatment for CARD9 deficiency.⁴

Hematopoietic Stem Cell Transplantation (HSCT)

For severe or recurring infections, hematopoietic stem cell transplantation (HSCT) can potentially cure the condition by providing patients with healthy, new immune cells. About 70% of people with CARD9 deficiency recover, and their immune functioning system starts to improve after a month. 

However, HSCT carries serious risks, such as fatal infections or rejection of the new cells. Not only that, HSCT is not a widely used treatment for CARD9 patients, therefore, experience utilising it is limited. For this reason, doctors must carefully decide/assess whether this treatment is worth the risk for each person.⁴

Future research or strategies

CRISPR/Cas9 is a powerful new gene editing tool that may help treat hereditary conditions like CARD9 deficiency. While it has shown promising results, more research is required to ensure its safety and effectiveness for long-term use. Researchers are working on improving how CRISPR is delivered to target cells, reducing undesirable adverse effects, and eliminating possible immune reactions. 

Future studies should test this technology extensively on animals and then in humans to ensure it functions well without causing harm to people. If effective, this strategy may result in long-term treatments for complex immune-related conditions like CARD9 deficiency.⁶

FAQs

What is CARD9 deficiency and what kind of infections does it cause?

It is an uncommon genetic condition that weakens your immunity, particularly against fungal infections. These infections may affect you deep skin, lungs and brain or spinal cord. They can be difficult to treat and may recur.

How do people get CARD9 deficiency?

It’s something you’re born with by inheriting it from your parents if both carry the faulty gene. It’s not something you catch later in life.

Is it dangerous? Can it be cured?

Yes, if not diagnosed and treated on time. It can cause life-threatening infections as it's not easy to cure. However, with the right treatment (either antifungal or bone marrow transplant) many people live healthier and better lives.

How do doctors find out if someone has CARD9 deficiency?

Doctors may suspect it if someone keeps getting serious fungal infections despite having treatment. A genetic test can confirm the condition by checking for changes in the CARD9 gene.

Is it common?

No, it’s very rare. Most people with strong immunity don’t have to worry about it.

When do symptoms usually start?

Symptoms often start in childhood or early adulthood, particularly after serious fungal infections. 

Can someone with CARD9 deficiency live a normal life?

Yes, many people manage their health well by early diagnosis, regular treatment and by avoiding high-risk fungal exposure like mold. 

Should other family members get tested?

Yes, family members, particularly siblings, might also carry the faulty gene. Genetic counselling and testing can help families understand the risks.

Summary

CARD9 deficiency is a rare genetic condition that lowers the immunity, making people highly susceptible to infections like fungal infections that are usually harmless in others. It happens due to mutations in the CARD9 gene, which affects important immune cells such as neutrophils and macrophages. Due to this, the body finds it difficult to detect and fight off some infections, which can lead to major health problems like meningitis, deep skin infections, and even infections of the brain or eyes. 

Immunological function and genetic testing are used in the diagnosis, particularly in patients with recurring or unexplained fungal infections. Current treatment therapies include antifungal medications, immune-boosting drugs like GM-CSF, and in severe cases, bone marrow transplants (HSCT). However, each of these options has limitations and risks.

Researchers are now investigating advanced therapies like CRISPR/Cas9 gene editing, which could potentially correct the underlying cause of genetic defects. Although it’s still in early stages, this technology may offer long-term hope for people with CARD9 deficiency, as scientists continue working to improve its safety, delivery and effectiveness.