Holocarboxylase Synthetase Deficiency

What else is it called?

  • Early-Onset Biotin-Responsive Multiple Carboxylase Deficiency 
  • Early-Onset Combined Carboxylase Deficiency 
  • HLCS deficiency 
  • Infantile Multiple Carboxylase Deficiency 
  • Multiple Carboxylase Deficiency, Neonatal Form 

Get in touch

Contact our caring team on 08452 412 173 for help and support. Our phone lines open 10am-4pm, Monday to Friday.

Prefer to email? Our email address is contact@metabolicsupportuk.org.

What causes it?

Holocarboxylase Synthetase Deficiency is caused by a problem in the HLCS gene. This gene produces the holocarboxylase synthetase enzyme which attaches the vitamin biotin to certain enzymes. This process is needed to produce and break down proteins, fats, and carbohydrates in your body. 

If you have a problem in the HLCS gene, you will not produce enough of the holocarboxylase synthetase enzyme. Therefore, you will be unable to manage your protein, fat, and carbohydrate intake enough to fulfil your body’s needs. In turn, this stops your cells from carrying out their roles, leading to a number of serious health problems. 

How common is it?

Holocarboxylase Synthetase Deficiency is a very rare disorder, and its exact prevalence is difficult to determine. It has been estimated that around 1 in every 87,000 people are affected by this disorder. 

The disorder appears to affect all ethnicities, and both males and females have been diagnosed with it. 

What are the signs and symptoms?

Typically, symptoms appear in the first few months of life, although this has been known to vary somewhat. Symptoms can include: 

  • Problems feeding 
  • Breathing difficulties 
  • Skin rashes 
  • Hair loss (alopecia) 
  • A lack of energy (lethargy) and sleeping longer than normal 
  • Vomiting 
  • Irritability 
  • Weakness in the muscles (hypotonia) 

If left untreated, the severity of Holocarboxylase Synthetase Deficiency can increase and cause seizures, delayed development, and even coma. Therefore, it is important to follow any management plan given to you by your medical team. 

Life expectancy is dependent on getting an early diagnosis and beginning treatment before the symptoms can progress. Although Holocarboxylase Synthetase Deficiency has a high mortality rate if not treated early, many people manage it very well and life healthy lives. 

How is it diagnosed?

Diagnosis for Holocarboxylase Synthetase Deficiency is carried out using a specialised blood and urine test.  

Your medical team may also carry out a muscle biopsy, which looks at your cells. This involves removing a very small amount of tissue from your body for microscopic analysis. It is a simple procedure and isn’t usually painful, but it may feel briefly uncomfortable. 

Your diagnosis will be confirmed using a genetic test. 

Some countries test for Holocarboxylase Synthetase Deficiency during their newborn screening process, including some states in the USA. However, it is not currently offered in the UK. 

Can it be treated?

There are treatments available for Holocarboxylase Synthetase Deficiency. Most commonly, you will be given oral biotin to replace the vitamins your body cannot produce. This will be prescribed by your doctor in the strength that is most suitable for you, and you will need to keep taking biotin for the rest of your life. This treatment is most effective when started before the symptoms of the disorder show. 

Supportive treatment will focus on reducing the impact of any symptoms you may have. This can include extra support to help with development, physical therapy to strengthen muscles and help movement, and medication to manage skin problems. These will be tailored to your specific needs which you can discuss with your medical team. 

Many people with Holocarboxylase Synthetase Deficiency go on to lead healthy lives, especially when treatment is started early. It is important to follow your medical team’s advice in order to stop the symptoms from getting worse. 

Do my family need to be tested?

Holocarboxylase Synthetase Deficiency is inherited from both parents in an autosomal recessive inheritance pattern. This means that both parents are carriers of the disorder through their genes.  

All genes come in pairs. Carriers of Holocarboxylase Synthetase Deficiency have one working HLCS gene and one faulty gene. Children inherit one gene from each parent to make their pair. Whether you have Holocarboxylase Synthetase Deficiency depends on the genes you inherit: 

  • There is a 50% chance that the child will inherit one faulty and one working gene, which means they will also be a carrier. 
  • There is a 25% chance that the child will inherit two working genes and not inherit or carry the faulty gene. 
  • There is a 25% chance that the child will inherit two faulty genes, which means they will have Holocarboxylase Synthetase Deficiency. 

If you think you, or somebody in your family, may have Holocarboxylase Synthetase Deficiency, or that you may be a carrier, contact your medical team who may refer you for genetic testing.

Relevant Organisations


References are available on request. Please contact Helen Morris by phoning 0845 241 2173 or emailing helen@metabolicsupportuk.org [Resource Library No: AAP002]. 

Skip to content