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Monogenic obesity (PCSK1)
This condition is caused by inherited changes in the PCSK1 gene, affecting a critical enzyme involved in processing many hormones and neuropeptides. It typically leads to significant weight gain starting in early childhood, alongside other symptoms like low blood sugar.
Overview
Monogenic obesity refers to severe obesity caused by a change in a single gene. One such form is associated with variants in the PCSK1 gene. This condition is considered very rare globally. Individuals with this type of monogenic obesity often experience significant and rapid weight gain from a very young age, usually within the first year of life [PMID:11328905].
The PCSK1 gene provides instructions for making an enzyme called proprotein convertase subtilisin/kexin type 1, also known as PC1/3. This enzyme plays a crucial role in activating various hormones and signalling molecules in the body, particularly those involved in regulating appetite, energy balance, and blood sugar levels. When the PC1/3 enzyme does not function correctly due to genetic changes, these hormones cannot be processed properly, leading to the clinical features of the condition.
Symptoms & clinical features
The primary symptom of PCSK1-related monogenic obesity is severe early-onset obesity. Affected individuals typically develop significant weight gain during infancy or early childhood [PMID:11328905]. Beyond weight gain, other symptoms may include an increased appetite, which can contribute to the obesity.
Some individuals may experience issues with blood sugar regulation, such as low blood sugar (hypoglycaemia) in the newborn period, although this can improve with age. Hormone deficiencies are also possible, including those affecting the adrenal glands (adrenal insufficiency) and reproductive hormones, which might lead to delayed puberty or fertility issues. Diabetes insipidus, a condition affecting water balance, has also been reported in some cases [PMID:24855928].
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Affected organs
Monogenic obesity due to PCSK1 primarily affects systems involved in metabolism and hormone regulation. The brain's hypothalamus, which controls appetite and energy expenditure, is significantly impacted because many of the hormones it relies on for signalling are processed by the PC1/3 enzyme. The pancreas, which produces insulin and other hormones, can also be affected, potentially leading to issues with blood sugar control.
Other endocrine glands, such as the pituitary and adrenal glands, may also be influenced, as the PC1/3 enzyme is essential for activating their respective hormones. This broad impact on the endocrine system underscores the wide-ranging effects of PCSK1 dysfunction on the body's internal balance.
Risks & severity
The severity of monogenic obesity caused by PCSK1 variants can vary, but it generally presents as severe early-onset obesity. The risk of developing associated health problems, often referred to as co-morbidities, is high due to the significant weight gain. These can include metabolic syndrome, type 2 diabetes, high blood pressure, and cardiovascular problems, which are commonly seen in individuals with severe obesity.
The age of onset for severe obesity is typically within the first year of life. While specific lifetime risk figures for each associated condition are not precisely established due to the rarity of PCSK1-related obesity, the general health risks associated with severe childhood obesity are well-documented. Management often focuses on reducing these long-term health complications.
Genetic causes
Monogenic obesity in this context is caused by pathogenic variants in the PCSK1 gene. The PCSK1 gene contains the instructions for making an enzyme known as proprotein convertase subtilisin/kexin type 1, or PC1/3. This enzyme acts like a molecular scissor, cutting inactive precursor proteins into their active, functional forms [PMID:11328905].
PC1/3 is particularly important for processing a wide range of neuropeptides and hormones. These include proinsulin (which is processed into insulin), proglucagon, pro-opiomelanocortin (POMC - important for appetite regulation), and various hormones from the pituitary gland. When variants in PCSK1 impair the function of PC1/3, these crucial hormones and signalling molecules are not properly activated. This disruption in hormone processing leads to the imbalance in appetite and energy metabolism observed in affected individuals [PMID:24855928].
- PCSK1 proprotein convertase subtilisin/kexin type 1
Inheritance pattern
Monogenic obesity due to PCSK1 variants follows an autosomal recessive inheritance pattern. This means that a person must inherit two altered copies of the PCSK1 gene - one from each parent - to develop the condition. Individuals who inherit only one altered copy of the PCSK1 gene are carriers; they typically do not show symptoms of the condition but can pass the altered gene copy on to their children.
If two carriers have children, there is a 1 in 4 (25%) chance with each pregnancy that their child will inherit two altered copies and be affected by the condition. There is a 2 in 4 (50%) chance that the child will be a carrier, and a 1 in 4 (25%) chance that the 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
Diagnosis of monogenic obesity, including the PCSK1-related form, typically begins with clinical suspicion based on severe early-onset obesity and other suggestive symptoms. Genetic testing is then used to confirm the diagnosis. In the UK, this would usually involve a referral to a clinical genetics service by a GP or paediatrician.
Genetic testing for PCSK1 variants falls under the NHS Genomic Medicine Service (GMS) pathways. Specific R-codes, such as R340 Severe obesity, may be relevant for commissioning genetic tests for obesity-related conditions. DNA sequencing is used to identify pathogenic variants within the PCSK1 gene. A confirmed genetic diagnosis can help guide management and provide important information for family planning.
Management & lifestyle
Management for monogenic obesity caused by PCSK1 variants is primarily supportive and aims to address the symptoms and potential complications. This often involves a multidisciplinary team, including paediatricians, endocrinologists, dietitians, and clinical geneticists, within the NHS framework. Dietary and lifestyle interventions are often recommended to help manage weight, though these can be challenging in cases of genetically driven obesity.
Specific hormonal deficiencies identified, such as adrenal insufficiency or diabetes insipidus, can be treated with hormone replacement therapy. Research into new therapeutic approaches targeting pathways affected by PC1/3 dysfunction is ongoing. Regular monitoring for metabolic complications like type 2 diabetes and cardiovascular risk factors is important, with appropriate interventions provided as needed.
UK care pathway
In the UK, individuals suspected of having a monogenic obesity condition like that caused by PCSK1 variants are typically referred through their GP or a specialist (e.g., paediatrician, endocrinologist) to a regional NHS Genomic Medicine Centre. Here, a clinical geneticist will evaluate the case and determine the most appropriate genetic testing, often guided by NHS R-codes such as R340 for severe obesity.
Following a diagnosis, ongoing care is generally provided by a multidisciplinary team within the NHS, often coordinated by a specialist paediatric or adult endocrine service. Access to genetic counsellors is also available, providing essential support and information to patients and their families regarding the genetic aspects of the condition, inheritance patterns, and reproductive options.
Frequently asked questions
What does 'monogenic' mean in monogenic obesity?
'Monogenic' means that the condition is caused by a change in a single gene. In the case of PCSK1-related obesity, it means that variants in the PCSK1 gene are responsible for the severe weight gain and associated symptoms, rather than multiple genes or environmental factors alone.
Can monogenic obesity caused by PCSK1 be prevented?
Since PCSK1-related monogenic obesity is a genetic condition inherited from parents, it cannot be prevented. However, early diagnosis can help in implementing management strategies to address symptoms and reduce the impact of the condition on health.
Is there a cure for PCSK1-related monogenic obesity?
Currently, there is no specific cure for monogenic obesity caused by PCSK1 variants. Management focuses on treating the symptoms, such as dietary modifications, lifestyle advice, and hormone replacement therapy if needed, to improve quality of life and reduce health complications.
How does PCSK1 affect my body to cause obesity?
The PCSK1 gene makes an enzyme that activates many important hormones, including those that regulate appetite and blood sugar. When PCSK1 doesn't work correctly, these hormones can't function properly, leading to increased hunger, rapid weight gain, and sometimes problems with blood sugar control.
What support is available for families affected by monogenic obesity in the UK?
Families in the UK can access support through NHS clinical genetics services, which offer genetic counselling. Patient support groups and charities focused on rare diseases or genetic conditions may also provide valuable resources and community connections.