Introduction

Menkes disease, also known as Menkes syndrome, is a rare genetic disorder. It affects copper levels in the body, leading to severe developmental and neurological impairments. 

It is caused by mutations in the ATP7A gene, which encodes a protein crucial for regulating copper transport. This malfunction leads to copper deficiency, affecting the development and function of various tissues, especially the nervous system. 

Menkes disease typically manifests in infancy. Examples of symptoms are sparse and kinky hair, failure to thrive, and developmental delays becoming evident within the first few months of life.

This article aims to provide a comprehensive overview of the neurological manifestations of Menkes disease. By exploring the pathophysiology and management strategies, this review seeks to highlight the critical aspects of the disease that impact the nervous system.¹

Background Information on Menkes disease

Genetic and biochemical basis

Menkes disease is a genetic disorder caused by mutations in the ATP7A gene, located on the X chromosome. This gene encodes a copper-transporting ATPase, an essential protein that regulates the distribution of copper in the body. 

Copper is a vital trace element necessary for the function of several enzymes involved in energy production, connective tissue formation (tissue that supports and gives structure to organs of the body), and neurotransmitter synthesis. 

In Menkes disease, the defective ATP7A protein leads to impaired copper absorption and transport. This results in systemic copper deficiency, and the accumulation of copper in certain tissues, including the intestines and kidneys.

Role of copper metabolism

Copper metabolism is integral to numerous physiological processes, including the nervous system development and maintenance. Copper-dependent enzymes play a critical role in neurotransmitter synthesis, myelination, and cellular respiration. 

Disruptions in copper homeostasis, as seen in Menkes disease, result in profound neurological impairments due to the failure of these enzymatic processes.²

Epidemiology

Menkes disease is a recessive disorder linked to the X chromosome, primarily affecting males. It is estimated to be approximately 1 in 100,000 to 250,000 live births. Due to its genetic basis, the disease can be inherited or result from new mutations. 

Females can be carriers of the ATP7A gene mutation and may exhibit mild symptoms due to X-chromosome inactivation patterns.

Clinical presentation and early symptoms

Menkes disease typically presents in the first few months of life. Early symptoms include:

• Sparse, kinky hair that is easily breakable
• Failure to thrive and poor weight gain
• Hypotonia (reduced muscle tone)
• Developmental delays
• Feeding difficulties
• Irritability and lethargy (decrease in consciousness)

These early signs often prompt further medical evaluation, leading to the suspicion of a metabolic or genetic disorder.

Diagnosis

Diagnosis of Menkes disease involves a combination of clinical evaluation, biochemical tests, and genetic analysis. 

Key diagnostic indicators include low serum copper (low concentration of copper in the blood) and ceruloplasmin levels, as well as genetic testing to identify mutations in the ATP7A gene. 

Early diagnosis is crucial for initiating treatment and involves the parenteral (outside of the intestine) administration of copper histidinate to bypass defective intestinal absorption.

Prognosis

The prognosis for individuals with Menkes disease varies but is generally poor. Despite early treatment, many children experience significant neurological impairments and developmental delays. 

Life expectancy is often reduced, with many affected individuals not surviving beyond early childhood. However, the degree of neurological involvement and overall health outcomes can vary depending on the timing of diagnosis and the effectiveness of treatment.³

Pathophysiology of Menkes disease

Copper transport and its role in the nervous system

Copper is a vital trace element that plays a crucial role in various biochemical processes within the nervous system. It is essential for the function of several enzymes involved in neurotransmitter synthesis, antioxidant defence, and myelination. 

In the context of Menkes disease, mutations in the ATP7A gene disrupt the normal transport and distribution of copper, leading to systemic deficiency. This impairment particularly affects the nervous system, where copper-dependent enzymes are critical for maintaining neural health and function.

Impact of copper deficiency on brain development

Copper is necessary for the activity of enzymes such as dopamine-β-hydroxylase, which is involved in the synthesis of the neurotransmitter norepinephrine (noradrenaline). The lack of copper impairs this enzyme's function, leading to deficits in neurotransmitter production essential for normal brain function.

It is also crucial for the function of cytochrome c oxidase, a key enzyme in the mitochondrial electron transport chain (a group of proteins bound to the inner mitochondrial membrane, electrons pass through these proteins in a series of reactions, and energy is released). This enzyme's impairment results in reduced cellular respiration and energy production, which are vital for the growth and maintenance of brain cells. 

Molecular mechanisms affecting neural tissues

Several molecular mechanisms underline the neurological manifestations of Menkes disease, primarily stemming from the disruption of copper-dependent processes:

• Enzyme dysfunction
• Oxidative stress
• Myelination defects
• Synaptic function and plasticity (changes affecting the strength and functioning of neuronal synapses)

Early neurological signs and symptoms

Developmental delays

One of the hallmark early neurological signs of Menkes disease is developmental delay. Affected infants typically show a delay in reaching key developmental milestones, such as sitting, crawling, and walking. 

These delays are often noticeable within the first few months of life. Cognitive development is also significantly impacted, with children displaying slower-than-expected progress in learning, language acquisition, and social interactions. 

The severity of developmental delays varies among individuals but generally correlates with the degree of copper deficiency and the effectiveness of early intervention.

Hypotonia

Hypotonia, or reduced muscle tone, is another prominent early symptom of Menkes disease. Infants with hypotonia appear “floppy” and have difficulty holding their heads up or maintaining posture. 

This condition is due to the impaired development of the central nervous system and the muscles’ inability to contract properly. Hypotonia can severely impact an infant’s ability to perform basic motor functions and contributes to the overall developmental delays seen in Menkes disease.

Seizures and epilepsy

Seizures are a common and serious early neurological manifestation of Menkes disease. They can present in various forms, including tonic-clonic seizures (shivering and twitching of muscles), myoclonic jerks (sudden muscle movements and jerks), and infantile spasms (tensing or jerking of muscles). 

These seizures often begin within the first few months of life and can be difficult to control with standard antiepileptic medications. Frequent and severe seizures contribute significantly to the neurological morbidity associated with the disease.

Movement disorders 

Movement disorders, such as Dystonia or Spasticity, are also commonly observed in infants with Menkes disease. Dystonia is characterised by involuntary muscle contractions causing repetitive movements or abnormal postures. 

Spasticity, or increased muscle stiffness and tone, can also develop, leading to difficulties with voluntary movements and motor control. These movement disorders result from the degeneration of neural pathways responsible for motor (movement) coordination and control, a consequence of impaired copper-dependent enzyme activity and subsequent neurodegeneration.⁴

Progression of neurological manifestations

Progressive neurodegeneration

As Menkes disease progresses, one of the most profound and devastating consequences is progressive neurodegeneration. This condition is characterised by the gradual loss of structure and function of neurons, leading to widespread brain atrophy (decrease in size). 

Neurodegeneration in Menkes disease is primarily driven by the deficiency of copper-dependent enzymes essential for normal brain function and maintenance. As neurons and other brain cells deteriorate, the affected individuals experience worsening motor and cognitive functions, leading to severe disability.

Myelination defects

Copper is critical for the proper development and maintenance of myelin, the protective sheath surrounding nerve fibres that facilitates efficient signal transmission in the nervous system.

In Menkes disease, the lack of copper disrupts the function of enzymes involved in myelination, leading to myelination defects. These defects result in hypomyelination, where the myelin sheath is thin or improperly formed, and demyelination, where existing myelin breaks down. 

These myelination issues severely impair neural communication, contributing to the progressive loss of motor skills, coordination, and sensory processing abilities seen in affected individuals.

Cognitive impairments and intellectual disability

Cognitive impairments and intellectual disability are significant neurological manifestations that worsen as Menkes disease progresses. 

Children with Menkes disease often exhibit severe delays in mental development, including difficulties with attention, memory, problem-solving, and learning. These impairments are directly related to the structural and functional abnormalities in the brain caused by copper deficiency.

Peripheral neuropathy

Peripheral neuropathy, characterised by damage to the peripheral nerves, is another neurological complication that can arise as Menkes disease progresses. This condition results from the degeneration of nerve fibres outside the central nervous system, leading to symptoms such as muscle weakness, loss of sensation, and abnormal reflexes. 

Peripheral neuropathy in Menkes disease is primarily due to the widespread impact of copper deficiency on neuronal health, affecting both motor and sensory nerves. This condition further contributes to the overall disability and reduced quality of life experienced by individuals with Menkes disease.⁵

FAQs

What is Menkes disease?

Menkes disease is a rare genetic disorder characterised by impaired copper absorption and transport due to mutations in the ATP7A gene. This results in severe copper deficiency, leading to progressive neurological degeneration and systemic manifestations.

What are the early signs and symptoms of Menkes disease?

Early signs and symptoms of Menkes disease include developmental delays, hypotonia, seizures, and movement disorders such as dystonia and spasticity. These symptoms typically manifest within the first few months of life.

How is Menkes disease diagnosed?

Menkes disease is diagnosed through genetic testing to identify mutations in the ATP7A gene. Additionally, biochemical tests measuring copper and ceruloplasmin levels in blood and urine can support the diagnosis. Neuroimaging, such as MRI, may reveal characteristic brain abnormalities associated with the disease.

What is the prognosis for individuals with Menkes disease?

The prognosis for individuals with Menkes disease varies depending on the severity of the genetic mutation and the effectiveness of early intervention. 

Without treatment, the disease is often fatal in early childhood. Early initiation of copper replacement therapy and supportive care can improve outcomes, but neurological impairments may still be profound.

Are there any treatments or therapies available for Menkes disease?

Current treatments for Menkes disease focus on copper replacement therapy to supplement deficient copper levels. This may involve oral or parenteral administration of copper histidine or other copper compounds. Emerging research is exploring potential therapies such as gene therapy to address the underlying genetic defect and improve neurological outcomes.

Summary

Menkes disease, a rare genetic disorder caused by mutations in the ATP7A gene, results in severe copper deficiency and profound neurological manifestations. Understanding the neurological aspects of the disease is crucial for early diagnosis, intervention, and management.

The disease manifests with developmental delays, hypotonia, seizures, and movement disorders early in life. As Menkes disease progresses, neurodegeneration, myelination defects, cognitive impairments, and peripheral neuropathy worsen, severely impacting motor and sensory functions.

However, recent research advancements have focused on genetic and molecular studies, potential therapies targeting neurological aspects, and the role of early intervention and gene therapy. These developments offer hope for improving outcomes and quality of life for individuals affected by Menkes disease.