What is the first step before ordering genetic testing?

Before considering genetic testing, a thorough disease and family history should be taken. It is often recommended that genetic counseling is performed, since both clear results and ambiguous incidental findings have psychological and possibly social consequences for the patient and the family. Other methods of diagnosis are often exhausted first before expensive genetic testing is ordered.

How is genetic testing used?

Genetic testing is used in hereditary neurological diseases to help with diagnosis, prognosis, and treatment planning. There are three ways in which genetic testing is performed:

• 
  

  Testing for a candidate gene—This is when a clinical presentation approximates a known clinical syndrome that has a known genetic cause.

  
  
• 
  

  Testing panels of genes—This is an expensive diagnostic exercise where all genes related to a category of presentation, for example myotonia or parkinsonism, are tested.

  
  
• 
  

  Whole-exome sequencing—This allows the sequencing of all genes that may be individually interrogated using bioinformatics methods. It increases the risk of incidental and ambiguous findings. It is cheaper than panel testing, yet it is often difficult to convince the insurance companies to order this test.

Where can one find more information on available testing and the laboratories that carry them out?

http://www-ncbi-nlm-nih-gov/gtr/ is a great resource. It also has links to OMIM and gene reviews, which are references that explain the genetics as well as the clinical presentations of genetic disease.

What are the modes of inheritance of neurological disease?

1. 
   

   Mendelian modes of inheritance:

   
   
   
   
   1. 
      

      Autosomal dominant (AD): The patient requires one copy of the gene to acquire the disease. It is often a gain of function change in the protein coded by the gene. For example, the aberrant protein may be toxic to nervous tissue. Each generation has 50% chance of inheriting the disease regardless of gender if one parent has the candidate gene (eg, GRN for frontotemporal dementia).

      
      
   2. 
      

      Autosomal recessive (AR): The patient requires 2 copies of the gene to acquire the disease. It is often a loss of function of the protein coded by the gene. For example, the physiological activity of a particular enzyme may be reduced. Each generation has 25% chance of having 2 copies of the gene, if the parents are both carriers. Another 50% become carriers (eg, Friedreich ataxia).

      
      
   3. 
      

      No male-to-male transmission, 100% male-to-female transmission, and 50% transmission from the affected mother. The more severe phenotypes are found in males.

      
      
   4. 
      

      X-linked recessive: Sons of female carriers have 50% chance of inheriting the condition. In some cases, the female carriers may exhibit a milder form of the same condition (eg, Fragile-X syndrome).

   
   
   
   
2. 
   

   Non-Mendelian modes of inheritance:

   
   
   
   
   1. 
      

      Imprinting: This is the phenomena when the same genetic defect has different phenotypes depending on from which parent the defect originates. The classic example is the fact that Prader–Willi (hypotonia, obesity, and hypogonadism—paternally inherited) and Angelman syndrome (epilepsy, tremor, and smiling facial expression—maternally inherited) are caused by the same deletion on chromosome 15.

      
      
   2. 
      

      Mitochondrial: Inherited only from the mother, for example mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS).

      
      
   3. 
      

      Multifactorial: Most diseases have both environmental and polygenetic causes. For example, patients with APOE4 genotype are at an increased risk of Alzheimer dementia, but the presence of this gene is neither necessary nor sufficient for the development of sporadic AD.

   
   

What complicates the transmission of Mendelian genes?

1. 
   

   De novo mutation: The mutation occurs in the germline of the previous generation, and the parents do not have the defective gene on testing.

   
   
2. 
   

   Incomplete penetrance: In many AD conditions, a proportion of offsprings inheriting the candidate gene do not develop the disease. It is said that disease has “skipped” that generation.

   
   
3. 
   

   Uncertain paternity: This is when the purported father of the child is not the biological father.

   
   
4. 
   

   Anticipation: See below.

   
   
5. 
   

   Germline mosaicism: This is when some gametes (sperm or ova) contain a particular mutation while others do not.

   
   
6. 
   

   Pleiotropy: A single locus appears to be responsible for diverging phenotypes. Note this is different to variable expressivity where the severity of the disease is different among individuals with the same genetic defect (eg, NF-1). Rather, in pleiotropy the syndrome appear completely separate on clinical grounds; for example, familial hemiplegic migraine, spinocerebellar atrophy type 6, and episodic ataxia type 2 are all caused by mutations in CACNA1A.

   
   
7. 
   

   Locus heterogeneity: This is when more than 1 gene causes a single disease (eg, TSC1 and TSC2 both cause tuberous sclerosis complex).

What is anticipation, and what conditions does it affect?

Anticipation occurs in disorders of trinucleotide repeats, for example CAG repeats or CTG repeats. With each generation, the length of the repeats increases. The severity increases, and often the condition appears at a younger age with each successive generation. Trinucleotide repeat conditions include:

• 
  

  Huntington disease (also see Chapter 34): This is an AD condition caused by CAG repeat in the HTT (Huntingtin) gene. The normal number of repeats is between 10 and 35. People with 36–39 repeats may be at risk of passing on the disease to their offspring. Repeat numbers more than 40 are consistent with a diagnosis of Huntington disease.²

  
  
• 
  

  Fragile X syndrome: It is the most common cause of intellectual developmental disorder (IDD) in males. The gene involved is FMR1, which is most avidly expressed in gonads and the brain. The change leading to pathogenesis is CGG triplet repeat expansion in this gene. Between 5 and 40 copies are found in normal populations. Repeats of more than 200 times are associated with mental retardation. People in between these two extremes have intermediate phenotypes. They may be normal or have learning disabilities. They are also at increased risk of ataxia in what is termed fragile X-associated tremor/ataxia syndrome.

  
  
• 
  

  Myotonic dystrophy: See below.

  
  
• 
  

  Cerebellar disorders (also see Chapter 30):

  
  
  
  
  • 
    

    Friedreich ataxia: This is the most common AR condition among Europeans and Middle Easterners. The defect is the expansion of GAA trinucleotide repeats.

    
    
  • 
    

    Spinocerebellar ataxias (SCAs): These are AD neurodegenerative disorders that predominantly affect the cerebellum. Anticipation is seen in SCAs but is not as strongly as the other conditions in this section. The responsible genes for many of SCAs are known (almost 30 at the time of writing) and often involve CAG repeats in disease-associated genes. SCA 8 is due to CTG repeats instead. As a general rule of thumb, repeat numbers above 35 are abnormal (SCA 3 being the exception with a higher threshold). Olivopontocerebellar atrophy (OPCA), a variant of the multisystem atrophies, now only refers to the nongenetic forms of SCA.

    
    
  • 
    

    Dentatorubral-pallidoluysian atrophy: This is for historical reasons not classified as one of the SCAs. It is also an AD condition caused by CAG repeat expansion in the ATN1 gene. It is associated with ataxia, hyperkinetic movement disorders, and intellectual and psychiatric deficits.

  
  
  
  
• 
  

  Spinal and bulbar muscular atrophy (Kennedy disease): This is a motor neuron disease caused by CAG expansion in the androgen receptor, which causes muscle atrophy mostly in males in adulthood.³

What are mitochondrial disorders, and how are they transmitted?

This is a trick question. Mitochondrial disorders are caused by the dysfunction of the mitochondria. The genes that encode for mitochondrial proteins may be part of mitochondrial DNA (in which case their transmission is “mitochondrial,” ie, matrilineal) or nuclear DNA in which case their inheritance may be AD, AR, or X-linked.

What are some of the characteristics of mitochondrial diseases?^4,5

• 
  

  May be associated with myopathy. The accumulation of abnormal mitochondria has a particular staining characteristic (ragged red fibers).

  
  
• 
  

  Optic nerve pathology is common.

  
  
• 
  

  Lactic acidosis can be caused by a defect in the respiratory chain.

  
  
• 
  

  Phenotypically very varied otherwise.

What is the differential diagnosis for this presentation?

The most common causes of sudden-onset focal neurological deficit in this age group are:

• 
  

  Ischemic stroke

  
  
• 
  

  Intracerebral hemorrhage

  
  
• 
  

  Demyelinating disease

  
  
• 
  

  Stroke-like event related to a mitochondrial disorder

In the aforementioned vignette, the time course, the presence of well-defined vascular distribution, and the report of a preceding event suggest an ischemic stroke in a young adult. Although rarer in the younger population, strokes are still among the more common etiologies of acute-onset focal neurological deficits. The difference is that the pathophysiology and the risk factors differ compared to strokes seen in the older population.

Why is it important to recognize stroke in the young adult as a specific category of disease and differentiate it from more common stroke syndromes?

The identification of stroke in the young population is important for three reasons:

• 
  

  Like all strokes, earlier intervention improves prognosis and outcomes.

  
  
• 
  

  Many conditions presenting as a stroke in the young adult are associated with high levels of morbidity and mortality.

  
  
• 
  

  Some of these conditions are treatable if detected early.

How common is stroke in the young adult?

The incidence of stroke increases with age:

• 
  

  Incidence: 0.6/100,000 for age 0–14 years

  
  
• 
  

  3/100,000 for age <35 years

  
  
• 
  

  20/100,000 for age 35–44 years

Epidemiologically, what are the most common causes of stroke in the young adult?

Virchow’s triad of coagulation (vessel abnormality, stasis of blood, and hypercoagulability) presents a good framework for remembering the common causes of stroke in the younger population:

• 
  

  Vessel related:

  
  
  
  
  • 
    

    Atherosclerosis, vasculopathy, and embolism account for 70% of these cases. Several risk factors of more interest in this age group:

    
    
    
    
    • 
      

      Cardiac embolism and patent foramen ovale

      
      
    • 
      

      Hyperlipidemia (possible inherited hyperlipidemia syndrome).

    
    
    
    
  • 
    

    Vasculitis:

    
    
    
    
    • 
      

      Infectious

      
      
    • 
      

      Necrotizing—polyarteritis nodosa, granulomatosis with polyangiitis, eosinophilic granulomatosis with polyangiitis (EGPA), lymphomatosis.

      
      
    • 
      

      Collagen vascular disease—SLE, RA, Sjögren’s disease, scleroderma.

      
      
    • 
      

      Systemic disease—Behçet disease, sarcoidosis, inflammatory bowel diseases.

      
      
    • 
      

      Giant-cell arteritis—(temporal arteritis), Takayasu arteritis.

      
      
    • 
      

      Hypersensitivity (drug, chemical)

      
      
    • 
      

      Neoplastic

      
      
    • 
      

      Primary angiitis of the CNS

    
    
    
    
  • 
    

    Nonatheroscelorotic/Noninflammatory narrowing of vasculature:

    
    
    
    
    • 
      

      Cervical artery dissection

      
      
    • 
      

      Moyamoya disease

      
      
    • 
      

      Fibromuscular dysplasia (FMD)

      
      
    • 
      

      Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL):⁷It is caused by the NOTCH-3 gene on 19 p13, and results in recurrent subcortical infarcts with spared U fibers.

      
      
    • 
      

      Connective tissue diseases (Ehlers–Danlos syndrome, Menkes syndrome, homocystinuria)

      
      
    • 
      

      Fabry disease (alpha-galactosidase A deficiency)

    
    

  
  

• 
  

  Hypercoagulable states:¹

  
  
  
  
  • 
    

    Protein C deficiency

    
    
  • 
    

    Protein S deficiency

    
    
  • 
    

    Anti-thrombin III deficiency

    
    
  • 
    

    Prothrombin gene mutation 20210A

    
    
  • 
    

    Dysfibrinogenemia

    
    
  • 
    

    Factor XII deficiency

    
    
  • 
    

    Antiphospholipid antibodies

    
    
  • 
    

    Fibrinolytic abnormalities

    
    
  • 
    

    Activated protein C resistance, Factor V Leiden mutation

    
    
  • 
    

    Hyperhomocysteinemia (gene on 1 q 36)

    
    
  • 
    

    MTHFR polymorphism

    
    
  • 
    

    Homocystinuria (cystathione synthase deficiency)

  
  
  
  
• 
  

  Reduced flow:

  
  
  
  
  • 
    

    Polycythemia vera

    
    
  • 
    

    Sickle cell disease

  
  
  
  
• 
  

  Other causes:

  
  
  
  
  • 
    

    Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS).

    
    
  • 
    

    Migraine (diagnosis of exclusion)

  
  

Can the ethnic origin of the patient point to increased probability of certain etiologies?

• 
  

  Moyamoya is more common in Asian populations and is well documented among the Japanese. Moyamoya is a vascular condition with risk of recurrent stroke. Primary moyamoya disease is an autosomal dominant disease most common in Japanese patients. Moyamoya is a Japanese word that translates as “puff of smoke,” which describes the angiographic blush that occurs due to extensive collateralization in response to occlusion of large intracranial arteries, often with bilateral carotid artery occlusion. Moyamoya syndrome can also occur secondary to sickle cell disease, Down syndrome, cranial radiation, neurofibromatosis type 1, and many other conditions.⁸

  
  
• 
  

  Sickle cell disease (SCD) is more common in the African American population. SCD is one of the most prevalent hematologic risk factors for stroke in the young adult. Nine percent of patients with SCD have an acute ischemic stroke by the age of 14, and approximately 20% have MRI evidence of silent cerebral ischemic events. The risk is highest among younger children, but events can occur later in life, especially if dehydration occurs. The sickled erythrocytes can cause thrombosis in large blood vessels or occlusion of small blood vessels.

What is the most common mimic of stroke in the young adult?

Demyelinating disorders are the most common mimics of strokes in the young adult. This is especially true for young females. Multiple sclerosis (MS) can present with a very rapid onset of symptoms. The distinguishing features may include the presence of a typically vascular syndrome and family history of strokes.

Rapid onset of symptoms may also occur with intracranial hemorrhage. What are some of the common causes of intracranial hemorrhage in young adults?

• 
  

  Trauma

  
  
• 
  

  Vascular anomalies (arteriovenous malformations (AVMs), cavernous malformations (CMs), and aneurysms):

  
  
  
  
  • 
    

    In those families with a history of subarachnoid hemorrhage (SAH) in more than one family member, the prevalence of unruptured aneurysms in other family members is markedly increased (4- to 10-fold increased prevalence).

    
    
  • 
    

    Autosomal dominant polycystic kidney disease (ADPKD) is unequivocally associated with a higher prevalence of intracranial aneurysms.

    
    
  • 
    

    Ehlers–Danlos syndrome type IV

    
    
  • 
    

    Alpha 1 antitrypsin deficiency

    
    
  • 
    

    Marfan syndrome

    
    
  • 
    

    Neurofibromatosis I

    
    
  • 
    

    Pseudoxanthoma elasticum

    
    
  • 
    

    Hereditary hemorrhagic telangiectasia

    
    
  • 
    

    CMs

  
  
  
  
• 
  

  High blood pressure—including when related to drugs with sympathomimetic activity

  
  
• 
  

  Bleeding diathesis

  
  
• 
  

  Toxemia of pregnancy

What are the findings typical of a mitochondrial disorder, which might cause or mimic a stroke?

MELAS is a hereditary mitochondrial disease that presents in childhood with proximal muscle weakness, episodic vomiting and lactic acidosis, migraine headaches, and stroke-like episodes.⁵ The areas of infarction are inconsistent with any single vascular distribution. Hearing and visual loss may occur as well. The disease is usually progressive. Diagnosis is made by muscle biopsy, which reveals ragged red fibers.

What are some of the common and often overlooked contributors to the risk of a stroke in the young adult?

The factors, which would contribute to the risk of stroke in the young adult, would mirror those seen in the older population. It would however be easier to overlook them in the young adult.

• 
  

  Obstructive sleep apnea (OSA) is associated with an increased risk of stroke. OSA increases platelet aggregation, causes relative dehydration, and creates recurrent spikes in blood pressure.

  
  
• 
  

  Drugs including cocaine, amphetamines, and so forth.

  
  
• 
  

  Dehydration

What may a basic workup of the patient in case 1 include?

• 
  

  Laboratory studies:

  
  
  
  
  • 
    

    CBC, comprehensive chemistry profile, prothrombin time (PT), activated partial thromboplastin time (aPTT), lipid profile, beta HCG (if applicable)

    
    
  • 
    

    Cardiac enzymes

    
    
  • 
    

    Urine drug screen

  
  
  
  
• 
  

  Imaging:

  
  
  
  
  • 
    

    CT: Initial screening for hemorrhage on initial presentation and to monitor edema in follow-up.

    
    
  • 
    

    CT angiography: This is a fast and accurate assessment of cerebral vasculature.

    
    
  • 
    

    MRI/MRA: Diffusion-weighted images may identify ischemic lesions within minutes of symptom onset. Diffusion perfusion imaging may identify a mismatch (potentially salvageable tissue).

    
    
  • 
    

    Carotid and vertebral (extracranial) ultrasound.

    
    
  • 
    

    Transthoracic echocardiogram (TTE) and/or transesophageal echocardiogram (TEE).

  
  

If the MRI imaging during the workup shows extensive white matter disease, what additional testing may be considered?

If extensive white matter disease is seen on the MRI, then other etiologies for the changes should be sought. The most common would be demyelinating disease of multiple sclerosis or acute demyelinating encephalomyelitis. The latter is often post-infection or post-vaccination and is associated with systemic symptoms including fever. Dysmyelinating diseases are rarer in this age group.