• Clinical science

Inborn errors of metabolism

Summary

Inborn errors of metabolism are a group of inherited disorders characterized by enzyme defects. Clinical manifestations are usually due to the accumulation of toxic substances in the body. While in many cases the disorder cannot be cured, disease outcomes and life expectancy can be improved with supportive care and the appropriate diet.

Alpha-1 antitrypsin deficiency (AAT deficiency)

The diagnosis of alpha-1 antitrypsin deficiency should be considered in all patients with emphysema under the age of 50.

References: [9][7][8][6][4][5][2][6][3][1]

Mitochondrial myopathies

General considerations

Subtypes of mitochondrial myopathies [10]

References:[13][14][15][16][17][11][18][10][12][19]

Disorders of amino acid metabolism

Alkaptonuria [20][21]

  • Definition: : congenital disorder of impairement to degrade tyrosine to fumarate
  • Etiology: mutation in HGD gene
  • Inheritance: autosomal recessive
  • Pathophysiology
    • Deficient activity of homogentisic acid dioxygenase → impaired conversion of homogentisate to 4-maleylacetoacetate
    • Accumulation of homogentisic acid → tissue discoloration and organ damage
  • Clinical features
  • Diagnostics
    • Urine turns black when left standing for prolonged time or when alkalinized.
    • Homogentisic acid in urine and serum; normal tyrosine levels
  • Treatment: diet low in tyrosine and phenylalanine → reduced formation of homogentisic acid

Homocystinuria [22]

  • Definition: a group of congenital disorders characterized by impaired homocysteine metabolism
  • Etiology: mutations in CBS, MTHFR, MTR, MTRR, and MMADHC genes
    • Methionine synthase deficiency
    • Cystathione synthase deficiency
    • Impaired affinity of cystathionine synthase for pyridoxal phosphate
    • Methylenetetrahydrofolate reductase (MTHFR) deficiency
  • Inheritance: autosomal recessive (all enzyme deficiencies that cause homocystinuria are AR)
  • Pathophysiology
    • Methionine synthase (homocysteine methyltransferase) deficiency → impaired conversion of homocysteine into methionine
    • Cystathionine synthase deficiency → impaired conversion of homocysteine into cystathionine
    • All forms result in accumulation of homocysteine.
  • Clinical features: Disease severity varies greatly.
  • Diagnostics
  • Treatment
    • Some patients respond to large doses of pyridoxine (vitamin B6). [25]
    • Methionine synthase deficiency
      • methionine diet
    • Cystathionine synthase deficiency
    • Impaired affinity of cystathionine synthase for pyridoxal phosphate
    • MTHFR deficiency

Marfan syndrome and homocystinuria both present with marfanoid habitus. Distinguishing features include intellectual disability, which is only seen in homocystinuria, and the direction of lens dislocation (downwards in homocystinuria; upwards in Marfan syndrome).

Remember HOMOCYstinuria: Homocystinuria, Osteoporosis, Marfanoid habitus, Ocular changes (lens subluxation), Cardiovascular (thromboembolism, accelerated atherosclerosis), kYphosis.

Hartnup disease [22][26]

  • Definition: : congenital disorder characterized by a defect in the renal and intestinal transport of neutral amino acids (e.g., tryptophan)
  • Etiology: mutation in the SLC6A19 gene
  • Inheritance: autosomal recessive
  • Pathophysiology:
    • Impaired Na+-dependent neutral amino acid transporterdecreased absorption of tryptophan from the gut and renal proximal tubules → inability to synthesize vitamin B3 (niacin)
  • Clinical features: symptoms of vitamin B3 deficiency
  • Diagnostics: amino acids in urine (neutral aminoaciduria)
  • Treatment
    • High-protein diet
    • Niacin supplementation

Phenylketonuria (PKU) [27][28][29][30]

  • Definition: : congenital disorder characterized by accumulation of phenylalanine
  • Epidemiology: incidence ∼ 1:10,000
  • Etiology: mutations in the PAH gene
  • Inheritance: autosomal recessive
  • Pathophysiology
    • Accumulation of phenylalanine due to impaired conversion to tyrosine
      • Defect of the liver enzyme phenylalanine hydroxylase (PAH) → impaired conversion of phenylalanine to tyrosinetyrosine becomes nutritionally essential (classical PKU)
      • Deficiency in tetrahydrobiopterin (BH4), a cofactor of phenylalanine metabolism (e.g., PAH) → decreased conversion of phenylalanine to tyrosine (malignant PKU)
        • Often due to a deficiency in dihydropteridine reductase: normally reduces dihydrobiopterin (DHB) to tetrahydrobiopterin (THB)
    • Excess of phenylalaninetransformed into phenylketone metabolites; (e.g., phenylpyruvate, phenylacetate, and phenyllactate) → excretion of metabolites in the urine
    • Tyrosine deficiency → decreased neurotransmitter, melanin, and thyroxine synthesis (see amino acid derivatives)
  • Clinical features
    • Symptoms may manifest within the first few months of life.
    • Psychomotor delay (starting as early as 4–6 months of age)
    • Seizures [31]
    • Blue eyes; light, pale hair
    • Eczema
    • Musty odor
    • Maternal PKU; : in pregnant women with PKU who do not follow proper diet during pregnancy; infant is at risk of microcephaly, growth restriction, facial dysmorphisms, congenital heart defects, intellectual disability
  • Diagnostics
    • Newborn screening: direct measurement of serum phenylalanine levels on the 2nd–3rd day after birth
    • If screening test is positive: oral tetrahydrobiopterin loading test
    • ↑ Phenylketones in urine: phenylacetate, phenyllactate, phenylpyruvate
  • Treatment
    • Low phenylalanine and high tyrosine diet
    • BH4 deficiency: supplementation of tetrahydrobiopterin

Patients with PKU should be advised to avoid aspartame, an artificial sweetener that contains phenylalanine!

Cystinosis

  • Definition: A congenital disorder characterized by impaired cystine storage.
  • Etiology: autosomal recessive inheritance
  • Epidemiology: Incidence of the most common form (infantile cystinosis): 1:100,000-200,000
  • Pathology:
    • Defective transport of cystine out of lysosomesaccumulation of cystine within lysosomes
    • Three clinical forms with variable age onset and severity → infantile form is the most severe form (infantile > juvenile > ocular (adult) cystinosis)
  • Clinical features
    • Failure to thrive
    • Vomiting, weakness, dehydration
    • Polyuria and polydipsia
    • Progressive renal failure
    • Photophobia due to corneal crystal formation
    • Additional organ involvement (e.g., hepatomegaly)
  • Diagnosis
  • Treatment

Cystinuria [35]

Histidinemia

  • Definition: A rare, benign congenital disorder characterized by an impaired histidine metabolism which leads to an elevation in histidine.
  • Epidemiology: 1:11,500 [36]
  • Etiology: autosomal recessive inheritance
  • Pathology: histidase deficiency → impaired histidine breakdown → histidine accumulates
  • Clinical symptoms
    • Mostly asymptomatic
    • Complications during or after birth (e.g., temporary lack of oxygen) → possible risk factor for the development of intellectual disability
  • Diagnostics
    • Histidine in serum
    • Histidine and imidazole metabolites in urine
  • Treatment: not required [36][37]

Maple syrup urine disease [22][38][39][40]

  • Definition: : congenital disorder characterized by the impaired break down of branched-chain amino acids
  • Etiology: mutations in BCKDHA, BCKDHB, and DBT genes
  • Inheritance: autosomal recessive
  • Pathophysiology: absent or deficient branched-chain alpha-ketoacid dehydrogenase → impaired degradation of branched-chain amino acids (valine, leucine, isoleucine) → elevated α-ketoacid formation
  • Clinical features
    • Symptom onset: early neonatal period
    • Dystonia
    • Vomiting, lethargy, poor feeding
    • Intellectual disability
    • Sweet-smelling urine (maple syrup or burnt sugar odor)
    • Death may occur without appropriate treatment.
  • Diagnostics
    • Part of newborn screening
    • Serum: increased alpha-ketoacids (especially leucine alpha-ketoacids); increased blood levels of leucine, isoleucine, and valine
    • Urine: presence of abnormal branched-chain hydroxy acids and ketoacids
    • Hypoglycemia
  • Treatment

Mnemonic for amino acids involved in maple syrup urine disease: I Love Vermont maple syrup from maple trees (with branches) → I = Isoleucine, Love = Leucine, Vermont = Valine, branches = branched-chain amino-acids

Pyruvate dehydrogenase complex deficiency [22][41]

Propionic acidemia [42][43]

  • Definition: : congenital disorder characterized by impaired metabolism of fats and proteins
  • Epidemiology: incidence ∼ 1:100,000
  • Etiology: mutations in the PCCA and PCCB genes
  • Inheritance: autosomal recessive
  • Pathophysiology:
    • Propionyl-CoA carboxylase deficiency → impaired conversion of propionyl-CoA to methylmalonyl-CoA → ↑ propionyl-CoA and ↓ methylmalonic acid → conversion into propionic acid, which accumulates in serum and urine
  • Clinical features
  • Diagnostics: ↑ propionic acid in urine and serum (organic acidosis)
  • Treatment
    • Low protein diet (e.g., avoidance of methionine, threonine, isoleucine, or valine)
    • Amino acid supplementation

References: [21][25][23][24][26][34][37][38][40][41][42][43]

Purine salvage deficiencies

Lesch-Nyhan syndrome [44][22][45]

To remember the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT), which is involved in Lesch-Nyhan syndrome: He's Got Purine Recovery Troubles OR Hyperuricemia, Gout, Pissed off (aggression, self-mutilation), Retardation (ID), DysTonia!

Adenosine deaminase deficiency [46]

References:[22][44][45]

Disorders of fatty acid metabolism

Medium-chain acyl-CoA dehydrogenase deficiency (MCAD deficiency) [47][48]

References:[49] [48]

Primary carnitine deficiency [48][22]

Carnitine palmitoyltransferase II deficiency (CPT II deficiency) [50][51][52]

Urea cycle disorders

Ornithine transcarbamylase deficiency (OTC deficiency) [53][54][55]

  • Definition: congenital disorder characterized by the inability to excrete ammonia
  • Epidemiology: most common urea cycle defect
  • Inheritance: X-linked recessive
  • Pathophysiology:
    • Defect in the enzyme ornithine transcarbamylase → impaired conversion of carbamoyl phosphate and ornithine to citrulline (and phosphate) → ammonia cannot be eliminated and accumulates
    • Accumulation of carbamoyl phosphate → conversion to orotic acid
  • Clinical features
    • Symptoms commonly manifest in the first days of life but can develop at any age.
    • Nausea, vomiting, irritability, poor feeding
    • Delayed growth and cognitive impairment
    • In severe cases, metabolic encephalopathy with coma and death
    • Does not cause megaloblastic anemia (as opposed to orotic aciduria)
  • Diagnostics
    • Hyperammonemia (usually > 100 μmol/L)
    • Orotic acid in urine and blood, BUN
    • Carbamoyl phosphate and ↓ citrulline in the serum
    • Enzyme analysis of OTC activity
  • Treatment
    • Strict low-protein diet
    • Reduce serum ammonia
      • Nitrogen scavengers such as sodium benzoate
      • Fluid management
      • Dialysis (in severe cases)
    • Arginine supplementation → promotes urea formation

OTC deficiency is the only urea cycle disorder that is X-linked recesssive. All of the other urea cycle disorders are autosomal recessive.

Arginase deficiency [53][56]

  • Definition: congenital disorder characterized by impaired arginase activity, resulting in the accumulation of nitrogen (in the form of ammonia)
  • Inheritance: autosomal recessive
  • Pathophysiology: absent or nonfunctional arginase enzyme → impaired conversion of arginine to ornithine → accumulation of ammonia and arginine in the serum
  • Clinical features
    • Acute: episodic hyperammonemia
      • Often asymptomatic
      • Triggered by metabolic stress (e.g., infections, trauma, surgery)
    • Chronic
      • Delayed growth (usually present by three years of age)
      • Progressive spasticity (especially of lower extremities), dystonia, ataxia, poor cognitive development, and missed developmental milestones
      • Seizures
      • By young adulthood: severe spasticity, inability to ambulate, complete loss of bowel and bladder control, severe intellectual disability
  • Diagnostics
  • Treatment
    • Reduce serum ammonia
      • Dialysis (severe cases)
      • Nitrogen scavengers such as sodium phenylacetate and sodium benzoate
      • Fluid management
    • Low-protein diet

References:[57][54][56] [53]

Orotic aciduria

Orotic aciduria can be distinguished from ornithine transcarbamylase deficiency by the presence of megaloblastic anemia and absence of hyperammonemia!