Homocystinuria: Genetic Causes to Methylmalonic Acidemia: Deficiency & Metabolism

Content August 26, 2025

Homocystinuria: Causes

Homocystinuria is an autosomal recessive disorder. Its causes include:

Cystathionine synthase deficiency (treatment: ↓ methionine, ↑ cysteine, ↑ B₆, B₁₂, and folate in diet)
Affinity of cystathionine synthase for pyridoxal phosphate (treatment: ↑↑ B₆ and ↑ cysteine in diet)
Methionine synthase (homocysteine methyltransferase) deficiency (treatment: ↑ methionine in diet)
Methylenetetrahydrofolate reductase (MTHFR) deficiency (treatment: ↑ folate in diet)

Homocystinuria: Clinical Features & Pathway

All forms of homocystinuria result in excess homocysteine. Clinically, patients present with:

↑↑ Homocysteine in urine
Osteoporosis
Marfanoid habitus
Ocular changes (downward and inward lens subluxation)
Cardiovascular effects (thrombosis and atherosclerosis, leading to stroke and MI)
Kyphosis
Intellectual disability
Hypopigmented skin

It’s important to note that in homocystinuria, lens subluxes “down and in,” which contrasts with Marfan syndrome where it subluxes “up and out.”

#boundingbox

The diagram illustrates the metabolic pathway where methionine is converted to homocysteine, which can then be converted to cystathionine by cystathionine synthase, or back to methionine by methionine synthase. Methyl folate and B12 are involved in the latter conversion, while B6 and serine are involved in the former.

Cystinuria: Defect and Stone Formation

#boundingbox

Cystinuria is a hereditary defect of the renal proximal convoluted tubule, or PCT, and intestinal amino acid transporter. This defect prevents the reabsorption of four specific amino acids: Cystine, Ornithine, Lysine, and Arginine, often remembered by the acronym COLA. Cystine itself is formed from two cysteine molecules connected by a disulfide bond. The excess cystine in the urine leads to recurrent precipitation of hexagonal cystine stones, as shown in the accompanying image.

Cystinuria: Diagnosis and Treatment

Cystinuria is an autosomal recessive condition, with a common prevalence of about 1 in 7000 individuals. Diagnosis is typically made with a urinary sodium-cyanide nitroprusside test and proton nuclear magnetic resonance spectroscopy of urine.

Treatment strategies focus on:

Urinary alkalinization using agents like potassium citrate or acetazolamide
Chelating agents, such as penicillamine, to increase the solubility of cystine stones
Ensuring good hydration
Maintaining a diet low in methionine

Organic Acidemias & Propionic Acidemia

Organic acidemias most commonly present in infancy with symptoms such as poor feeding, vomiting, hypotonia, high anion gap metabolic acidosis, hepatomegaly, and seizures. The accumulation of organic acids has two main effects:

It inhibits gluconeogenesis, leading to decreased fasting blood glucose levels, increased ketoacidosis, and high anion gap metabolic acidosis.
It inhibits the urea cycle, resulting in hyperammonemia.

Specifically, in Propionic acidemia, there is a deficiency of propionyl-CoA carboxylase, which leads to an increase in propionyl-CoA and methylmalonic acid. Treatment involves a low-protein diet, specifically limiting substances that metabolize into propionyl-CoA, such as Valine, Odd-chain fatty acids, Methionine, Isoleucine, and Threonine, often remembered by the acronym VOMIT.

Methylmalonic Acidemia & Pathway

Methylmalonic acidemia is caused by a deficiency of either methylmalonyl-CoA mutase or its cofactor, vitamin B₁₂.

#boundingbox

The metabolic pathway illustrates how several precursors—Valine, Odd-chain fatty acids, Methionine, Isoleucine, and Threonine, derived from protein metabolism—are converted to Propionate. Propionate is then converted to Propionyl-CoA, which is acted upon by Propionyl-CoA carboxylase, requiring Biotin, to form Methylmalonyl-CoA. Methylmalonyl-CoA mutase, requiring Vitamin B₁₂, then converts Methylmalonyl-CoA to Succinyl-CoA, which can enter the TCA cycle. A deficiency in methylmalonyl-CoA mutase or Vitamin B₁₂ leads to the accumulation of methylmalonic acid.

Topics Covered: