Inborn Errors & Lysosomal Storage Diseases

1. What this chapter covers, and how NEET PG actually tests it

Inborn errors appear to be the most memory-intensive topic in biochemistry, with dozens of enzymes and syndromes.

They are governed by one rule, and applying it converts most of the memorisation into reasoning.

A blocked enzyme causes deficiency of everything downstream and accumulation of everything upstream, and the accumulated substrate is often diverted into a minor pathway that is normally insignificant.

The clinical picture is therefore the sum of three effects: what is missing, what has built up, and what the overflow pathway produces.

Phenylketonuria illustrates all three at once. Tyrosine becomes deficient, phenylalanine accumulates, and the overflow produces phenylketones.

A second organising idea covers the lysosomal diseases. These are not metabolic blocks in the usual sense but failures of degradation, so the substrate accumulates inside lysosomes and the affected organ is whichever one handles most of that substrate.

This chapter covers amino acid disorders, the urea cycle and ammonia, carbohydrate and glycogen storage defects, and lysosomal storage diseases.

2. Amino acid disorders

2.1 Phenylketonuria

Phenylalanine hydroxylase converts phenylalanine to tyrosine, and its deficiency is the classic inborn error.

Apply the rule in three parts.

Deficiency downstream: tyrosine can no longer be made, so it becomes an essential amino acid. Since tyrosine is the melanin precursor, patients are characteristically fair-skinned and fair-haired.

Accumulation upstream: phenylalanine builds up and is directly neurotoxic, producing intellectual disability, seizures and microcephaly if untreated.

Overflow pathway: excess phenylalanine is transaminated to phenylketones, giving urine and sweat a musty or mousy odour.

Treatment is a low-phenylalanine diet with tyrosine supplementation, and aspartame must be avoided since it is a phenylalanine source.

A minority of cases are not enzyme deficiency at all but a defect in the cofactor tetrahydrobiopterin, which is more severe because the same cofactor is needed for dopamine and serotonin synthesis.

Maternal phenylketonuria is a separate entity worth knowing. A treated woman who relaxes her diet in pregnancy exposes a genetically normal fetus to high phenylalanine, causing microcephaly, intellectual disability and cardiac defects.

2.2 The other amino acid disorders

Maple syrup urine disease is worth linking back to pyruvate dehydrogenase, because branched-chain alpha-ketoacid dehydrogenase uses the same five cofactors — which is why thiamine-responsive variants exist.

Homocystinuria is most often confused with Marfan syndrome, and the discriminating features are worth holding precisely.

The lens dislocates downward in homocystinuria and upward in Marfan syndrome. Intellectual disability and thrombosis occur in homocystinuria and not in Marfan syndrome.

Thrombosis is the feature that kills, since homocysteine damages vascular endothelium, and it is the reason treatment is urgent rather than cosmetic.

3. The urea cycle and ammonia

3.1 Why ammonia is toxic

Ammonia is detoxified by conversion to urea in the liver, and any block causes hyperammonaemia.

The toxicity mechanism is worth knowing because it explains the neurological picture.

Ammonia is consumed by combining with alpha-ketoglutarate to form glutamate, and then with glutamate to form glutamine.

Both reactions drain alpha-ketoglutarate out of the citric acid cycle, so cerebral energy production fails.

Glutamine also accumulates within astrocytes and acts as an osmole, drawing in water and producing cerebral oedema — the immediate cause of death in acute hyperammonaemia.

3.2 Ornithine transcarbamylase deficiency and the orotic aciduria discriminator

Ornithine transcarbamylase deficiency is the commonest urea cycle disorder and the only X-linked one, the rest being autosomal recessive.

Carbamoyl phosphate accumulates above the block and, having nowhere else to go, spills out of the mitochondrion into the pyrimidine synthesis pathway.

That overflow produces orotic acid, which appears in the urine.

Orotic aciduria therefore has two possible causes, and one laboratory value separates them.

The megaloblastic anaemia is the discriminator, and it makes sense: in hereditary orotic aciduria the block is within pyrimidine synthesis itself, so pyrimidines for DNA are genuinely lacking. In ornithine transcarbamylase deficiency the pyrimidine pathway is intact and merely overloaded.

Hereditary orotic aciduria is treated with uridine, which bypasses the block.

3.3 A framework for the collapsing neonate

Many inborn errors present identically — a previously well newborn deteriorates once feeding is established, because the offending substrate is now being supplied.

Three laboratory values sort almost all of them, and this framework is worth more than any individual disease.

The logic is that a urea cycle defect blocks nitrogen disposal without generating acid, so ammonia rises in isolation and the hyperventilation it provokes produces a respiratory alkalosis rather than an acidosis.

An organic acidaemia — propionic, methylmalonic or isovaleric — accumulates an organic acid, so there is a raised anion gap acidosis with ketosis. Ammonia also rises, because the accumulated acids secondarily inhibit the urea cycle.

A fatty acid oxidation defect gives the reverse pattern, with hypoglycaemia and acidosis but conspicuously absent ketones, because ketogenesis depends on the blocked pathway.

A respiratory alkalosis in a collapsing neonate is therefore a strong clue rather than an incidental finding, and it points specifically at the urea cycle.

Emergency management is common to all: stop protein and any offending substrate, provide glucose to suppress catabolism, and remove ammonia with sodium benzoate or phenylacetate, or with dialysis if severe.

4. Carbohydrate and glycogen disorders

4.1 Galactose and fructose: two pairs, mild and severe

Each sugar has two disorders, and in both cases the second enzyme in the pathway produces the severe disease.

Galactokinase deficiency is mild. Galactose accumulates and is reduced by aldose reductase to galactitol, which osmotically damages the lens, so cataracts are the main finding.

Classic galactosaemia results from deficiency of galactose-1-phosphate uridyltransferase and is severe.

Galactose-1-phosphate accumulates inside cells and is directly toxic, producing vomiting, failure to thrive, liver failure, cataracts, intellectual disability and a characteristic susceptibility to Escherichia coli sepsis in neonates.

Essential fructosuria, from fructokinase deficiency, is benign and often an incidental finding.

Hereditary fructose intolerance, from aldolase B deficiency, is severe, and its mechanism is instructive.

Fructose-1-phosphate accumulates and traps inorganic phosphate, so the cell cannot regenerate ATP. That phosphate depletion inhibits both glycogenolysis and gluconeogenesis simultaneously.

So the patient develops hypoglycaemia they cannot correct, along with vomiting and liver failure, after any fructose or sucrose exposure.

Symptoms appear when weaning introduces fruit or sucrose, and affected children characteristically develop an aversion to sweet foods.

4.2 Glycogen storage diseases

Four are regularly tested, and each maps to a specific defect.

Von Gierke's lactic acidosis follows directly from the block. Glucose-6-phosphate cannot become glucose, so it is diverted into glycolysis and accumulates as lactate.

Cori disease has normal lactate because gluconeogenesis is intact — only glycogen breakdown is impaired — which is the single value distinguishing the two.

Pompe disease is the odd one out because the enzyme is lysosomal rather than cytoplasmic, so it is simultaneously a glycogen storage disease and a lysosomal storage disease. The cardiac involvement is what makes it lethal.

McArdle's second-wind phenomenon has a precise explanation. After several minutes of exercise, fatty acid delivery and hepatic glucose supply increase, so muscle switches to fuels it can actually use and symptoms improve.

An ischaemic forearm exercise test shows a flat lactate curve, since muscle glycogen cannot be mobilised.

5. Lysosomal storage diseases

5.1 The sphingolipidoses

These result from failure to degrade sphingolipids, and the accumulating substrate determines which organ suffers.

The cherry-red macula pair is the single most reliably tested discrimination in this section.

Both Tay-Sachs and Niemann-Pick produce it, because storage material accumulates in retinal ganglion cells everywhere except the fovea, which has none — so the normal choroidal red shows through against a pale surround.

Hepatosplenomegaly separates them: present in Niemann-Pick, absent in Tay-Sachs.

Note that a cherry-red spot also occurs in central retinal artery occlusion, by an entirely different mechanism, and the clinical setting distinguishes that easily.

5.2 The mucopolysaccharidoses

Two are tested, and they form a clean pair.

Hurler syndrome results from alpha-L-iduronidase deficiency, is autosomal recessive, and produces coarse facies, corneal clouding, hepatosplenomegaly, intellectual disability and early death.

Hunter syndrome results from iduronate-2-sulfatase deficiency, is X-linked, is milder, and characteristically has no corneal clouding but notable aggressive behaviour.

The two discriminators are corneal clouding and inheritance, and they travel together — Hurler is recessive with clouding, Hunter is X-linked without it.

A useful hook is that Hunter needs clear vision to hunt.

Both accumulate the same substrates — dermatan and heparan sulphate — which is why the two diseases resemble each other so closely despite different enzymes and different inheritance.

5.3 Inheritance as a discriminator

Almost every lysosomal storage disease is autosomal recessive, which makes the exceptions valuable.

Fabry and Hunter are the two X-linked ones.

Because they are so few, a pedigree showing male-only involvement with no male-to-male transmission narrows the answer to those two immediately, and corneal clouding or angiokeratomas then separate them.

5.4 I-cell disease

I-cell disease is conceptually different and worth understanding rather than memorising.

The defect is not in a lysosomal enzyme at all, but in the Golgi enzyme that attaches mannose-6-phosphate tags to lysosomal enzymes.

That tag is the address label directing enzymes to the lysosome.

Without it, the enzymes are secreted from the cell instead, so lysosomes are empty of enzymes while plasma lysosomal enzyme levels are strikingly high.

Clinically it resembles a severe mucopolysaccharidosis, with coarse facies, corneal clouding, restricted joint movement and gingival hyperplasia.

The raised plasma enzyme levels are the diagnostic signature, and they are counterintuitive enough that questions rely on them.

5.5 Why enzyme replacement works for some and not others

Enzyme replacement therapy exists for Gaucher, Fabry and Pompe disease but not for Tay-Sachs or Krabbe disease, and the reason is instructive rather than arbitrary.

Infused enzyme is taken up by cells through the mannose-6-phosphate receptor and delivered to lysosomes — the same pathway that fails in I-cell disease.

That works well for visceral organs, which is why hepatosplenomegaly, bone disease and renal involvement respond.

It fails for the brain, because the infused enzyme cannot cross the blood-brain barrier.

So a lysosomal disease whose burden is primarily visceral is treatable, while one that is primarily neurodegenerative is not.

Gaucher type 1 is the clearest success, being visceral without central nervous system involvement. Tay-Sachs is purely neuronal, and no amount of circulating enzyme reaches the affected cells.

This single principle predicts which of these diseases has a disease-modifying therapy, and it explains why substrate reduction and gene therapy approaches are pursued for the neuronopathic forms instead.

7. Common exam traps

• Confusing Tay-Sachs with Niemann-Pick. Both give a cherry-red macula; only Niemann-Pick gives hepatosplenomegaly.
• Reversing the lens dislocation direction. Downward in homocystinuria, upward in Marfan syndrome.
• Forgetting that orotic aciduria has two causes. Megaloblastic anaemia indicates hereditary orotic aciduria; hyperammonaemia indicates ornithine transcarbamylase deficiency.
• Assuming all urea cycle disorders are recessive. Ornithine transcarbamylase deficiency is X-linked and is also the commonest.
• Treating the two galactose or two fructose disorders as equivalent. The second enzyme in each pathway causes the severe disease.
• Expecting glucagon to help in von Gierke disease. Glucose-6-phosphatase is absent, so glucose cannot be released whatever the signal.
• Missing that Pompe disease is lysosomal. It is both a glycogen and a lysosomal storage disease, and cardiomegaly is what kills.
• Expecting low plasma enzyme levels in I-cell disease. They are high, because untagged enzymes are secreted rather than delivered.

Summary

• Every inborn error follows one rule: deficiency downstream, accumulation upstream, and diversion of the excess into an overflow pathway.
• Phenylketonuria makes tyrosine essential, so patients are fair, while accumulated phenylalanine is neurotoxic and its ketones cause the musty odour.
• Cofactor-deficient phenylketonuria is more severe because tetrahydrobiopterin is also needed for dopamine and serotonin synthesis.
• Maternal phenylketonuria harms a genetically normal fetus, causing microcephaly and cardiac defects.
• Homocystinuria is distinguished from Marfan syndrome by downward lens dislocation, intellectual disability and thrombosis, and is often pyridoxine-responsive.
• Ammonia toxicity depletes alpha-ketoglutarate from the citric acid cycle and causes astrocytic glutamine accumulation with cerebral oedema.
• Ornithine transcarbamylase deficiency is the commonest urea cycle disorder and the only X-linked one, causing orotic aciduria with hyperammonaemia but no megaloblastic anaemia.
• Hereditary orotic aciduria gives orotic aciduria with megaloblastic anaemia and normal ammonia, and responds to uridine.
• Galactokinase deficiency causes cataracts alone through galactitol, while transferase deficiency causes liver failure, cataracts and susceptibility to Escherichia coli sepsis.
• Essential fructosuria is benign, but aldolase B deficiency traps inorganic phosphate and blocks both glycogenolysis and gluconeogenesis, giving refractory hypoglycaemia.
• Von Gierke disease causes fasting hypoglycaemia with lactic acidosis and no glucagon response; Cori disease is similar but with normal lactate.
• Pompe disease is lysosomal, causing cardiomegaly and hypotonia, and McArdle disease causes exercise intolerance with a second wind and a flat lactate curve.
• Gaucher disease is the commonest lysosomal storage disease, with Erlenmeyer flask femora and crumpled tissue paper macrophages.
• A cherry-red macula with hepatosplenomegaly is Niemann-Pick disease; without it, Tay-Sachs disease.
• Fabry and Hunter are the only X-linked lysosomal storage diseases, and Hurler has corneal clouding while Hunter does not.
• I-cell disease is a failure of mannose-6-phosphate tagging, so lysosomal enzymes are secreted and plasma levels are high rather than low.
• In a collapsing neonate, three values sort the causes: high ammonia without acidosis means a urea cycle defect, acidosis with ketosis means an organic acidaemia, and acidosis without ketones means a fatty acid oxidation defect.