LEC 7 - URINARY SCREENING OF METABOLIC DISORDERS

What causes overflow disorders?

They result from the disruption of a normal metabolic pathway that causes increased plasma concentrations of nonmetabolized substances

What is an inborn error of metabolism?

It is the disruption of enzyme function caused by failure to inherit the gene to produce a particular enzyme

What causes renal disorders?

They are caused by malfunctions or problems in the renal tubular reabsorption mechanism

How is blood collected for the newborn screening test?

By heel puncture

What method is used to perform the newborn screening test?

Tandem mass spectrophotometry

What are the two major groups of metabolic disorders?

1. Disorders causing accumulation of one or more early amino acid degradation products (e.g., Maple Syrup Urine Disease or MSUD)

2. Disorders causing accumulation of organic acids produced further down in the amino acid metabolic pathway

What are the tryptophan disorders?

Indicanuria and 5-hydroxyindolacetic acid disorders

What are the cysteine disorders?

Cystinosis and homocystinuria

How common is phenylketonuria?

It occurs in 1 of every 10,000 to 20,000 births

What causes phenylketonuria?

The absence of the enzyme phenylalanine hydroxylase

What is the major clinical finding in phenylketonuria?

Mental retardation

What physical characteristics are associated with PKU?

Fair complexion, lighter hair, and lighter eyes

What is the characteristic odor of urine and sweat in PKU patients?

Mousy or musty odor

What does the ferric chloride tube test screen for?

Phenyl pyruvic acid in urine

What happens when ferric chloride is added to urine containing phenyl pyruvic acid?

A permanent blue-green color is produced

What is tyrosyluria (tyrosinemia with tyrosyluria)?

It occurs when the metabolism of tyrosine from the diet or from phenylalanine is abnormal

What is the appearance of tyrosine crystals in urine?

Fine, silky crystals that are scattered singly or aggregated

What is the color of tyrosine crystals in urine?

Brown to black

In what condition do tyrosine crystals precipitate and in what are they soluble?

They precipitate in acid and are soluble in alkali

In whom does transitory hypertyrosinemia occur and how serious is it?

It occurs in low-birthweight and premature infants as a benign condition

What are the clinical and laboratory features of transitory hypertyrosinemia?

Asymptomatic, absence of liver or renal disease, and elevated plasma tyrosine and phenylalanine levels

What enzymes are defective in Type I hereditary tyrosinemia?

Fumarylacetoacetate hydrolase (Type Ia) and maleylacetoacetate isomerase (Type Ib)

What compounds accumulate in Type I hereditary tyrosinemia?

Succinylacetoacetone and succinylacetone

What are the clinical and laboratory features of Type I hereditary tyrosinemia?

Liver failure, hepatoma (late complication), renal dysfunction, rickets, acute intermittent porphyria-like symptoms, phosphaturia, and glycosuria

What enzyme deficiency causes Type II tyrosinemia?

Tyrosine aminotransferase deficiency

What are the clinical and laboratory features of Type II tyrosinemia?

Elevated urinary phenolic acids and erosions of the cornea, soles, and palms

What enzyme deficiency causes Type III tyrosinemia?

p-Hydroxyphenylpyruvic acid dioxygenase deficiency

What are the clinical and laboratory features of Type III tyrosinemia?

Intellectual disability, seizures, intermittent ataxia, elevated plasma tyrosine, and elevated urinary phenolic acids

What is the screening test for Type III tyrosinemia, and what indicates a positive result?

The Nitroso-Naphthol Test; an orange-red color indicates tyrosine metabolites

What is melanuria?

It is the increased excretion of melanin in the urine

What condition does melanuria indicate?

It is a serious finding that indicates the proliferation of melanin-producing cells (melanocytes), often associated with malignant melanoma

What compound is secreted by tumors in melanuria and what happens to it?

5,6-dihydroxyindole is secreted by tumors, which oxidizes to melanogen and then to melanin

What causes alkaptonuria?

It is caused by a deficiency of the enzyme homogentisic acid oxidase

Why is the condition called "alkaptonuria"?

The name comes from the observation that the urine darkens (brown to black) after becoming alkaline upon standing, hence "alkali lover" or alka

What accumulates in the body in alkaptonuria?

Homogentisic acid accumulates in the blood, tissues, and urine

What is the key clinical feature of alkaptonuria related to tissue pigmentation?

Ochronosis — brown pigmentation in cartilage and connective tissue

How does the urine of a patient with alkaptonuria appear?

It turns brown-black on standing or when the pH becomes alkaline

What are the two main screening tests for alkaptonuria?

1. Ferric Chloride Test - Observe for a transient dark blue color.
2. Homogentisic Acid (Silver Nitrate) Test - Observe for black color formation.

What is a simple screening method for alkaptonuria using fresh urine?

Add alkali to freshly voided urine and observe for darkening of color

What enzyme deficiency causes MSUD?

Deficiency of branched-chain α-ketoacid dehydrogenase complex

Which amino acids are affected in MSUD?

Leucine, Isoleucine, and Valine

What causes the accumulation of keto acids in MSUD?

Failure of oxidative decarboxylation of branched-chain keto acids

What is the characteristic odor of urine in MSUD?

Maple syrup odor

What screening test is used for MSUD?

2,4-Dinitrophenylhydrazine (DNPH) Test — observe for yellow turbidity or precipitate

What are the most commonly encountered organic acidemias?

Isovaleric acidemia, Propionic acidemia, and Methylmalonic acidemia

What are the general clinical and laboratory findings in organic acidemias?

Vomiting, Metabolic acidosis, Hypoglycemia, Ketonuria, and Elevated serum ammonia

What enzyme deficiency causes isovaleric acidemia?

Isovaleryl coenzyme A dehydrogenase deficiency

What is the characteristic odor associated with isovaleric acidemia?

Sweaty feet odor in urine

What causes the sweaty feet odor in isovaleric acidemia?

Accumulation of isovaleryl glycine and isovaleric acid due to the enzyme deficiency

What leads to the buildup of isovaleric acid and other toxic substances in this condition?

The failure to metabolize isovaleryl-CoA properly because of the enzyme defect

What causes propionic and methylmalonic acidemia?

Errors in the metabolic pathway that converts isoleucine, valine, threonine, and methionine to succinyl coenzyme A

What enzyme deficiency causes Propionic Acidemia?

Propionyl coenzyme A carboxylase deficiency

What enzyme deficiency causes Methylmalonic Acidemia?

Methylmalonyl coenzyme A mutase deficiency

What is the main cause of indicanuria?

Increased tryptophan converted to indican

Name the conditions that can cause indicanuria.

Obstruction, abnormal bacteria, malabsorption syndromes, Hartnup disease

Why does indican turn blue?

It's colorless until oxidized to indigo blue when exposed to air

What is "blue diaper syndrome"?

Blue staining of diapers due to oxidized indican

What cells produce serotonin from tryptophan?

Argentaffin cells

What is the role of platelets in serotonin physiology?

They carry serotonin throughout the body

What is 5-HIAA?

The breakdown product of serotonin, excreted in urine

How do carcinoid tumors affect serotonin and 5-HIAA?

They cause excess serotonin production, leading to elevated urinary 5-HIAA

What urinary 5-HIAA level suggests a carcinoid tumor?

25 mg/24h (normal: 2-8 mg/24h)

What is the screening test for elevated 5-HIAA?

Silver Nitroprusside Test (purple-black color indicates positive)

What causes cystinuria?

The renal tubules cannot reabsorb cystine filtered by the glomerulus

What screening test is used for cystinuria?

Cyanide-Nitroprusside Test

What color indicates a positive Cyanide-Nitroprusside Test?

Red-purple color

What substances can cause false positives in the Cyanide-Nitroprusside Test?

Ketones and homocysteine

What causes cystinosis?

Defective lysosomal membranes prevent cystine from leaving the lysosome for metabolism

What gene is mutated in cystinosis and what does it encode?

CTNS gene → encodes cystinosin, which transports cystine out of lysosomes

What are the two main types of cystinosis?

1. Nephropathic - affects kidneys, more severe
2. Non-nephropathic - spares kidneys, relatively benign

List the key clinical features of cystinosis.

Photophobia, renal failure, rickets, growth failure

What enzyme deficiency causes homocystinuria?

Cystathionine β-synthase

What reaction does cystathionine β-synthase catalyze?

Converts homocysteine + serine → cystathionine

What problems can increased homocysteine cause?

Failure to thrive, cataracts, intellectual disability, thromboembolic problems

What screening test is used for homocystinuria?

Silver-Nitroprusside Test for homocysteine

What color indicates a positive Silver-Nitroprusside Test?

Red-purple color

What are porphyrins?

Intermediate compounds in heme production

What causes porphyria?

Blockage in the heme pathway → accumulation of the product before the blockage

What are the two types of porphyrias?

Inherited - enzyme gene not inherited; Acquired - from liver/erythrocyte problems or toxins

How can porphyrinuria be recognized?

Urine turns red/port wine color after exposure to air

Which porphyrins are most soluble and appear in urine?

ALA, porphobilinogen, uroporphyrin

Which porphyrins are less soluble or not found in urine?

Coproporphyrin - less soluble but present; Protoporphyrin - not in urine

What is the Ehrlich Reaction used for?

Detects ALA, porphobilinogen, urobilinogen using p-dimethylaminobenzaldehyde

Name two differentiation tests for porphobilinogen.

Watson-Schwartz Test, Hoesch Test

How can porphyrins be detected without identifying the type?

Fluorescence under UV light (550-600 nm)

What are current laboratory methods for porphyrin detection?

HPLC, ion exchange chromatography, spectrometry

What are mucopolysaccharides (GAGs)?

Large compounds mainly in connective tissue, made of a protein core with many polysaccharide branches

What causes mucopolysaccharidoses (MPS)?

Inherited disorders that prevent full breakdown of polysaccharides → accumulation in lysosomes → increased urinary excretion

Which GAGs are most frequently found in urine in MPS?

Dermatan sulfate, keratan sulfate, heparan sulfate

Key features of Hurler Syndrome?

Sex-linked recessive (rare in females), severe intellectual disability, abnormal skeleton, GAG accumulation in cornea

Key feature of Sanfilippo Syndrome?

Intellectual disability

Screening test for MPS?

Cetyltrimethylammonium bromide (CTAB) Turbidity Test

What is melituria?

Presence of increased urinary sugar

What is galactosuria?

Inability to metabolize galactose → glucose; may cause galactosemia with toxic intermediates

What are the main clinical consequences of galactosuria in infants?

Failure to thrive, liver disorders, severe intellectual disability

Effects of specific enzyme deficiencies in galactose metabolism:

GALT deficiency → severe/fatal symptoms
GALK deficiency → adult cataracts
UDP-galactose-4-epimerase deficiency → mild or asymptomatic

Other causes of melituria?

Lactose (lactosuria) → pregnancy
Fructose (fructosuria) → parenteral feeding
Pentose (pentosuria) → high fruit intake

What screening test is used for urinary sugars?

Clinitest (Copper Reduction Test)

How does the Clinitest work?

Reduces copper sulfate in urine (redox reaction) → effervescence → color change (blue/orange/red) compared to chart

What are the main ingredients in a Clinitest tablet?

Copper sulfate, sodium carbonate, sodium citrate, sodium hydroxide