AUTHOR AND EDITOR INFORMATION

Section 1 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

INTRODUCTION

Section 2 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Background

In 1961, Childs et al published the earliest clinical report of a patient who was ultimately found to be affected by a deficiency of propionyl coenzyme A (CoA) carboxylase. These authors noted a series of severe ketoacidotic episodes in the child that were precipitated by protein ingestion (specifically, methionine and threonine administration) but manifested by marked elevations in plasma and urinary glycine levels. Because of these observations, the disease was given the name ketotic hyperglycinemia, a phenomenological term that inadvertently drew investigators' efforts toward a defect in glycine metabolism and delayed elucidation of the biochemical basis. The clinical hallmark of the disease is severe ketoacidosis of an episodic nature.

In 1969, Hsia et al described the underlying defect in propionate carboxylation that occurs in patients with ketotic hyperglycinemia.¹ Simultaneously, Morrow et al described the concurrence of methylmalonic acidemia and ketotic hyperglycinemia; thus, although the condition had been previously considered a single disorder, it was subsequently recognized on clinical grounds to be composed of least 2 different diseases.²

In 1971, subsequent studies by Hsia et al of the original patient's sister demonstrated a specific defect in propionyl CoA carboxylase.³ The study also delineated propionic acidemia from methylmalonic acidemia as a distinct biochemical disorder. Subsequent work led to further delineation of another disorder, initially called multiple carboxylase deficiency, which includes deficiency of propionyl CoA carboxylase activity in addition to defects in other carboxylases.

The defect may be present at either of 2 different gene loci. One locus, on chromosome 13, controls synthesis of the asubunit of the tetrameric enzyme apoprotein; the second locus, on chromosome 3, controls synthesis of the bsubunit. The 2 types of mutations are categorized as PCCA and PCCB (also PCCC) complementation groups, distinguishable from each other by complementation studies of cultured fibroblasts in vitro.

Pathophysiology

The formation of propionyl CoA in human metabolism is derived from many sources, chiefly catabolism of a number of essential amino acids (isoleucine, valine, threonine, methionine). Other sources of propionyl CoA include odd chain-length fatty acids and the side chain of cholesterol, although these probably contribute very little in relation to the amino acid sources. Accumulation of the 3-carbon fatty acyl-CoA within the mitochondrion leads to decreased free CoA for other reactions, which is alleviated by conversion of propionyl CoA to propionyl-carnitine.

Propionyl-carnitine is transported out of the cell and excreted in urine, while the mitochondrial CoA, thus freed, can participate in other reactions or once again become involved in formation of propionyl CoA. The relative reaction rates of these simultaneous processes stand between a tenuously balanced state of equilibrium and severe ketoacidosis. Carnitine deficiency can precipitate a clinical episode by disruption of the balance. Obviously, enhanced dietary protein intake has the same net effect by flooding the mitochondrion with propionyl CoA.

A common clinical finding is mild-to-moderate blood ammonia elevation, which may contribute by direct neurotoxicity to changes in a patient's mentation. Studies suggest that the underlying cause of the hyperammonemia is the inhibition of N-acetylglutamate synthase (NAGS) activity by free propionic acid. Since N-acetylglutamate (NAG) is the allosteric activator of carbamoylphosphate synthase, the entry step into the urea cycle, decreased ureagenesis occurs with accumulation of free ammonia.

Additional evidence suggests that plasma glutamine-to-glutamate ratios decrease with increasing plasma ammonia concentration; simultaneously, urinary methylcitrate excretion increases while urinary citrate diminishes. Taken together, these data might suggest mitochondrial impairment, with inability to produce adequate alpha-ketoglutarate as substrate for formation of glutamate. This hypothesis, while attractive, has not been validated. The free organic acid has also been demonstrated to inhibit bone marrow production of leukocytes, red cells, and platelets. Pancytopenia is common and usually occurs 2-3 days after the acute presentation.

The reason for elevation of serum glycine levels is unclear, although studies show inhibition of glycine cleavage to 1-carbon fragments. Unlike ketoacidosis and hyperammonemia, elevation of glycine in the circulation is not known to be harmful. Thus, propionic acidemia manifests as clinical signs and symptoms of acidosis and hyperammonemia, including tachypnea, vomiting, lethargy, irritability, shock, coma, and death.

Frequency

United States

The incidence rate is estimated to be approximately 1 per 100,000 live births. The caveat here is that population screening programs for the disease are not available in most areas; thus, this figure rests on clinical diagnosis. Because mild and even asymptomatic cases in which persons are genetically affected have been reported, the incidence is likely an under-representation of the true occurrence rate of homozygosity in the US population.

International

Although the disease is generally quite rare, the incidence rate has been reported to be as high as 1 per 2000 to 1 per 5000 births in areas of the world with restricted gene pools, such as Saudi Arabia.

Mortality/Morbidity

• In most patients in whom presentation occurs in infancy with full-blown symptoms, the morbidity is very high. Severe ketoacidosis with pH values as low as 6.80 may cause circulatory shock, hypoxia, and irreparable brain damage.
• Repetitive hyperammonemia causes neurotoxicity with neuronal cell death leading to mental retardation.
• The pancytopenia commonly seen after clinical presentation renders the patient vulnerable to infection, which may become overwhelming and result in death.
• Death can occur at any time from an acute episode.

Sex

Propionyl CoA carboxylase deficiency is inherited as an autosomal recessive trait; therefore, no sex bias is observed.

Age

Like many autosomal recessive traits, multiple mutations at a single gene locus can account for various clinical severities, especially given the potential for mixed heterozygosity.

CLINICAL

Section 3 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

History

• Many affected infants initially present in the first month of life, often with failure to thrive due to feeding intolerance and vomiting. Somnolence is also often a part of the history; thus, poor feeding may be erroneously attributed to CNS disorders.
• Other infants have a fulminant initial presentation, with rapidly developing ketoacidosis, dehydration, shock, and a precedent history of lethargy, poor feeding, and rapid breathing that only extends over 1-2 days.
• As a rare autosomal recessive disease, a family history of similarly affected infants is extremely unlikely.
• Occasionally, an older infant or young child may have a lifelong history of episodic lethargy, anorexia, vomiting, and acidosis that has responded to short hospital stays with intravenous glucose and bicarbonate administration.

Physical

• Affected newborns have been protected from their disease by the maternal circulation and metabolism; therefore, no relevant physical findings present upon neonatal examination.
• Carefully assess infants who present with unexplained vomiting for signs of ketoacidosis; urinalysis is particularly important because neonates normally do not excrete large quantities of ketones.
• CNS depression, which signifies either severe acidosis or hyperammonemia, may be apparent upon examination.
• Any infant with an inborn error can also be affected by other disorders. Suspicion of sepsis based on the typical nonspecific signs must not eliminate the possibility of underlying disease, such as propionic acidemia, from the differential.

Causes

Propionyl CoA carboxylase is a tetrameric enzyme, comprising 4 chains of 2 a and 2 b polypeptides. The gene for production of the b chain has a locus of 13q32, whereas the gene for production of the b chain has a locus of 3q21-q22. Thus, a mutation at either locus affects enzyme activity, but only changes that have occurred in either the a or the b chain affect the respective enzyme activity.  In a mixed heterozygotic individual, with mutations at each gene locus, both types of monomeric constituent polypeptides are affected.

DIFFERENTIALS

Section 4 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Other Problems to be Considered

Multiple carboxylase (holocarboxylase) deficiency

WORKUP

Section 5 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Lab Studies

• A basic metabolic panel is indicated. Serum electrolyte measurement is important. A child who is feeding poorly and vomits may have significant electrolyte abnormalities. Accumulation of free organic acids (anions) significantly increases the anion gap. Therefore, an anion gap larger than 16 mEq/L may indicate propionic acidemia. On rare occasions, affected babies do not present with an increased anion gap.
• Because free propionic acid is known to suppress bone marrow, assessing the status of circulating elements, including platelets, is important.
• Specific gravity, obtained through a routine urinalysis, is important in assessing the degree of dehydration. The presence of ketones in association with an anion gap (as mentioned above) is strongly suggestive of ketoacidosis. A low urine pH lends additional weight to this suspicion.
• Obtaining blood ammonia levels is important in the assessment of the overall metabolic status of the patient, as well as to help in determination of causes for mental status changes. Blood ammonia levels are often secondarily elevated.
• Obtaining plasma lactate levels is helpful in the determination of the causes for an observed anion gap. Lactate levels are often elevated but are not sufficiently high enough to account for the increase in anion gap, which should then prompt further investigation.
• Assessing the urinary organic acid levels is the definitive clinical diagnostic study. Most frequently, it demonstrates large increases in beta-hydroxy propionic acid, lactic acid, and methylcitrate excretion levels.
• Leukocyte propionyl CoA carboxylase activity is the study required for definitive biochemical diagnosis and appropriate genetic counseling.

Other Tests

ECG is recommended in all patients because of the frequency of prolonged QTc intervals and decreased left ventricular contractility reported in patients with propionic acidemia. Further evaluation with continuous ECG monitoring and echocardiography should be considered.

Histologic Findings

A diagnosis of propionic acidemia that is missed in life is extremely difficult to make postmortem.

TREATMENT

Section 6 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Medical Care

• Most patients are so ill at presentation that they have already been admitted to a hospital, which should facilitate appropriate diagnosis and early treatment.
• Because the usual major metabolic precursors of propionic acid are the essential amino acids (isoleucine, valine, threonine, methionine), halt all protein ingestion and emphasize alternative sources of calories on a temporary basis.
• Ketoacidosis is best treated with increased carbohydrate calories, bicarbonate replacement, and increased fluids to enhance excretion. In severely ill patients, metabolic reversal can be expedited by an insulin drip, but this should only be administered in an intensive care setting.
• Reinitiate protein feeding to a level of protein no greater than 1.5 g/kg/d after the patient's condition has normalized. From this point, the patient should be under the care of a biochemical geneticist who may prescribe a special diet prior to discharge.

Consultations

• Biochemical geneticist
• Nutritionist

Diet

Appropriate dietary management is the mainstay of treatment. Several commercially produced formulas are available that provide a protein supplement without any of the 4 amino acids that result in propionate production. However, because they are all essential in humans, closely monitored quantities of isoleucine, valine, threonine, and methionine must be added. For this reason, collaboration between the biochemical geneticist and the nutritionist is imperative.

Activity

No restriction is necessary.

MEDICATION

Section 7 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Some authorities recommend oral biotin supplements in pharmacological dosage (10 mg/d). Although no complication of biotin administration is known, even in such large doses, no good clinical evidence suggests that such treatment is effective.

Some authorities also advocate carnitine supplementation. Although such treatment makes sense biochemically, little evidence suggests any efficacy.

FOLLOW-UP

Section 8 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Further Outpatient Care

• Under no circumstances should patients with propionic acidemia be monitored without the close and frequent input of a biochemical geneticist.
• Frequently assess plasma amino acid concentrations for the need to alter dietary composition and consult a nutritionist in making such changes.
• Recent observations suggest a propensity for optic nerve damage in long-term survivors. Thus, at least annual follow-up by an ophthalmologist is advisable.

In/Out Patient Meds

• See Medication.

Complications

• In most cases, the initial presentation is severe enough to cause significant developmental delay due to brain damage.
• The propionic acidemia may cause leukopenia and permit sepsis, which is devastating in an infant who is already sick.
• Dietary indiscretion, intercurrent illness, and inadequate essential amino acid supplementation may precipitate a severe recurrence of the initial episode.
• Optic nerve atrophy has been observed in male long-term survivors.

Prognosis

• Although less-severely affected patients have been reported, most individuals with propionic acidemia have a classic presentation and course and a guarded prognosis. Survival is in question, and significant brain damage is likely.

Patient Education

• Parents must be taught to strictly adhere to the dietary regimen as prescribed. They also must be made aware of the importance of follow-up for adjustment of diet to meet the requirements for growth.
• Patients must be seen as early as feasible during the course of any intercurrent illness. Treat patients with intravenous glucose and bicarbonate immediately if indicated.

MISCELLANEOUS

Section 9 of 10  

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

Medical/Legal Pitfalls

• Failure to recognize an anion gap, which should lead to correct diagnosis
• Failure to recognize the potential for optic nerve atrophy and, thus, to advise parents and provide appropriate follow-up care

Special Concerns

• As an autosomal recessive disease, propionic acidemia recurs with a 25% risk at each conception in an affected family. Because of the generally dismal prognosis, note that prenatal diagnosis is available to obligate heterozygous couples. Several techniques are in use, including direct assay of carboxylase activity in cultured amniocytes or chorionic villi, estimation of carbon dioxide fixation in amniocytes, and measurement of methylcitrate in amniotic fluid. Making prior arrangements with the appropriate laboratory is always essential before sampling in order to ensure proper handling and reliable results.

REFERENCES

Section 10 of 10

• Authors and Editors
• Introduction
• Clinical
• Differentials
• Workup
• Treatment
• Medication
• Follow-up
• Miscellaneous
• References

1. Hsia YE, Scully KJ, Rosenberg LE. Defective propionate carboxylation in ketotic hyperglycinaemia. Lancet. Apr 12 1969;1(7598):757-8. [Medline].
2. Morrow G 3rd, Barness LA, Auerbach VH, et al. Observations on the coexistence of methylmalonic acidemia and glycinemia. J Pediatr. May 1969;74(5):680-90. [Medline].
3. Hsia YE, Scully KJ, Rosenberg LE. Inherited propionyl-Coa carboxylase deficiency in "ketotic hyperglycinemia.". J Clin Invest. Jan 1971;50(1):127-30. [Medline]. [Full Text].
4. Baumgartner D, Scholl-Burgi S, Sass JO, et al. Prolonged QTc intervals and decreased left ventricular contractility in patients with propionic acidemia. J Pediatr. Feb 2007;150(2):192-7. [Medline].
5. Feliz B, Witt DR, Harris BT. Propionic acidemia: a neuropathology case report and review of prior cases. Arch Pathol Lab Med. Aug 2003;127(8):e325-8. [Medline].
6. Filipowicz HR, Ernst SL, Ashurst CL, Pasquali M, Longo N. Metabolic changes associated with hyperammonemia in patients with propionic acidemia. Mol Genet Metab. Jun 2006;88(2):123-30. [Medline].
7. Gravel RA, Lam KF, Scully KJ, Hsia Y. Genetic complementation of propionyl-CoA carboxylase deficiency in cultured human fibroblasts. Am J Hum Genet. Jul 1977;29(4):378-88. [Medline].
8. Ianchulev T, Kolin T, Moseley K, Sadun A. Optic nerve atrophy in propionic acidemia. Ophthalmology. Sep 2003;110(9):1850-4. [Medline].
9. Lamhonwah AM, Gravel RA. Propionicacidemia: absence of alpha-chain mRNA in fibroblasts from patients of the pccA complementation group. Am J Hum Genet. Dec 1987;41(6):1124-31. [Medline].
10. Meyburg J, Hoffmann GF. Liver transplantation for inborn errors of metabolism. Transplantation. Sep 27 2005;80(1 Suppl):S135-7. [Medline].
11. Nyhan WL, Bordern M, Childs B. Idiopathic hyperglycinemia: a new disorder of amino acid metabolism. II. The concentrations of other amino acids in the plasma and their modification by the administration of leucine. Pediatrics. Apr 1961;27:539-50. [Medline].
12. Perez-Cerda C, Perez B, Merinero B, et al. Prenatal diagnosis of propionic acidemia. Prenat Diagn. Dec 15 2004;24(12):962-4. [Medline].
13. Saunders M, Sweetman L, Robinson B, et al. Biotin-response organic aciduria. Multiple carboxylase defects and complementation studies with propionic acidemia in cultured fibroblasts. J Clin Invest. Dec 1979;64(6):1695-702. [Medline]. [Full Text].
14. Wolf B, Willard HF, Rosenberg LE. Kinetic analysis genetic complementation in heterokaryons of propionyl CoA carboxylase-deficient human fibroblasts. Am J Hum Genet. 1980;32(1):16-25. [Medline].

Article Last Updated: Aug 24, 2007