Investigation of patients with possible peroxisomal disorders,

including X-ALD and Refsum's disease.

Diagnosis of Refsum's disease and to aid in the assessment of

peroxisomal function

Peroxisomes are single-membrane organelles present in all

mammalian cells with the exception of mature erythrocytes.

Size and number vary considerably from tissue to tissue, they

are most abundant in liver and kidney cells. Peroxisomal

proteins are encoded by nuclear genes, synthesized in the

cytosol, and imported posttranslationally into peroxisomes

based on a targeting signal at the C-terminus of a protein

sequence. New peroxisomes form by budding of preexisting

ones after accumulation of newly synthesized matrix and

membrane proteins. Peroxisome degradation occurs

randomly, as wholes, by autophagy. Their half-life is

approximately 1.5 to 2 days.

Peroxisomes carry out important anabolic and catabolic functions,

the latter including beta-oxidation of C(8)- C(26) fatty acids to provide

acetyl-CoA for anabolic reactions when the cells are well supplied

with energy. Impairment of this pathway leads to accumulation of

very long-chain fatty acids (VLCFA). Accordingly, plasma VLCFA

represent the most important biochemical markers of peroxisomal

disorders, including X-linked adrenoleukodystrophy (X-ALD) and

Refsum's disease (phytanic acid oxidase deficiency).

Peroxisomal disorders include 2 major groups: disorders of

peroxisomal biogenesis (caused by defective assembly of

the entire organelle) and singe peroxisomal defects.

The differential diagnosis of these disorders is based on

recognition of clinical phenotypes combined with a series of

biochemical tests to assess peroxisomal function and structure.

These include measurements of very VLCFA, pipecolic acid

(performed separately), phytanic acid and its metabolite

pristanic acid. 

C22:0   < or = 96.3 umol/L

C24:0   < or = 91.4 umol/L

C26:0   < or = 1.30 umol/L

C24:0/C22:0 ratio   < or = 1.39

C26:0/C22:0 ratio   < or = 0.023

Pristanic Acid                                                                                                               

0-4 months:  < or = 0.60 umol/L

5-8 months:  < or = 0.84 umol/L

9-12 months:  < or = 0.77 umol/L

13-23 months:  < or = 1.47 umol/L

> or = 24 months:  < or = 2.98 umol/L

Phytanic Acid                                                                                                                

0-4 months:  < or = 5.28 umol/L

5-8 months:  < or = 5.70 umol/L

9-12 months:  < or = 4.40 umol/L

13-23 months:  < or = 8.62 umol/L

> or = 24 months:  < or = 9.88 umol/L

Pristanic/Phytanic Acid Ratio

0-4 months:  < or = 0.35

5-8 months:  < or = 0.28

9-12 months:  < or = 0.23

13-23 months:  < or = 0.24

> or = 24 months:  < or = 0.39                     

Reports include concentrations of C22:0, C24:0, C26:0 species,

phytanic acid and pristanic acid, and calculated C24:0/C22:0,

C26:0/C22:0, and phytanic acid/pristanic acid ratios.

When no significant abnormalities are detected, a simple

descriptive interpretation is provided.

A profile of elevated phytanic acid, low-normal pristanic acid, and

normal VLCFA is consistent with a diagnosis of Refsum's disease,

phytanic acid oxidase deficiency. Serum phytanic acid concentration

may also be increased in disorders of peroxisomal biogenesis and

should be considered in the differential diagnosis of peroxisomal

disorders.

When abnormal results are detected, a detailed interpretation is given

including an overview of the results and of their significance, a correlation

to available clinical information, elements of differential diagnosis

(including the calculation of a discriminating function, see "Clinical

Reference"), recommendations for additional biochemical testing

and in vitro confirmatory studies (enzyme assay, molecular analysis),

name and phone number of key contacts who may provide these studies

at Mayo or elsewhere, and a phone number to reach 1 of the laboratory

directors in case the referring physician has additional questions.

If results are indicative of hemizygosity for X-ALD, we also include the

calculated value of a discriminating function used to more accurately

segregate hemizygous individuals from normal controls.

When a diagnosis of X-linked ALD (hemizygous male) or heterozygosity

for X-ALD is suspected, the report suggests to contacting the coordinator

of a multicenter study. 

In rare instances, patients with X-ALD may have only minimally

elevated values, 15% to 20% of women heterozygous for X-ALD have normal plasma VLCFA levels.

Dietary treatment (Lorenzo's oil, ketogenic diet) and drugs

(lovastatin) could result in values of difficult interpretation.

Moser AB, Kreiter N, Bezman L, et al:  Plasma very

long chain fatty acid assay in 3,000 peroxisome disease

patients and 29,000 controls. Ann Neurol 1999;45:100-110