Pelizaeus-Merzbacher Disease Workup

Molecular diagnostic tests

Molecular diagnostic tests for mutations of the PLP1 gene are the definitive studies for the diagnosis of Pelizaeus-Merzbacher disease. Duplications of the gene can usually be identified by fluorescent in situ hybridization (FISH) testing on interphase leukocytes.

Evoked potentials

Auditory evoked-potential testing shows normal latency of wave 1, and possibly of wave 2 as well, but with prolongation or abolition of central waves 3-5. Caution should be used when interpreting this test because functional hearing is often present, even in the absence of undetectable evoked responses. ^[14]

Visual evoked-potential testing demonstrates increased latency of P100. Somatosensory evoked-potential testing reveals normal peripheral latencies with prolonged or absent central latencies.

Evoked potentials of other leukodystrophies typically have delayed peripheral, as well as central, components.

Nerve conduction studies

Nerve conduction test results are usually normal in Pelizaeus-Merzbacher disease, but patients with null mutations (ie, those that prevent any PLP1 expression) have a mild, multifocal, demyelinating peripheral neuropathy. In contrast, other leukodystrophies, such as Krabbe disease, Cockayne disease, metachromatic leukodystrophy, and adrenoleukodystrophy, have diffusely slow nerve conduction velocities.

Imaging

In studies of the cerebrum, brainstem, and cerebellum, MRI reveals widespread, symmetrical abnormality of the white matter.

Additional tests

Testing for lysosomal storage diseases (particularly for arylsulfatase A, galactosylceramide beta-galactosidase, and hexosaminidase), Salla disease (urine sialic acid), and adrenoleukodystrophy (very ̶ long-chain fatty acids) should be done to exclude these disorders.

If prominent peripheral, as well as central, dysmyelination is present, along with facial features of Waardenburg-Hirschsprung syndrome, then screening for mutations of the SOX10 gene should be considered.

Individuals with a clinical history and signs that are typical for Pelizaeus-Merzbacher disease, but for whom results of routine testing for PLP1 mutations is negative, should be referred to a research laboratory for possible research testing and additional mutation screening.

MRI is the most useful imaging study in Pelizaeus-Merzbacher disease, demonstrating symmetrical and widespread abnormality of the white matter of the cerebrum, brainstem, and cerebellum. Given the heterogeneous nature of the disease process, with multiple genetic and molecular mechanisms causing Pelizaeus-Merzbacher disease, white matter atrophy is a major pathological determinant of the clinical disability in most patients. ^[15]

White matter has increased signal intensity on T2-weighted and inversion-recovery images (see the images below) and is hypointense on T1 images. These changes may not be readily evident or as confidently detected until after age 1 year, because the newborn brain is not well myelinated at birth.

T2-weighted magnetic resonance imaging (MRI) scan of a child aged 10 months with duplication of the proteolipid protein (PLP) gene; note the high-intensity signal throughout the cerebral white matter.

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T2-weighted magnetic resonance imaging (MRI) scan of a man aged 41 years with duplication of the proteolipid protein (PLP) gene; note the increased white matter signal, as well as diffuse atrophy.

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The normal differentiation of white from gray matter is most easily observed after age 1 year, by which time, normally, myelination is actively proceeding. However, the brainstem and cerebellum are partially myelinated at birth, and the posterior limbs of the internal capsule, splenium, and genu are normally myelinated at age 3 months; therefore, absence of the normal myelin MRI signals in these areas should raise suspicion of Pelizaeus-Merzbacher disease in an appropriate clinical setting.

In addition to a diffuse, increased T2-signal intensity, another MRI characteristic often seen in Pelizaeus-Merzbacher disease is a reduction in the absolute volume of white matter; this reduction is most severe in patients with the connatal form of the disorder (see the first image below). Patients with spastic paraplegia type 2 may have only patchy areas of increased T2 signal. Patients with the null mutation may have a more subtle increase in signal intensity, relative to that seen in other patients with Pelizaeus-Merzbacher disease, and the volume of white matter may be normal (see the second image below).

T2-weighted magnetic resonance imaging (MRI) scan of a man aged 20 years with connatal Pelizaeus-Merzbacher disease due to a Pro14Leu mutation; note the severe reduction in white matter volume, as well as the increased white matter signal.

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T2-weighted magnetic resonance imaging (MRI) scan of a boy aged 17 years with null mutation of the proteolipid protein (PLP) gene; note the more subtle increase in signal intensity relative to that seen in the previous images, and observe that the volume of white matter is normal.

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MR spectroscopy

In a recent study by Mori et al, they revealed that the changes in metabolite concentrations during growth can reflect the pathological condition of Pelizaeus-Merzbacher disease. Furthermore, the lack of change in the concentration of choline-containing compounds can be useful for differentiating Pelizaeus-Merzbacher disease from other white matter disorders. ^[16]

Using molecular diagnostic testing to detect mutations of the PLP1 gene is the definitive way to diagnose Pelizaeus-Merzbacher disease. Most patients (about 70%) have duplications (or, rarely, triplication or quintuplication) of the gene, which can usually be identified by FISH testing on interphase leukocytes. Other cells, such as buccal epithelia, chorionic villus cells, and amniocytes, can be tested as well. Duplications can also be identified using the Southern blot test or quantitative polymerase chain reaction (Q-PCR) assay.

Chromosomal microarray analysis (CMA) or comparative genomic hybridization (CGH) testing can also identify duplications or other changes in dosage of the PLP1 gene. FISH testing of metaphase chromosomes can be helpful in identifying rare cases in which the duplicated PLP1 gene is inserted in anomalous sites, such as distant loci of the X or Y chromosome or autosomes.

About 15-20% of patients have small and, typically, single nucleotide mutations that result in missense substitutions. Since FISH testing, CMA, and CGH testing do not identify these mutations, patients suspected of having Pelizaeus-Merzbacher disease who do not have PLP1 duplications should have PLP1 sequence analysis performed.

Mutations have been described that cause nonsense, frameshift, and splicing changes, in addition to complete gene duplications and deletions.

The remaining 5-10% of patients, without duplication or other mutations, may have mutations in PLP1 remote from those regions that are routinely examined in testing laboratories.

Locus heterogeneity (ie, the presence of additional genes that can cause a Pelizaeus-Merzbacher disease–like syndrome) is also observed. Mutations that affect a gap junction protein, GJA12 (also known as connexin 46.6), cause a syndrome virtually identical to Pelizaeus-Merzbacher disease. Patients with this autosomal recessive syndrome have nystagmus, motor and cognitive impairment, and diffuse leukodystrophy on MRI scans. Peripheral neuropathy and seizures are more prevalent in this Pelizaeus-Merzbacher disease–like syndrome. ^[12]

White matter areas in Pelizaeus-Merzbacher disease classically show a tigroid or patchy pattern of staining with myelin stains, but individuals who are more severely affected may have uniform and total loss of myelin staining.

Oligodendrocyte numbers are reduced in most patients. However, patients with null or other relatively mild mutations have normal to near-normal oligodendrocyte numbers and are able to make normal amounts of myelin, although it stains poorly with conventional histochemical myelin stains, such as Luxol fast blue. Some patients have loss of axons, especially of the longer tracts.

Patients with null mutations of PLP1 develop patchy demyelination of the peripheral nerves, typically at sites prone to compression, such as the elbow and wrist.