Pelizaeus–Merzbacher Disease due to PLP1 Frameshift Mutation in a Female with Nonrandom Skewed X-Chromosome Inactivation

Dear Sir,

Pelizaeus–Merzbacher disease (PMD) is an X-linked inherited hypomyelinating leukodystrophy caused by point mutations or copy number variants (including deletion, duplication, and triplication) of PLP1 that encodes an essential protein of the myelin sheath called proteolipid protein (PLP).[1] The severity of the disease is variable, depending on the peculiar underlying molecular and pathogenic mechanism.[1] [2] [3] Duplications and triplications cause severe PMD, with increasing clinical severity as the number of gene copies grows, whereas milder phenotypes are associated with deletions and point mutations.[1] Classic PMD is an early-onset (before the 1st year of age) and progressive disorder characterized by nystagmus, developmental delay, hypotonia, spasticity, and a distinctive central nervous system (CNS) hypolmyelination.[4] Only few symptomatic female carriers have been reported so far. Following the identification of a pathogenic variant in PLP1 in a female with classic PMD, we reviewed the literature on affected female carriers ([Table 1]).

Table 1 Genetic, neuroradiological, and clinical features of classic PMD in female patients

Authors (y)	Pts. (n)	Genetic findings	Clinical manifestations	Brain MRI findings
Hodes et al (1995)	1	PLP1: c.41C > T, p.(Pro14Leu)	Ny, mild DD, mild dysmetria and ataxia, abnormal BAEPs	Diffuse hypomyelination
Carrozzo et al (1997)	1	X dupl (pter→q24::q21.32→qter) with random X-inactivation	Severe ID/DD, dysmorphic features, multiple congenital anomalies, ocular albinism	Diffuse hypomyelination
Fattal-Valevski et al (2009)	1	PLP1: c.619T > C, p.(Tyr207His)	Ny, DD, hypotonia, normal speech, ambulation with support, fine motor difficulties, dysmetria	Diffuse supratentorial dysmyelination
Yiu et al (2009)	1	Xq22.2 dupl encompassing PLP1	Ny, hypotonia, appendicular hypertonia, dystonia, impaired speech, hyperreflexia	Diffuse hypomyelination with normal spectroscopy
Fonseca et al (2013)	1	t(X;22)(q22;q13) with Xq22 dupl involving PLP1	Ny, DD, seizures, hypotonia, choreoathetosis, impaired speech, able to sit with support	Severe diffuse cerebral hypomyelination
Lassuthova' et al (2014)	1	PLP1: c.453_453 + 6del7insA	Ny, lower extremities spasticity, cerebellar ataxia, hypotonia, no cognitive impairment	Diffuse white matter T2-weighted hyperintensity
Brender et al (2015)	1	712 kb interstitial Xq22.2 del involving PLP1 (exons 1–7) with nonrandom X inactivation	Ny, DD, progressive spasticity, absent speech, loss of ambulation, seizures, self-destructive behavior	Chronic, non progressive diffuse leukodystrophy and supratentorial atrophic changes
Masliah-Planchon et al (2015)	1	Xq22 dupl encompassing PLP1	Ny, DD, ataxia, pyramidal signs, abnormal BAEPs	Diffuse hypomyelination
Ji H et al (2017)	1	PLP1 dupl	Severe DD, Ny, hypotonia, precocious death at 23 mo	−

Abbreviations: BAEPs, brainstem auditory evoked potentials; DD, developmental delay; del, deletion; dupl, duplication; ID, intellectual deficiency; mo, months; MRI, magnetic resonance imaging; n, number; Ny, nystagmus; PMD, Pelizaeus–Merzbacher disease; Pts., patients; y, years.

This patient is the third born to unrelated healthy parents. Her older brothers are healthy. She was delivered at term after uncomplicated pregnancy and neonatal course was uneventful. In the first few months of life, global developmental delay (DD) was observed. At the age of 10 months, neurologic examination revealed spastic tetraparesis with hyperreflexia. The patient was nonverbal and lacked postural control. Brain magnetic resonance imaging (MRI) revealed white matter signal abnormalities consistent with delayed myelination with relative sparing of the subcortical white matter and the posterior cranial fossa structures. Metabolic tests for metachromatic leukodystrophy, Krabbe's leukodystrophy, and Tay–Sachs disease yielded normal results. Brain MRI at the age of 2.5 years demonstrated a hypomyelinating leukodystrophy with decreased N-acetylaspartate (NAA) peak and altered choline (Cho)/NAA ratio at MR spectroscopy ([Fig. 1]). Nerve conduction studies and visual evoked potentials were normal, and brainstem auditory evoked potentials were mildly altered.

Fig. 1 Brain MRI performed at 2.5 years of age. (A–C) Axial and (D) coronal T2-weighted images show a pattern of hypomyelination with relative sparing of the subcortical white matter (arrowheads) and of the pontine and cerebellar white matter (empty arrows). (E) MR spectroscopy performed at the level of the right centrum semiovale reveals mild reduction of N-acetyl-aspartate (arrow). MRI, magnetic resonance imaging.

Comparative genomic hybridization (CGH) array was normal. Molecular analysis of GJC2 in suspicion of PMD-like disease 1 yielded negative results. Eventually, PLP1 sequencing led to the identification of the heterozygous frameshift variant c.8delT, p.(L3Cfs*2). Parental testing confirmed that this variant occurred de novo in the proband. X-inactivation analysis excluded nonrandom skewed X-inactivation. After peripheral blood genomic DNA digestion with restriction endonucleases sensitive to cytosine methylation (HpaII) and polymerase chain reaction (PCR) amplification of “CAG” (cytosine-adenine-guanine) polymorphism in AR gene, X-inactivation ratio (95:5) resulted in favor of the mother. This finding suggested a preferential inactivation of maternal X-chromosome.

Although PMD is an X-linked disorder, female carriers may occasionally be affected ([Table 1]). During CNS development, olygodendrocytes expressing severe PLP1 mutant alleles are negatively selected (apoptosis) in favor of wild-type cells, with a cell-type specific skewed X-inactivation.[2] Conversely, milder PLP1 mutant alleles can be tolerated and affected cells contribute to the myelination process, leading to the synthesis of an unstable myelin.[2] Accordingly, females carrying milder PLP1 pathogenic variants show a variably penetrating X-dominant PMD with late-onset neurodegeneration, whereas female carriers of variants causing severe oligodendrocyte damage are usually asymptomatic. In these individuals, an X-recessive PMD may develop only if nonrandom skewed inactivation occurs, similarly to what occurred in our patient.[1] [5] [6] Indeed, despite harboring a frameshift deletion mutation usually associated with the milder PLP null phenotype,[1] she was severely symptomatic due to the unfavorable X-inactivation pattern.

In conclusion, we highlight the relevant role of PMD in the differential diagnosis of hypomyelinating leukodystrophies in females, especially if clinicoradiological phenotype is suggestive. Despite symptomatic female carriers represent a small portion of PMD affected individuals, an early genetic diagnosis supported by accurate X-inactivation studies may significantly contribute to improve the clinical management of these patients.

• References

• 1 Hobson GM, Garbern JY. Pelizaeus-Merzbacher disease, Pelizaeus-Merzbacher-like disease 1, and related hypomyelinating disorders. Semin Neurol 2012; 32 (01) 62-67
• 2 Hurst S, Garbern J, Trepanier A, Gow A. Quantifying the carrier female phenotype in Pelizaeus-Merzbacher disease. Genet Med 2006; 8 (06) 371-378
• 3 Biancheri R, Grossi S, Regis S. , et al. Further genotype-phenotype correlation emerging from two families with PLP1 exon 4 skipping. Clin Genet 2014; 85 (03) 267-272
• 4 Osório MJ, Goldman SA. Neurogenetics of Pelizaeus-Merzbacher disease. Handb Clin Neurol 2018; 148: 701-722
• 5 Garbern J, Cambi F, Shy M, Kamholz J. The molecular pathogenesis of Pelizaeus-Merzbacher disease. Arch Neurol 1999; 56 (10) 1210-1214
• 6 Brender T, Wallerstein D, Sum J, Wallerstein R. Unusual presentation of pelizaeus-merzbacher disease: female patient with deletion of the proteolipid protein 1 gene. Case Rep Genet 2015; 2015: 453105