Advances in the diagnosis of leukodystrophies

The realm of leukodystrophies can often be intimidating; this perception has probably never been truer, with the recent addition of many newly discovered leukodystrophies, confounded by the countless described phenotypes for previously recognized leukodystrophies. This review aims to provide readers with a simplified and clear-cut approach to these rare, yet fascinating, disorders and to summarize the recent discoveries in the field.

Leukodystrophies are a heterogeneous group of inherited neurodegenerative disorders affecting the CNS white matter [1,2]. Most leukodystrophies are inherited in an autosomal recessive fashion (e.g., Krabbe, metachromatic and so on) [3,4], while others are inherited in an X-linked (e.g., adrenoleukodystrophy, Pelizaeus–Merzbacher disease) [5,6] or autosomal dominant (e.g., adult-onset autosomal dominant leukodystrophy) [7] fashion. Leukodystrophies can be classified as either demyelinating (or classic) or hypomyelinating based on their MRI features [2]. The tables and figures presented in this article are meant to help clinicians to orient their diagnostic process when a leukodystrophy is suspected; the order of tables and figures follows that of the clinical approach and investigations as they are usually performed in daily clinical practice.

Clinical approach

The clinical presentation of the different types of leukodystrophies is often similar among the same age group. Figure 1 presents the main forms of leukodystrophies according to their age of onset, while Table 1 summarizes and compares their clinical presentation.

Age of onset

Among the important clinical features to consider when formulating a differential diagnosis of a leukodystrophy is the age of onset. Indeed, all leukodystrophies have a typical age of onset (Figure 1). In general, hypomyelinating leukodystrophies present early in life, as early as the neonatal period. Certain forms of hypomyelinating leukodystrophies can, however, present later. As for demyelinating leukodystrophies, they have very variable ages of onset, ranging from childhood to adulthood, with some forms having different ages at presentation (e.g., metachromatic leukodystrophy) while others have a preferential or unique age of onset (e.g., adult-onset autosomal dominant leukodystrophy in adulthood).

Clinical features

The clinical presentation of the different leukodystrophies is often similar for a given age group. Patients with a neonatal or infantile presentation will typically present with axial hypotonia, which will evolve over time into spastic quadraparesis. They can also present other features such as nystagmus or seizures, amongst others. Patients with onset during childhood typically present with motor delay and regression, with progressive upper motor neuron signs, with or without other manifestations, such as ataxia, dysarthria and so on. The adolescent or adult presentations are usually characterized by cognitive regression and psychiatric manifestations, including behavioral abnormalities, while the motor manifestations are typically more subtle.

Despite the fact that the clinical presentations are very similar for each age group, some clinical features may be more specific to one or more disorders and can help orient the investigations. Table 1 can help the clinician differentiate between different leukodystrophies based on the presence of such clinical features, classified by specific organ/system involvement. For example, in a neonatal or infantile presentation, the presence of congenital cataracts in a hypomyelinating leukodystrophy would strongly suggest the diagnosis of hypomyelination with congenital cataracts (HCC) [8]. On the other hand, the absence of such a feature has been described in HCC and cannot be used to rule out the diagnosis. Another example would be the presence of macrocephaly, which is typically seen in Alexander, Canavan and megalencephalic leukoencephalopathy with subcortical cysts (MLC) [9].

Neuroradiology: the importance of MRI pattern recognition

When a leukodystrophy is suspected, the brain MRI becomes a crucial tool to formulate a diagnostic hypothesis. The first distinction to make when looking at the MRI is whether the white matter abnormalities correspond to a demyelinating or hypomyelinating process (Figure 2). Demyelinating leukodystrophies are characterized by prominent hyperintensity of the white matter in T2-weighted and prominent hypointensity in T1-weighted images compared with gray matter structures, while in hypomyelinating leukodystrophies, the white matter abnormalities appears mildly hyperintense in T2-weighted images and have a variable signal (hyper-, iso- or hypo-intense) in T1-weighted images [2,10].

van der Knaap introduced the concept of pattern recognition in the early 1990s and currently, it is well recognized that MRI features can greatly help to orient molecular testing, ultimately leading to a precise diagnosis [11]. Indeed, many entities have been described using this approach (e.g., MLC [12], vanishing white matter disease [13], leukoencephalopathy with brainstem and spinal cord involvement and lactic acidosis [14], and leukoencephalopathy with thalamus and brainstem involvement and high lactate [LTBL] [15]).

MRI pattern recognition is also useful for discriminating between the many hypomyelinating disorders [16]. Beyond the common diffuse hypomyelination [16], each of these disorders have other specific characterizing features on MRI, which can help to differentiate one from the others [16]. It is important to note that whether the MRI pattern shows demyelination or hypomyelination, the differential diagnosis should include, when appropriate, primary neuronal, hereditary and acquired disorders [2,17,18].

When faced with a first MRI showing hypomyelination, especially in a child younger than 2 years, it is important to consider that this could represent a delay in myelination rather than a true hypomyelination [2,10,19]. A practical way to distinguish the two conditions is by repeating a brain MRI 6 months after the first, in order to identify whether there is a progression in myelination. In myelination delay, the myelination progresses, while in hypomyelination, it does not [2]. This distinction is crucial to make as the differential diagnosis for the two entities is quite different. Indeed, myelination delay is, in general, considered a nonspecific finding associated with global developmental delay [19]. Various etiologies have been described as causing myelination delay, such as chromosomal abnormalities (e.g., trisomy 21), inborn errors of metabolism (e.g., methylmalonic acidemia [20] and phenylketonuria [21]) and acquired causes (e.g., hypoxic–ischemic encephalopathy [22]).One exception to this is the X-linked disorder, Allan–Herndon–Dudley syndrome, formerly called MCT8-specific thyroid hormone cell transporter deficiency, a disorder characterized by myelination delay, but that presents clinically very similarly to a hypomyelinating leukodystrophy, with infantile hypotonia, progressing to spastic quadraparesis with associated movement disorders and/or intellectual disabilities with or without seizures [23,24]. Serum thyroid function tests show normal/slightly elevated thyroid stimulating hormone, high serum T3, low serum reverse T3 and normal/low T4. Thus, the investigations of children with myelination delay should include what is recommended for global developmental delay, as well as thyroid stimulating hormone, T3, reverse T3 and T4 [25].

Once a leukodystrophy has been classified either as demyelinating or hypomyelinating, the distribution of the white matter signal abnormalities will help to orient the diagnosis (Figures 3 & 4)[2,16,26,27].

Moreover, sometimes peculiar MRI features can be useful in the diagnostic process. This is the case of contrast enhancement in adrenoleukodystrophy [28] and in Alexander disease [29] or of the presence of cysts as found in MLC [30] and in RNASET2-deficient leukoencephalopathy [31]. Rarefaction of the cerebral white matter is pathognomonic in vanishing white matter disease [32]. Besides MRI, brain CT scans can provide important information, and in particular, can show cerebral calcifications that are a hallmark of some leukodystrophies (i.e., Aicardi–Goutières syndrome [33] and RNASET2-deficient leukodystrophy) [31].

Genetics

Once the differential diagnosis has been narrowed to one or more diagnostic hypotheses based on salient clinical features and more importantly on MRI characteristics, molecular studies can confirm it, when available. Tables 2 & 3 present the inheritance, mutated genes, screening and molecular genetic tests and indicate whether the latter is clinically available or not, for the different demyelinating and hypomyelinating leukodystrophies, respectively.

Recent advances in leukodystrophies

In this section, the authors will review the recent discoveries in the field of leukodystrophies, with an emphasis on the recently described syndromes or recently discovered genes.

LTBL

LTBL is a newly described mitochondrial disease caused by recessive mutations in EARS2 leading to a mitochondrial translation deficit [27]. The clinical presentation is usually set in the first year of life and is characterized by neurological decline, including progressive spasticity. A subsequent clinical improvement and partial recovery is frequently noticed and has been correlated to the degree of brain involvement. Brain MRI shows diffuse abnormal signals in the cerebral white matter, with relative sparing of the periventricular region, associated with a striking signal abnormality in the thalami and mesencephalon. An incomplete development of the posterior part of the corpus callosum has been reported. Magnetic resonance spectroscopy reveals a peak of lactate. The abovementioned neuroradiological findings can improve in subsequent follow-up examinations [27]. The description of LTBL has shed new light into mitochondrial translation deficits such as those due to mutations in DARS2 (leukoencephalopathy with brainstem and spinal cord involvement and high lactate) [34] and RARS2 (pontocerebellar hypoplasia type 6) [35].

eIF2B-related disorders: extending the phenotype

The classic phenotype of vanishing white matter disease [13] or childhood ataxia with central hypomyelination has expanded. Nowadays, the term ‘eIF2B-related disorders’ appears more appropriate as it includes all the different phenotypes. Besides the typical late infantile onset, characterized by neurologic deterioration following even mild infections or head traumas, the age of onset of eIF2B disorders can vary from the neonatal period (such as what is seen in the allelic disorder Cree leukoencephalopathy) to slowly progressive adult forms [36,37], including the disorder formerly called ‘ovarioleukodystrophy’ [37] characterized by the presence of premature menopause due to ovarian failure. Some adult cases have also been reported with a classic history of complicated migraines preceding the neurological deterioration [36,38]. The brain MRI reveals a demyelinating leukodystrophy with characteristic white matter rarefaction [13]. Recessive mutations in the five genes (EIF2B1–5) coding for the subunits of eIF2B are responsible for the disorder. Some genotype–phenotype correlations have been observed, including some mutations in EIF2B5 associated to late-onset forms [38].

MLC – focus on the new phenotype

MLC is an autosomal recessive disorder firstly described in 1995 by van der Knaap et al. in patients with macrocephaly, insidious neurological deterioration and MRI evidence of white matter swelling and subcortical cysts, particularly of the temporal poles [12]. Subsequently, recessive mutations in the MLC1 gene were found to cause the disease in almost 75% of the affected subjects [30]. The recent discovery that mutations in the HEPACAM gene are responsible for MLC in patients negative to MLC1 analysis has provided new insight in the knowledge of the disease [39]. MLC has a typical onset in infancy, with macrocephaly being the first clinical sign, followed by slow neurological deterioration in the classical phenotype. In the last few years, a new phenotype has been reported, characterized by the lack of the clinical decline and improvement of the neuroradiological findings [40]. When inherited in an autosomal recessive fashion, mutations in HEPACAM have been correlated with the classic phenotype of MLC, while the autosomal dominant transmission has been associated with MLC2, which is characterized by the absence of clinical deterioration and with transient MRI findings. Autosomal dominant mutations in HEPACAM have also been associated with other phenotypes: familiar macrocephaly, as well as macrocephaly and mental retardation with or without autistic features [39].

Hereditary diffuse leukoencephalopathy with spheroids

Hereditary diffuse leukoencephalopathy with spheroids (HDLS) is an adult-onset autosomal dominant white matter disease, associated with progressive cognitive and motor dysfunction [41]. The typical age of onset is from 20 to 60 years of age [42]. Clinical features include initial personality and behavioral changes, as well as cognitive dysfunction (e.g., memory problems), followed by limb spasticity, ataxia and seizures [42]. Considering its autosomal dominant inheritance, HDLS should certainly be included in the differential diagnosis of patients with a strong family history of early-onset, predominantly frontal dementia. Notable MRI findings include bilateral, patchy and often asymmetrical T1-hypointense and T2-hyperintense signal of the white matter, with frontal predominance [43]. Frontal lobe atrophy can also be seen in advanced stages of the disease. The brainstem can also be affected with mainly corticospinal tract involvement [43]. Until recently, the diagnosis of HDLS was solely made on pathology, with the presence of abundant axonal spheroids in the cerebral white matter. CSF1R has been identified as the causative gene responsible for HDLS [41]. It encodes for a colony stimulating factor 1 receptor that is thought to play a crucial role in the mediation of microglial proliferation and differentiation [41].

Pelizaeus–Merzbacher-like disease caused by mutations in GJC2

Pelizaeus–Merzbacher-like disease is an autosomal recessive leukodystrophy presenting both clinical and radiological similarities to Pelizaeus–Merzbacher disease, the prototypical hypomyelinating leukodystrophy. Pelizaeus–Merzbacher-like disease typically presents in early infancy with nystagmus [44]. The patients then develop axial hypotonia, progressive limb spasticity, cerebellar dysfunction and movement disorders [45]. As it is the case with PMD, a milder phenotype of hereditary spastic paraparesis (SPG44) has been described [46]. Similarly to PMD, the MRI brain typically shows diffuse hypomyelination [2] but with a typical involvement of the pons, which is usually not seen in PMD [16]. Pelizaeus–Merzbacher-like disease is caused by GJC2 mutations (formerly called GJA12) on chromosome 1 [44–46]. GJC2 encodes for the gap junction protein γ-2 (also known as connexin 46.6 or 47), which is thought to play a key role in central myelination and to some extent in peripheral myelination [45].

Polymerase III-related leukodystrophies

This novel group of disorders includes five clinically distinct hypomyelinating leukodystrophies, which share some clinical and radiological features, and are all caused by recessive mutations in POLR3A and POLR3B[47–49]. The first of these disorders to be described was leukodystrophy with oligodontia, by Atrouni et al. in 2003 [50]. Since then, four other disorders have been described, namely hypomyelination, hypodontia and hypogonadotropic hypogonadism (4H syndrome) [51], ataxia, delayed dentition and hypomyelination [52], tremor-ataxia with central hypomyelination [53] and hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum [47]. These disorders are likely representing a spectrum of clinical presentations, and for this reason, are referred to as polymerase III (Pol III)-related leukodystrophies.

Pol III-related leukodystrophies have a variable age of onset, ranging from infancy to adolescence [54]. Their core clinical features include motor delay and/or regression, progressive spasticity, cerebellar ataxia, tremor, abnormal dentition (e.g., delayed dentition, hypodontia, oligodontia and so on) and hypogonadotropic hypogonadism [54,55]. The brain MRI demonstrates diffuse hypomyelination, typically associated with T2-hypointensities of the anterolateral nuclei of the thalami, the optic radiations, the dentate nuclei, as well as the pyramidal tracts at the level of the posterior limb of the internal capsules. Other possible findings on MRI are cerebellar atrophy, white matter atrophy and thinning of the corpus callosum [16,53,54]. This group of disorders was recently found to be caused by recessive mutations in POLR3A (chromosome 10) [47,48] and POLR3B (chromosome 12) [47,49], encoding for the two largest subunits of the Pol III, an essential macromolecule responsible for the transcription of DNA into RNA.

Peripheral demyelinating neuropathy, central dysmyelinating leukodystrophy, Waardenburg syndrome, Hirschsprung disease or SOX10-related disorders

SOX10 is a gene coding for a transcription factor important for neural crest and glia development. Mutations in SOX10 have been reported to cause certain cases of the neurological variant of Waardenburg syndrome type IV (Waardenburg–Shah syndrome), also known as peripheral demyelinating neuropathy, central dysmyelinating leukodystrophy, Waardenburg syndrome and Hirschsprung disease [56]. Recently, heterozygous mutations in SOX10 have been associated with an expanding clinical spectrum. In fact, they have been found in patients with Waardenburg syndrome type 2, type 4, and peripheral demyelinating neuropathy, central dysmyelinating leukodystrophy, Waardenburg syndrome and Hirschsprung disease [57]. Classically, the age of onset of SOX10-related disorders is in the first year of life; however, cases have been reported with onset in late infancy. Other than the classic Waardenburg syndrome features (pigmentary abnormalities and sensorineural deafness), the clinical picture is characterized by developmental delay and hypotonia with or without peripheral neuropathy. Early-onset nystagmus, ataxia and spasticity often complete the clinical picture [58]. Other possible phenotypes are Waardenburg syndrome type 2E and Waardenburg syndrome type 4C [57]. Interestingly, Waardenburg syndrome type 2E is also characterized by pigmentary abnormalities and sensorineural deafness with or without neurological signs (mental retardation, ataxia and nystagmus) [56].

Diffuse central hypomyelination with features reminiscent of Pelizaeus–Merzbacher disease characterizes the neuroradiological picture [59]. The presence of hypomyelination and/or demyelination on MRI brain imaging has been described in some Waardenburg syndrome type 2E cases [56].

Hypomyelinating leukoencephalopathy affecting early myelinating structures

A new distinct pattern of hypomyelination has been recently described in four boys [26]. Hypomyelinating leukoencephalopathy affecting early myelinating structures is presumed to be X-linked. All patients presented a specific MRI involvement, with hypomyelination (mild T2-hyperintensity, T1-hyper-, iso- or mild hypo-intensity) of structures that are known to myelinate early in life. In particular, the optic radiations and the frontoparietal periventricular white matter were involved in all cases; the brainstem and the cerebellar white matter were also hypomyelinated, as well as the thalamus. The posterior limb of the internal capsule showed a striped altered signal in T2-weighted images in the majority of cases. The clinical presentation is characterized by onset around the age of 6–20 months with nystagmus. Cerebellar signs (ataxia and dysarthria) were reported in all patients.

It is interesting to note that this entity differs from all other hypomyelinating disorders. In fact, hypomyelinating leukodystrophies usually present with a pattern of hypomyelination in which the early myelinating structures are the most myelinated in comparison to regions of late myelination. In other words, the myelination sequence is respected in hypomyelinating leukodystrophies, except in hypomyelinating leukoencephalopathy affecting early myelinating structures where the structures involved are those of early myelination.

Future perspective

Leukodystrophies are a group of disorders where research is advancing quickly. The challenges are huge, and range from clinical description of new disorders, identification of their causal genes, understanding their epidemiology, clinical features and natural evolution to the understanding of their pathophysiology and development of therapeutic strategies. Hopefully, the next few decades will be as rich in discoveries in this field as the last few have been.

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