Argentinian clinical genomics in a leukodystrophies and genetic leukoencephalopathies cohort: Diagnostic yield in our first 9 years

1 INTRODUCTION

Genetic leukodystrophies and leukoencephalopathies constitute a vast group of neurological diseases that affect all age groups, with the involvement of cerebral white matter as a common denominator. Under the term leukodystrophies are those genetic disorders induced by primary damage of glial cells or myelin of the central nervous system, with or without involvement of the peripheral nervous system. On the other hand, genetic leukoencephalopathies include hereditary disorders of the white matter that cannot be included in the term leukodystrophies, such as those entities with primary involvement of the gray matter or systemic pathologies with secondary neurological involvement (example: mitochondrial diseases with hepatic, muscular, or cardiac involvement and additional commitment of central nervous white matter) (Vanderver et al., 2015). Between 1990 and 2010, many of the genes linked to these pathologies were identified. In the last few years, thanks to the incorporation of massive sequencing techniques, a noticeable expansion in the knowledge of the genetic causes of white matter diseases was made possible, thus allowing the identification of more than 100 leukodystrophies and genetic leukoencephalopathies.

Despite these significant advances, the molecular diagnostic yield is still less than 50% (Kevelam et al., 2016). The nonspecificity of the symptoms, the clinical superposition between one entity and another, the heterogeneous presentation, the limited existence of biochemical markers, and the little guidance in the findings in neuroimaging several of the disorders added to the high number of genes involved generate marked difficulties when making a diagnostic approach, even for medical specialists in the field. The need arises to find an ideally simple, economic, and reproducible procedure that allows us to reach the etiological diagnosis in a greater number of cases, which would have profound implications in the clinical follow-up, family counseling, and eventual therapeutic behavior, depending on the case. On the other hand, although there have been isolated reports of individual patients with leukodystrophies and genetic leukoencephalopathies, and small series of cases in the country and region (Amorosi et al., 2012; Arroyo, Grippo, Taratuto, Duffau, & Chamoles, 1991; de Kremer et al., 2002; Guarnaschelli & Sotelo, 2017; Rodríguez-Quiroga et al., 2015), there has not been a systematic analysis evaluating the diagnostic performance of a comprehensive molecular diagnosis approach in the field of these disorders in Argentina.

Therefore, the objectives of our work were (1) to characterize and stratify a population of adult and pediatric patients with leukodystrophies and genetic leukoencephalopathies, and (2) to develop and evaluate the diagnostic performance of a clinical-molecular procedure, which used classical and next-generation sequencing (NGS) techniques in a prospective cohort of patients with neurogenetic disorders.

2 MATERIALS AND METHODS

2.1 Patients: Diagnostic procedure

A prospective, observational, and descriptive cohort study was conducted, including adult and pediatric patients enrolled at the Neurogenetic Clinic of the J.M. Ramos Mejía Hospital in Buenos Aires, from May 2008 to December 2016, with a clinical diagnosis of leukodystrophy or genetic leukoencephalopathy, based on the definition of both concepts by Vanderver et al. (2015). All the patients, prior to their participation in the study, provided informed consent through a form approved by the Ethics Committee of the institution, and in the case of minors, the consent was given by their parents. Demographic, family history, clinical, and paraclinical data were obtained.

For the selection of the molecular tests to be performed in each patient, the clinical characteristics, biochemical markers, and neuroimaging findings of each individual case were considered. In those who presented with a phenotype characteristic of a pathology linked to a single candidate gene and capable of being analyzed by the Sanger method, such as, but not limited to, X-linked adrenoleukodystrophy, metachromatic leukodystrophy, cerebral autosomal-dominant arteriopathy with subcortical infarcts (CADASIL), and hereditary diffuse leukoencephalopathy with spheroids, we proceeded to the sequencing of the etiopathogenic gene using the technique described for ABCD1, ARSA, NOTCH3, and CSF1R, respectively. In cases with nonspecific phenotype and/or complex, genetic heterogeneity, or candidate gene not amenable to analysis by the Sanger method, a multigenic panel was used with the inclusion of more than 100 genes linked to leukodystrophies and genetic leukoencephalopathies, with NGS techniques. Unfortunately, funding did not permit multigene panel testing in those subjects (n = 9) for whom the Sanger method did not confirm an etiology (Figure 1; Supplementary Table 1).

3 RESULTS

3.1 Cohort

Of the 882 patients evaluated in the Neurogenetic Clinic of our hospital from May 2008 to December 2016, we included 46 subjects with diagnostic presumptions of leukodystrophy or genetic leukoencephalopathy.

The implementation of the diagnostic procedure presented in Figure 1, based on the analysis of clinical data, family history, and paraclinical studies, allowed us to systematize the study of our population, dividing them into two large groups:

1. a group of patients with a phenotype characteristic of a pathology linked to a single candidate gene, and capable of being analyzed by the Sanger method, in which individual sequencing was carried out using the technique described; such is the case of X-linked adrenoleukodystrophy, metachromatic leukodystrophy, CADASIL, vanishing white matter disease, Alexander's disease, and diffuse hereditary leukoencephalopathy with axonal spheroids and pigmented cells, and

2. a group of patients with nonspecific phenotype and/or complex, genetic heterogeneity, or candidate genes not amenable to analysis by the Sanger method, who were evaluated through extended gene sequencing with next-generation techniques.

The methodology used and the cohort studied by strata are represented in Figure 2 with the diagnostic yield obtained in each case. The demographic and clinical characteristics of the included patients are shown in Figure 3. The details of the clinical presentation, radiology, family history, diagnostic presumption, and method of molecular study used in each patient, are presented in Table 1.

Table 1. Patients with clinical diagnosis of leukodystrophy or genetic leukoencephalopathy: Demographic, clinical presentation, brain MRI, family history/inherited pattern, suspected clinical diagnosis based on phenotype, and gene sequencing method used to achieve a molecular diagnostic

Patient number	Gender	Age of onset (years)	Clinical	Brain MRI	Family history inherited pattern	Clinical diagnosis	Gene sequencing method
1	M	10	Epilepsy: spastic quadriparesis, language disorder, evolutive, Addison syndrome, increased VLCFA	White matter images: hyperintense in T2 and hypointense in T1, bilateral parietal and occipital, Gd (+)	Six cousins affected X- linked	X-linked adrenoleukodystrophy	Sanger method: ABCD1 gene
2	M	9	Epilepsy, spastic quadriparesis, language and cognitive disorder, evolutive, Addison syndrome increased VLCFA	White matter images, hyperintense in T2 and hypointense in T1, bilateral parietal and occipital	Grandfather and brother affected X-linked	X-linked adrenoleukodystrophy	Sanger method: ABCD1 gene
3	M	7	Epilepsy, spastic quadriparesis, language and cognitive disorder, evolutive. Addison syndrome, increased VLCFA	White matter images, hyperintense in T2 and hypointense in T1, bilateral parietal and occipital, Gd (+)	Sister carrier, X-linked	X-linked adrenoleukodystrophy	Sanger method: ABCD1 gene
4	M	6	Epilepsy, spastic quadriparesis, language and cognitive disorder, evolutive, Addison syndrome, increased VLCFA	White matter images, hyperintense in T2 and hypointense in T1, bilateral frontal, parietal and occipital, Gd (+)	Uncle affected, sister carrier, X-linked	X-linked adrenoleukodystrophy	Sanger method: ABCD1 gene
5	M	18	Learning disorders, pyramidal syndrome, periferic neuropathy, evolutive, increased VLCFA	White matter images, hyperintense in T2 and hypointense in T1, bilateral parietal and occipital with involvement of brainstem and cerebellum	Brother with increased VLCFA	X-linked adrenoleukodystrophy	Sanger method: ABCD1 gene
6	M	20	Peripheral neuropathy, pyramidal syndrome, language and cognitive disorder, evolutive, Addison syndrome, increased VLCFA	Right periventricular white matter image hyperintense in T2, tractography: decrease in the fibers of the pyramidal path	Sister carrier. X-linked	X-linked adrenoleukodystrophy	Sanger method: ABCD1 gene
7	M	3	Epilepsy, without symptoms for 13 years, then ataxia, spastic quadriparesis, language disorder, evolutive, increased VLCFA	White matter images, hyperintense in T2 and hypointense in T1, bilateral parietal and occipital	Mother with peripheral neuropathy, X-linked	X-linked adrenoleukodystrophy	Sanger method: ABCD1 gene
8	F	47	TIA, migraine with aura, mood disorders	Bihemispheric white matter images hyperintense in T2, with temporal pole involvement	Autosomal dominant	CADASIL	Sanger method: NOTCH3 gene
9	M	34	TIA, migraine without aura	Bihemispheric white matter images hyperintense in T2, with temporal pole involvement	Sporadic	CADASIL	Sanger method: NOTCH3 gene
10	F	32	Epilepsy, migraine with aura	Bihemispheric white matter images hyperintense in T2, with temporal pole involvement	Autosomal dominant	CADASIL	Sanger method: NOTCH3 gene
11	M	27	TIA	Bihemispheric white matter images hyperintense in T2, with temporal pole involvement	Autosomal dominant	CADASIL	Sanger method: NOTCH3 gene
12	F	30	Stroke, migraine without aura, epilepsy, cognitive disorder	Bihemispheric white matter images hyperintense in T2, with temporal and frontal pole involvement	Autosomal dominant	CADASIL	Sanger method: NOTCH3 gene
13	F	21	Stroke	Bihemispheric white matter images hyperintense in T2, with supra and infratentorial involvement.	Sporadic	CADASIL	Sanger method: NOTCH3 gene
14	F	40	Migraine, cognitive disorder	Bihemispheric white matter images hyperintense in T2, with temporal pole involvement	Sporadic	CADASIL	Sanger method: NOTCH3 gene
15	M	44	Migraine, arterial hypertension	Bihemispheric white matter images hyperintense in T2, with temporal pole and external capsule involvement	Autosomal dominant	CADASIL	Sanger method: NOTCH3 gene
16	M	46	Migraine, cognitive disorder	Bihemispheric white matter images hyperintense in T2, with temporal pole and external capsule involvement	Autosomal dominant	CADASIL	Sanger method: NOTCH3 gene
17	M	43	Stroke, migraine with aura, arterial hypertension	Bihemispheric white matter images hyperintense in T2, with temporal pole and external capsule involvement	Sporadic	CADASIL	Sanger method: NOTCH3 gene
18	M	3	Epilepsy, intellectual disability, low weight and short stature, macrocephaly	Bilateral periventricular white matter images, hyperintense in T2, with infratentorial and spinal cord involvement	Sporadic	Unspecific phenotype	Panel gene sequencing with NGS technique
19	M	1	Epilepsy, intellectual disability, macrocephaly	Bihemispheric white matter images, hyperintense in T2, with frontal lobe involvement	Sporadic	Alexander disease	Sanger method: GFAP gene
20	F	0	Epilepsy, intellectual disability, macrocephaly, spastic quadriparesis, ataxia	Bihemispheric white matter images, hyperintense in T2, with frontal lobe involvement	Sporadic	Alexander disease	Sanger method: GFAP gene
21	F	1	Epilepsy, intellectual disability, microcephaly, spastic quadriparesis, ataxia, strabismus and nystagmus, hypoacusis, low weight and short stature, dysmorphism, hypogonadism, hypoparathyroidism	Diffuse cortical atrophy, hyperintensity of lenticular nucleus in T1, bilateral periventricular images hyperintense in T2 hypointense in T1, corpus callosum atrophy	Sporadic	Unspecific phenotype	Panel gene sequencing with NGS technique
22	M	6	Ataxia, cognitive and conductal disorder, visual involvement and nystagmus, hypoacusis	Bilateral periventricular images hyperintense in T2 hypointense in T1, corpus callosum and brainstem atrophy	Sporadic	Unspecific phenotype	Panel gene sequencing with NGS technique
23	M	1	Epilepsy, intellectual disability, ataxia, spastic quadriparesis, hypodontia, dysmorphia	Bihemispheric white matter images hyperintense in T2, hypointense in T1	Sporadic	4 “H” leukodystrophy	Plan: panel gene sequencing with NGS technique (genetic heterogeneity) (this patient could not be studied)
24	M	10	Epilepsy, intellectual disability, ataxia, spastic quadriparesis evolutive, decreased in arylsulfatase A activity and increased in urine sulfatides	Diffuse cortical atrophy, bilateral periventricular images hyperintense in T2 hypointense in T1, corpus callosum atrophy	Autosomal recessive	Metachromatic leukodystrophy	Sanger method: ARSA gene
25	M	1	Intellectual disability, spastic quadriparesis evolutive, decreased in arylsulfatase A activity and increased in urine sulfatides	Bilateral periventricular images hyperintense in T2 hypointense in T1	Sporadic	Metachromatic leukodystrophy	Sanger method: ARSA gene
26	M	1	Epilepsy, intellectual disability, ataxia, spastic quadriparesis evolutive, decreased in arylsulfatase A activity and increased in urine sulfatides	Bilateral periventricular images hyperintense in T2 hypointense in T1 with infratentorial and corpus callosum involvement	Sporadic	Metachromatic leukodystrophy	Sanger method: ARSA gene
27	M	27	Evolutive, spastic quadriparesis, saccadic eyes movement disorder, cognitive impairment	Asymmetric bihemispheric white matter images, hyperintense in T2, with frontal lobe involvement	Autosomal dominant	Hereditary diffuse leukoencephalopathy with spheroids and pigmented cells	Sanger method: CSF1R gene
28	F	71	Ataxia, cognitive disorder, evolutive	Asymmetric bihemispheric white matter images, hyperintense in T2, with frontal lobe involvement	Sporadic	Hereditary diffuse leukoencephalopathy with spheroids and pigmented cells	Sanger method: CSF1R gene
29	F	1	Intellectual disability, epilepsy, macrocephaly, Ataxia	Bihemispheric hypomyelination with subcortical temporal cysts	Unknown	Megalencephalic leukodystrophy with subcortical cysts	Panel gene sequencing with NGS technique (this gene is not candidate to individual sequencing)
30	F	15	Epilepsy, left hemiparesis, pyramidal syndrome, ataxia, cognitive disorder, evolutive	Asymmetric bihemispheric white matter images, hyperintense in T2	Sporadic	Unspecific phenotype	Panel gene sequencing with NGS technique
31	F	3	Ataxia, status epilepticus and posterior progressive encephalopathy, spastic quadriparesis, microcephaly, Primary amenorrhea and absence of secondary sexual characteristics	Bihemispheric white matter images, hyperintense in T2 with tendency to cysts formations, diffuse atrophy	Sporadic	Unspecific phenotype	Panel gene sequencing with NGS technique
32	F	5	Brain injury then ataxia and posterior recovery, acute left hemiparesis, migraine, epilepsy, premature ovarian failure	Asymmetric bihemispheric white matter images, hyperintense in T2 with tendency to cysts formations, diffuse atrophy	Sporadic	Vanishing white matter disease	Sanger method: EIF2B5 gene, exon 7
33	M	2	Brain injury then ataxia and posterior recovery, other episode of brain injury then left hemiparesis and posterior partial recovery, pyramidal syndrome	Bihemispheric white matter images, hyperintense in T2 with tendency to cysts formations, diffuse atrophy	Sporadic	Vanishing white matter disease	Panel gene sequencing with NGS technique (genetic heterogeneity)
34	M	4	Evolutive cognitive disorder	Bihemispheric white matter images, hyperintense in T2 with tendency to cysts formations	Sporadic	Vanishing white matter disease	Plan: panel gene sequencing with NGS technique (genetic heterogeneity) (this patient could not be studied)
35	M	1	Intellectual disability, spastic quadriparesis, visual disorders	Bihemispheric white matter images, hyperintense in T2 with tendency to cyst formation, diffuse atrophy	Sporadic	Vanishing white matter disease	Plan: panel gene sequencing with NGS technique (genetic heterogeneity) (this patient could not be studied)
36	M	3	Epilepsy, spastic quadriparesis, ataxia, cognitive disorder, hypoacusis, evolutive	Bihemispheric white matter images, hyperintense in T2 with corpus callosum and infratentorial involvement	Sporadic	Unspecific phenotype	Plan: panel gene sequencing with NGS technique (this patient could not be studied)
37	M	33	Visual disorders, transient focal neurological disorders, migraine, hypoacusis	Bihemispheric white matter images, hyperintense in T2 with tendency to cysts formations	Sporadic	Unspecific phenotype	Plan: panel gene sequencing with NGS technique (this patient could not be studied)
38	M	0	Epilepsy, intellectual disability, spastic quadriparesis, ataxia, cognitive disorder	Bihemispheric white matter images, hyperintense in T2 with supra and infratentorial involvement	Sporadic	Unspecific phenotype	Plan: panel gene sequencing with NGS technique (this patient could not be studied)
39	F	4	Epilepsy, spastic quadriparesis, language disorder, evolutive, systemic lupus erythematosus	Bihemispheric white matter images, hyperintense in T2 with basal ganglia calcification	Sporadic	Aicardi-Goutières	Plan: panel gene sequencing with NGS technique (this patient could not be studied)
40	M	40	Episodic vertigo, hypothyroidism	Bihemispheric white matter images, hyperintense in T2 with supra and infratentorial involvement	Sporadic	Unspecific phenotype	Panel gene sequencing with NGS technique
41	M	1	Epilepsy, intellectual disability, spastic quadriparesis, cognitive disorder, microcephaly	Bilateral periventricular white matter images, hyperintense in T2	Autosomal recessive	Unspecific phenotype	Panel gene sequencing with NGS technique
42	F	28	Epilepsy, ataxia, cognitive disorder, evolutive, premature ovarian failure, hypothyroidism	Bihemispheric white matter images, hyperintense in T2 with supra and infratentorial involvement	Sporadic	Unspecific phenotype	Plan: panel gene sequencing with NGS technique (this patient could not be studied)
43	M	1	Epilepsy, intellectual disability, spastic quadriparesis, cognitive and visual disorders	Bihemispheric white matter images, hyperintense in T2 with tendency to cysts formations	Sporadic	Unspecific phenotype	Panel gene sequencing with NGS technique
44	M	10	Migraine, cognitive disorder, pyramidal syndrome	Bihemispheric white matter images, hyperintense in T2	Sporadic	Unspecific phenotype	Panel gene sequencing with NGS technique
45	M	2	Progressive spastic paraparesis, hollow foot	Supratentorial bihemispheric white matter images, hyperintense in T2	Sporadic	Unspecific phenotype	Panel gene sequencing with NGS technique
46	F	1	Intellectual disability, evolutive, ataxia, spastic quadriparesis	Bihemispheric white matter images, hyperintense in T2 with corpus callosum involvement	Autosomal recessive	Unspecific phenotype	Panel gene sequencing with NGS technique

• Abbreviations: MRI, Magnetic resonance imaging; M, male; F, female; VLCFA, very long-chain fatty acids; TIA, transient ischemic attack; CADASIL, cerebral autosomal-dominant arteriopathy with subcortical infarcts; Gd (+), gadolinium positive.

3.3 Illustrative cases

3.3.1 Case No. 1 (Patient No. 1)

A male patient, at 10 years old, presented with convulsions, progressive neurologic deterioration manifesting as pyramidal motor compromise, changes in skin color, visual symptoms, and a family history suggestive of X-linked inheritance (Figure 4a). Neurological examination revealed severe phonological deglutition disorder, emission of incomprehensible sounds, bilateral visual acuity affectation with homonymous hemianopsia and convergent strabismus, spastic quadriparesis with pyramidal signs, and cervical dystonic posture. MRI of the brain showed confluent images in posterior periventricular white matter, hyperintense in T2 and spin echo sequences enhanced in T2, hypointense in T1, with peripheral enhancement after contrast administration (Figure 4b). Laboratory findings included increased adrenocorticotrophin and decreased cortisol compatible with adrenal insufficiency, and a neurometabolic study indicated an increase in very long-chain fatty acids. With presumptive diagnosis of X-linked adrenoleukodystrophy, the ABCD1 gene was sequenced, and c.320T > C variant (p.Leu107Pro) was detected in exon 1, classifiable as pathogenic and previously reported in the literature (Figure 4c; Korenke et al., 1997; Krasemann, Meier, Korenke, Hunneman, & Hanefeld, 1996).

3.3.2 Case No. 2 (Patient No. 30)

A 32-year-old female patient had presented at 15 years of age with progressive neurological symptoms, characterized by focal seizures, ataxia, motor deficit in the left side of the body, and cognitive impairment, with no family history (Figure 5a). A metabolic laboratory without relevant findings, and brain MRI with confluent lesions in bilateral periventricular and subcortical white matter, hyperintense in T2 and hypointense in T1, with tractography showed an asymmetric bilateral pyramidal pathway compromise (Figure 5b). Given that the phenotype did not approach a defined clinical-radiological syndrome, a multigenic panel sequencing was resorted to using next-generation techniques. A pathogenic variant in homozygosis was identified in exon 3 of the EIF2B3 gene (*606273): NM_001261418: c.260C > T (p.A87V), which has been previously reported in the literature as pathogenic (van der Knaap et al., 2002), concluding in the diagnosis of vanishing white matter disease.

4 DISCUSSION

In the present work, a pediatric and adult population of patients with genetic leukoencephalopathies and leukodystrophies was characterized, obtaining a genetic diagnosis in a significant number of individuals. This has been possible because of the development of a diagnostic procedure by means of which the molecular study path is addressed according to the presumption that arises from the phenotypic evaluation.

Although these entities are characterized by a marked clinical heterogeneity and involve a high number of candidate genes (Lynch et al., 2017), the identification of the causal defect was achieved in 22 of the 38 patients studied. Our results confirm the usefulness of the proposed diagnostic procedure, expressed by the significant diagnostic yield (57.9%). Noteworthy, the yield for Sanger sequencing in the CADASIL phenotype was 40%, whereas the yield for the remaining phenotypes was higher at 80%.

In patients with an uncharacteristic and/or complex phenotype, the use of a directed capture of genes through the NGS technology reaffirmed the performance of these techniques for the diagnostic approach in these pathologies (Wang et al., 2016). This constituted a quick and effective instrument, allowing us to arrive at a definite etiology even when the clinical manifestation is not eloquent (Gordon, Letsou, & Bonkowsky, 2014), reducing costs by shortening the long process of complementary methods of study that patients are often subjected to (Córdoba, González Morón, Rodríguez-Quiroga, & Kauffman, 2014).

This work covers the largest series of patients affected by genetic diseases of the cerebral white matter reported in Argentina, with the inclusion of clinical and molecular data. In relation to similar publications in other countries, we found similarities in clinical manifestations and in the genes studied, with individual differences in the frequency of each of the pathologies, probably because of the different epidemiological characteristics of the populations evaluated (Lynch et al., 2017; Stellitano, Winstone, van der Knaap, & Verity, 2016). Likewise, there was a greater proportion of adult patients because of the high number of people belonging to this age group being evaluated at our center, which is reflected in the diagnostic approach to pathologies characteristic of this group, such as CADASIL, and diffuse hereditary leukoencephalopathy with axonal spheroids and pigmented cells.

In other cohorts that studied populations of similar ages, the suspicion based on the clinical-radiological characterization was similar, with higher frequency of adult-onset vanishing white matter disease, and other pathologies such as leukodystrophy with ovarian failure related to the mutation in the AARS2 gene, which we did not have in our series (Lynch et al., 2017). The sequence of the NOTCH3 gene confirmed the diagnosis in only four of the patients studied with suspected CADASIL, unlike other reports where the diagnostic yield was higher (Chen et al., 2017). In our particular case, we recognize the laxity in the selection criteria, as we studied those patients who met at least one clinical and one radiological criterion described by Davous (1998) without being strict in the sum of 15 or more points on the diagnostic probability scale of CADASIL proposed by Pescini, Nannucci, & Pantoni (2013), there is a better performance in the molecular study of the NOTCH3 gene.

On the other hand, with respect to the patients studied with suspicion of X-linked adrenoleukodystrophy, in a single patient no pathogenic variants were detected in the ABCD1 gene, presuming that they may be found in noncoding regions of the gene or because of the presence of a deletion that escapes detection by the Sanger method. A similar case occurs in the patient with suspected metachromatic leukodystrophy in whom a confirmatory molecular diagnosis could not be reached because a single heterozygous pathogenic variant was detected. Therefore, it is believed that there may be another variant in noncoding regions or structural anomalies not identifiable by this method, requiring additional techniques such as MLPA assays not performed by our group. Finally, the case with suspicion of diffuse hereditary leukoencephalopathy with axonal spheroids and pigmented cells that we could not confirm, corresponded to a patient with a nonconclusive clinical phenotype, with difficulty in the subsequent follow-up.

The illustrated cases are paradigmatic. The first patient with phenotypic characteristics allowed suspicion of a particular entity, leading to the sequencing of an individual gene by classical techniques, with a high probability of achieving a definitive etiological diagnosis through a single molecular study. The second case represents a sufficiently complex phenotype to require a broader approach, feasibly to be analyzed through a next-generation technique as a directed panel of candidate genes, which in a single step allowed obtaining a molecular diagnosis, confirming a defined pathological entity, reducing time and costs (Richards, Korgenski, Taft, Vanderver, & Bonkowsky, 2015). The configuration of a diagnostic procedure allows the treating physician to have a better approach to this complex group of pathologies, optimizing the diagnostic resources.

The access to an etiological diagnosis derives in medical actions that originate from it, through the early detection or monitoring of the different comorbidities (Parikh et al., 2015). The possibility of providing adequate genetic counseling through the guidance on risk of recurrence is equally important, and, most of all, the obtaining of a greater proportion of molecular diagnoses expands the knowledge of the clinical spectrum of each genetic leukoencephalopathy, configuring a registry of the individualized causal defect for future therapeutic opportunities in current development. Moreover, comprehensive diagnostic approaches, such as the one used by our group, reduces the time that these patients must wait before getting a diagnosis thereby ending odysseys of many years, impacting in disorders such as ALD-X on their medical management, and optimizing the genetic counseling for these families. However, we must keep working on new methods that allow a rapid diagnosis of the genetic conditions of the cerebral white matter to reduce the individual and family impact.

The vast amount of knowledge that has emerged in recent years about the genes linked to leukodystrophies and genetic leukoencephalopathies will provide genomics an increasingly essential role as a tool for diagnostic use, which will lead to the growing need to develop therapeutic strategies to avoid the significant morbidity and mortality of these pathologies.

Despite the notorious advances achieved with the increasing use of massive sequencing techniques, a high percentage of affected individuals remain without genetic diagnosis. It is necessary to have larger groups of patients through collaborative work between the different groups of studies, to have a deeper and more extensive knowledge of these conditions. Research in the molecular diagnosis of genetic leukoencephalopathies and leukodystrophies continues to be a great challenge in this new era where knowledge and promising therapeutic advances evolve daily.

CONFLICT OF INTEREST

The authors declare that they have no conflicts of interest.