Brain white matter abnormalities associated with copy number variants

Unclassified white matter disorders: A diagnostic journey requiring close collaboration between clinical and laboratory services

BACKGROUND

Next generation sequencing studies have revealed an ever-increasing number of causes for genetic disorders of central nervous system white matter. A substantial number of disorders are identifiable from their specific pattern of biochemical and/or imaging findings for which single gene testing may be indicated. Beyond this group, the causes of genetic white matter disorders are unclear and a broader approach to genomic testing is recommended.

AIM

This study aimed to identify the genetic causes for a group of individuals with unclassified white matter disorders with suspected genetic aetiology and highlight the investigations required when the initial testing is non-diagnostic.

METHODS

Twenty-six individuals from 22 families with unclassified white matter disorders underwent deep phenotyping and genome sequencing performed on trio, or larger, family groups. Functional studies and transcriptomics were used to resolve variants of uncertain significance with potential clinical relevance.

RESULTS

Causative or candidate variants were identified in 15/22 (68.2%) families. Six of the 15 implicated genes had been previously associated with white matter disease (COL4A1, NDUFV1, SLC17A5, TUBB4A, BOLA3, DARS2). Patients with variants in the latter two presented with an atypical phenotype. The other nine genes had not been specifically associated with white matter disease at the time of diagnosis and included genes associated with monogenic syndromes, developmental disorders, and developmental and epileptic encephalopathies (STAG2, LSS, FIG4, GLS, PMPCA, SPTBN1, AGO2, SCN2A, SCN8A). Consequently, only 46% of the diagnoses would have been made via a current leukodystrophy gene panel test.

DISCUSSION

These results confirm the importance of broad genomic testing for patients with white matter disorders. The high diagnostic yield reflects the integration of deep phenotyping, whole genome sequencing, trio analysis, functional studies, and transcriptomic analyses.

CONCLUSIONS

Genetic white matter disorders are genetically and phenotypically heterogeneous. Deep phenotyping together with a range of genomic technologies underpin the identification of causes of unclassified white matter disease. A molecular diagnosis is essential for prognostication, appropriate management, and accurate reproductive counseling.

Epidemiological, clinical, and genetic characteristics of paediatric genetic white matter disorders in Northern Finland

AIM

To examine the epidemiological, clinical, and genetic characteristics of paediatric patients with genetic white matter disorders (GWMDs) in Northern Finland.

METHOD

A longitudinal population-based cohort study was conducted in the tertiary catchment area of Oulu University Hospital from 1990 to 2019. Patients were identified retrospectively by International Statistical Classification of Diseases and Related Health Problems codes in hospital records and prospectively by attending physicians. Inclusion criteria were children younger than 18 years with defined GWMDs or genetic disorders associated with white matter abnormalities (WMAs) on brain magnetic resonance imaging.

RESULTS

Eighty patients (mean age [SD] at the end of the study 11y [8y 6mo], range 0-35y; 45 males, 35 females) were diagnosed with a defined GWMD. The cumulative childhood incidence was 30 per 100 000 live births. Regarding those patients with 49 distinct GWMDs, 20% had classic leukodystrophies and 80% had genetic leukoencephalopathies. The most common leukodystrophies were cerebral adrenoleukodystrophy, Krabbe disease, and Salla disease. Additionally, 29 patients (36%) had genetic aetiologies not previously associated with brain WMAs or they had recently characterised GWMDs, including SAMD9L- and NHLRC2-related neurological disorders. Aetiology was mitochondrial in 21% of patients. The most common clinical findings were motor developmental delay, intellectual disability, hypotonia, and spasticity.

INTERPRETATION

The cumulative childhood incidence of childhood-onset GWMDs was higher than previously described. Comprehensive epidemiological and natural history data are needed before future clinical trials are undertaken. What this paper adds Forty-nine distinct genetic white matter disorders (GWMDs) were identified, with 20% of cases being classic leukodystrophies. The cumulative childhood incidence of GWMDs was higher than described previously. A considerable proportion (36%) of GWMDs were previously undefined or recently characterised GWMDs. Mitochondrial aetiology was more common (21%) than previously reported.

Chromosomal Microarray Analysis Has a Poor Diagnostic Yield in Children with Developmental Delay/Intellectual Disability When Concurrent Cerebellar Anomalies Are Present

Chromosomal microarray analysis is commonly used as screening test for children with neurodevelopmental issues, also in case of complex neurological phenotypes. Developmental delay/intellectual disability is a common presentation sign in pediatric ataxias, diseases with high clinical and genetic heterogeneity. In order to determine the diagnostic yield of Array-CGH in such conditions, all the tests performed in the last 10-year activity of a single referral center in children who present, besides the neurodevelopmental impairment, cerebellar abnormalities have been systematically gathered. The study demonstrates that, except for Dandy-Walker malformation or poly-malformative phenotypes, chromosomal microarray analysis should be discouraged as first-line diagnostic test in pediatric ataxias with neurodevelopmental disability.

Obstetrics, genetics, and litigation

INTRODUCTION

Claims of medical negligence are universal. Unexpected adverse pregnancy outcome may trigger litigation. Such outcomes, especially with neurodevelopmental sequelae, may be compounded by a genetic disorder, congenital abnormality, or syndrome.

MATERIAL AND METHODS

This is a report of 297 cases in which a pregnancy complication, error, or incident occurred that was followed by progeny with a genetic disorder, congenital abnormality, or syndrome that spawned litigation. The author assessed, opined, and in many cases, testified about causation.

RESULTS

Pregnancies complicated by hypoxic ischemic encephalopathy were not infrequently compounded by offspring with a genetic disorder, congenital abnormality, or syndrome. Multiple cases were brought because of missed ultrasound or laboratory diagnoses, or failures in carrier detection. Teratogenic medication prescribed before or during pregnancy invited legal purview. Failure to refer (or confer) for genetic evaluation or counseling in the face of significant risk, occurred repeatedly. Ethical breaches and hubris promptly led to litigation.

CONCLUSIONS

Many lessons and recommendations emerge in this report. These include the realization that the vast majority of errors in this series involved at least two caregivers, serial ultrasound studies are important, decreased fetal movements may signal a genetic disorder, congenital abnormality, or syndrome, family history and ethnicity are vital, cognitive biases profoundly affect decision-making. Finally, the simplest of errors have the potential for causing life-long grief.

Phenotypic and Molecular Cytogenetic Analysis of a Case of Monosomy 1p36 Syndrome due to Unbalanced Translocation

Monosomy 1p36 syndrome is one of the most common submicroscopic deletion syndromes, which is characterized by the presence of delayed developmental milestones, intellectual disability, and clinically recognizable dysmorphic craniofacial features. The syndrome comprises 4 cytogenetic groups including pure terminal deletions, interstitial deletions, complex rearrangements, and derivative chromosomes 1 due to unbalanced translocations, where unbalanced translocations represent the least percentage of all cases of monosomy 1p36 (7%). Most patients with monosomy 1p36 due to an unbalanced translocation can be cytogenetically diagnosed using conventional techniques. However, chromosomal microarray analysis is mandatory in these cases to detect copy number variance and size of the deletion and allows for setting a phenotype-genotype correlation. Here, we studied a 1.5-year-old female patient who showed intellectual disability, delayed milestones, hypotonia, seizures, and characteristic dysmorphic features including brachycephaly, straight eyebrows, deep-set eyes, downslanting palpebral fissures, midface hypoplasia, depressed nasal bridge, long philtrum, and pointed chin. Conventional cytogenetic analysis (CCA), microarray study, and fluorescence in situ hybridization (FISH) analysis were performed. CCA showed a translocation involving chromosomes 1 and 21, 45,XX,der(1)t(1;21)(p36.32;q21.1)dn. Microarray analysis revealed copy number losses at both 1p36 and proximal 21q. FISH confirmed the presence of the 1p36 deletion, but was not performed for 21q. We have concluded that phenotype-genotype correlation for monosomy 1p36 syndrome can be performed for the fundamental clinical manifestations; however, the final aspect of the syndrome depends on composite factors. Monosomy 1p36 due to unbalanced translocation may present either classically or with additional altered features of various severity based on the copy number variations involving different chromosomes.