A Proposed Clinical Classification and a Diagnostic Approach for Congenital Ataxias

Compound heterozygous TMEM67 biallelic variants including a novel frameshift mutation in two Filipino adolescent siblings with Joubert syndrome

Joubert Syndrome (JS) is a congenital cerebellar ataxia typically inherited in an autosomal recessive pattern, although rare X-linked inheritance can occur. It is characterized by hypotonia evolving into ataxia, global developmental delay, oculomotor apraxia, breathing dysregulation, and multiorgan involvement. To date, there are 40 causative genes implicated in JS, all of which encode proteins of the primary cilium. Primary cilia play a crucial role in the normal development and function of many organs, including parts of the brain (cerebellum and brainstem), kidneys, and the retina. This likely explains the multiorgan involvement seen in JS. In this report, we present the first genetically confirmed case of JS in two Filipino adolescent siblings who had early onset ataxia, hepatomegaly, and global developmental delay. A cranial CT scan revealed the Molar Tooth Sign (MTS). Whole Exome Sequencing (WES), performed via buccal swab, showed biallelic pathogenic variants at NM_153704.6:c.2086 C > T (NP_714915.3:p.Leu696Phe) and NM_153704.6:c.431del (NP_714915.3:p.Leu144CysfsTer19) in TMEM67, which are associated with Joubert Syndrome 6 (OMIM:610688) in a compound heterozygous state. The prevalence of NM_153704.6:c.2086 C > T (NP_714915.3:p.Leu696Phe) in TMEM67 variant is very rare (< 0.001%), and the NM_153704.6:c.431del (NP_714915.3:p.Leu144CysfsTer19) has not been recorded. This case contributes valuable information to the expanding knowledge of JS and its related disorders.

Three Cases of Spinocerebellar Ataxia Type 2 (SCA2) and Pediatric Literature Review: Do Not Forget Trinucleotide Repeat Disorders in Childhood-Onset Progressive Ataxia

Background: Childhood-onset progressive ataxias are rare neurodegenerative disorders characterized by cerebellar signs, sometimes associated with other neurological or extra-neurological features. The autosomal dominant forms, known as spinocerebellar ataxias (SCAs), linked to trinucleotide (i.e., CAG) repeat disorders, are ultra-rare in children. We describe three patients from two unrelated families affected by spinocerebellar ataxia type 2 (SCA2) and present a literature review of pediatric cases. Methods: The patients' clinical and genetic data were collected retrospectively. Results: The first case was a 9.5-year-old boy, affected by ataxia with oculomotor apraxia and cerebellar atrophy, subcortical myoclonus, and peripheral axonal sensitive polyneuropathy caused by a pathologic expansion in ATXN2, inherited from his asymptomatic father. Two brothers with familial SCA2 presented neurodegeneration leading to early death in one case and progressive ataxia, parkinsonism, and epilepsy with preserved ambulation at age 18 years in the second. To date, 19 pediatric patients affected by SCA2 have been reported, 3 of whom had a phenotype consistent with progressive ataxia with shorter CAG repeats, while 16 had more severe early-onset encephalopathy, with longer alleles. Conclusions: Although they are ultra-rare, trinucleotide repeat disorders must be considered in differential diagnosis of hereditary progressive ataxias in children, especially considering that they require targeted genetic testing and can manifest even before a parental carrier becomes symptomatic. Thus, they must also be taken into account with negative family history and when Next-Generation Sequencing (NGS) results are inconclusive. Notably, the association between cerebellar ataxia and other movement disorders should raise suspicion of SCA2 among differential diagnoses.

Clinical and Genetic Characterization of a Cohort of Brazilian Patients With Congenital Ataxia

Background and Objectives

Congenital ataxias are rare hereditary disorders characterized by hypotonia and developmental motor delay in the first few months of life, followed by cerebellar ataxia in early childhood. The course of the disease is predominantly nonprogressive, and many patients are incorrectly diagnosed with cerebral palsy. Despite significant advancements in next-generation sequencing in the past few decades, a specific genetic diagnosis is seldom obtained in cases of congenital ataxia. The aim of the study was to analyze the clinical, radiologic, and genetic features of a cohort of Brazilian patients with congenital ataxia.

Methods

Thirty patients with a clinical diagnosis of congenital ataxia were enrolled in this study. Clinical and demographic features and neuroimaging studies were analyzed. Genetic testing (whole-exome sequencing) was also performed.

Results

A heterogeneous pattern of genetic variants was detected. Eighteen genes were involved: ALDH5A1, BRF1, CACNA1A CACNA1G, CC2D2A, CWF19L1, EXOSC3, ITPR1, KIF1A, MME, PEX10, SCN2A, SNX14, SPTBN2, STXBP1, TMEM240, THG1L, and TUBB4A. Pathogenic/likely pathogenic variants involving 11 genes (ALDH5A1, CACNA1A, EXOSC3, MME, ITPR1, KIF1A, STXBP1, SNX14, SPTBN2, TMEM240, and TUBB4A) were identified in 46.7% of patients. Variants of uncertain significance involving 8 genes were detected in 33.3% of patients. Congenital ataxias were characterized by a broad phenotype. A genetic diagnosis was more often obtained in patients with cerebellar-plus syndrome than in patients with a pure cerebellar syndrome.

Discussion

This study re-emphasizes the genetic heterogeneity of congenital ataxias and the absence of a clear phenotype-genotype relationship. A specific genetic diagnosis was established in 46.7% of patients. Autosomal dominant, associated with sporadic cases, was recognized as an important genetic inheritance. The results of this analysis highlight the value of whole-exome sequencing as an efficient screening tool in patients with congenital ataxia.

Inherited Ataxias in Children

The purpose of this review is to describe the current diagnostic approach to inherited ataxias during childhood. With the expanding use and availability of gene testing technologies including large sequencing panels, the ability to arrive at a precise genetic diagnosis in this group of disorders has been improving. We have reviewed all the gene sequencing studies of ataxias available by a comprehensive literature search and summarize their results. We provide a logical algorithm for a diagnostic approach in the context of this evolving information. We stress the fact that both autosomal recessive and autosomal dominant mutations can occur in children with ataxias and the need for keeping in mind nucleotide repeat expansions, which cannot be detected by sequencing technologies, as a possible cause of progressive ataxias in children. We discuss the traditional phenotype-based diagnostic approach in the context of gene testing technologies. Finally, we summarize those disorders in which a specific therapy may be indicated.

Liver Involvement in Congenital Disorders of Glycosylation: A Systematic Review

An ever-increasing number of disturbances in glycosylation have been described to underlie certain unexplained liver diseases presenting either almost isolated or in a multi-organ context. We aimed to update previous literature screenings which had identified up to 23 forms of congenital disorders of glycosylation (CDG) with associated liver disease. We conducted a comprehensive literature search of three scientific electronic databases looking at articles published during the last 20 years (January 2000-October 2020). Eligible studies were case reports/series reporting liver involvement in CDG patients. Our systematic review led us to point out 41 forms of CDG where the liver is primarily affected (n = 7) or variably involved in a multisystem disease with mandatory neurological abnormalities (n = 34). Herein we summarize individual clinical and laboratory presentation characteristics of these 41 CDG and outline their main presentation and diagnostic cornerstones with the aid of two synoptic tables. Dietary supplementation strategies have hitherto been investigated only in seven of these CDG types with liver disease, with a wide range of results. In conclusion, the systematic review recognized a liver involvement in a somewhat larger number of CDG variants corresponding to about 30% of the total of CDG so far reported, and it is likely that the number may increase further. This information could assist in an earlier correct diagnosis and a possibly proper management of these disorders.