Megalencephalic leukoencephalopathy with subcortical cysts: a variant update and review of the literature.

The leukodystrophy megalencephalic leukoencephalopathy with subcortical cysts (MLC) is characterized by infantile-onset macrocephaly and chronic edema of the brain white matter. With delayed onset, patients typically experience motor problems, epilepsy and slow cognitive decline. No treatment is available. Classic MLC is caused by bi-allelic recessive pathogenic variants in MLC1 or GLIALCAM (also called HEPACAM). Heterozygous dominant pathogenic variants in GLIALCAM lead to remitting MLC, where patients show a similar phenotype in early life, followed by normalization of white matter edema and no clinical regression. Rare patients with heterozygous dominant variants in GPRC5B and classic MLC were recently described. In addition, two siblings with bi-allelic recessive variants in AQP4 and remitting MLC have been identified. The last systematic overview of variants linked to MLC dates back to 2006. We provide an updated overview of published and novel variants. We report on genetic variants from 508 patients with MLC as confirmed by MRI diagnosis (258 from our database and 250 extracted from 64 published reports). We describe 151 unique MLC1 variants, 29 GLIALCAM variants, 2 GPRC5B variants and 1 AQP4 variant observed in these MLC patients. We include experiments confirming pathogenicity for some variants, discuss particularly notable variants, and provide an overview of recent scientific and clinical insight in the pathophysiology of MLC.

Case report: XMEN disease: a patient with recurrent Hodgkin lymphoma and immune thrombocytopenia.

Here we present the case of a 28-year-old man with X-linked immunodeficiency with magnesium defect, Epstein-Barr virus (EBV) infection and neoplasia (XMEN) disease. He presented with immune thrombocytopenia within 1 year after successful autologous hematopoietic stem cell transplantation for recurrent EBV-associated classical Hodgkin lymphoma (CHL). The combination of EBV- associated malignancy, autoimmunity, recurrent airway infections at young age and bronchiectasis, prompted immunological investigation for an inborn error of immunity (IEI). Genetic testing revealed XMEN disease. XMEN disease is characterized by a glycosylation defect due to mutations in the MAGT1 gene. Germline mutations in the MAGT1 gene disrupt glycosylation of the NKG2D receptor in immune cells, including natural killer and CD8-positive T cells, vital for immune surveillance, especially against EBV. Consequently, individuals with XMEN disease, are prone to EBV-associated lymphoproliferative disorders in addition to auto-immunity. Early recognition of adult onset IEI-related B-lymphoproliferative disorders, including CHL is of vital importance for treatment decisions, including (allogeneic) haematopoietic stem cell transplantation and family screening.

Endocrine and Growth Abnormalities in 4H Leukodystrophy Caused by Variants in POLR3A, POLR3B, and POLR1C.

CONTEXT: 4H or POLR3-related leukodystrophy is an autosomal recessive disorder typically characterized by hypomyelination, hypodontia, and hypogonadotropic hypogonadism, caused by biallelic pathogenic variants in POLR3A, POLR3B, POLR1C, and POLR3K. The endocrine and growth abnormalities associated with this disorder have not been thoroughly investigated to date. OBJECTIVE: To systematically characterize endocrine abnormalities of patients with 4H leukodystrophy. DESIGN: An international cross-sectional study was performed on 150 patients with genetically confirmed 4H leukodystrophy between 2015 and 2016. Endocrine and growth abnormalities were evaluated, and neurological and other non-neurological features were reviewed. Potential genotype/phenotype associations were also investigated. SETTING: This was a multicenter retrospective study using information collected from 3 predominant centers. PATIENTS: A total of 150 patients with 4H leukodystrophy and pathogenic variants in POLR3A, POLR3B, or POLR1C were included. MAIN OUTCOME MEASURES: Variables used to evaluate endocrine and growth abnormalities included pubertal history, hormone levels (estradiol, testosterone, stimulated LH and FSH, stimulated GH, IGF-I, prolactin, ACTH, cortisol, TSH, and T4), and height and head circumference charts. RESULTS: The most common endocrine abnormalities were delayed puberty (57/74; 77% overall, 64% in males, 89% in females) and short stature (57/93; 61%), when evaluated according to physician assessment. Abnormal thyroid function was reported in 22% (13/59) of patients. CONCLUSIONS: Our results confirm pubertal abnormalities and short stature are the most common endocrine features seen in 4H leukodystrophy. However, we noted that endocrine abnormalities are typically underinvestigated in this patient population. A prospective study is required to formulate evidence-based recommendations for management of the endocrine manifestations of this disorder.

POLR3A variants with striatal involvement and extrapyramidal movement disorder.

Biallelic variants in POLR3A cause 4H leukodystrophy, characterized by hypomyelination in combination with cerebellar and pyramidal signs and variable non-neurological manifestations. Basal ganglia are spared in 4H leukodystrophy, and dystonia is not prominent. Three patients with variants in POLR3A, an atypical presentation with dystonia, and MR involvement of putamen and caudate nucleus (striatum) and red nucleus have previously been reported. Genetic, clinical findings and 18 MRI scans from nine patients with homozygous or compound heterozygous POLR3A variants and predominant striatal changes were retrospectively reviewed in order to characterize the striatal variant of POLR3A-associated disease. Prominent extrapyramidal involvement was the predominant clinical sign in all patients. The three youngest children were severely affected with muscle hypotonia, impaired head control, and choreic movements. Presentation of the six older patients was milder. Two brothers diagnosed with juvenile parkinsonism were homozygous for the c.1771-6C > G variant in POLR3A; the other seven either carried c.1771-6C > G (n = 1) or c.1771-7C > G (n = 7) together with another variant (missense, synonymous, or intronic). Striatal T2-hyperintensity and atrophy together with involvement of the superior cerebellar peduncles were characteristic. Additional MRI findings were involvement of dentate nuclei, hila, or peridentate white matter (3, 6, and 4/9), inferior cerebellar peduncles (6/9), red nuclei (2/9), and abnormal myelination of pyramidal and visual tracts (6/9) but no frank hypomyelination. Clinical and MRI findings in patients with a striatal variant of POLR3A-related disease are distinct from 4H leukodystrophy and associated with one of two intronic variants, c.1771-6C > G or c.1771-7C > G, in combination with another POLR3A variant.

Clinical spectrum of POLR3-related leukodystrophy caused by biallelic POLR1C pathogenic variants.

OBJECTIVE: To determine the clinical, radiologic, and molecular characteristics of RNA polymerase III-related leukodystrophy (POLR3-HLD) caused by biallelic POLR1C pathogenic variants. METHODS: A cross-sectional observational study involving 25 centers worldwide was conducted. Clinical and molecular information was collected on 23 unreported and previously reported patients with POLR3-HLD and biallelic pathogenic variants in POLR1C. Brain MRI studies were reviewed. RESULTS: Fourteen female and 9 male patients aged 7 days to 23 years were included in the study. Most participants presented early in life (birth to 6 years), and motor deterioration was seen during childhood. A notable proportion of patients required a wheelchair before adolescence, suggesting a more severe phenotype than previously described in POLR3-HLD. Dental, ocular, and endocrine features were not invariably present (70%, 50%, and 50%, respectively). Five patients (22%) had a combination of hypomyelinating leukodystrophy and abnormal craniofacial development, including 1 individual with clear Treacher Collins syndrome (TCS) features. Brain MRI revealed hypomyelination in all cases, often with areas of pronounced T2 hyperintensity corresponding to T1 hypointensity of the white matter. Twenty-nine different pathogenic variants (including 12 new disease-causing variants) in POLR1C were identified. CONCLUSIONS: This study provides a comprehensive description of POLR3-HLD caused by biallelic POLR1C pathogenic variants based on the largest cohort of patients to date. These results suggest distinct characteristics of POLR1C-related disorder, with a spectrum of clinical involvement characterized by hypomyelinating leukodystrophy with or without abnormal craniofacial development reminiscent of TCS.

Interferon-α and the calcifying microangiopathy in Aicardi-Goutières syndrome.

Aicardi-Goutières syndrome is a leukoencephalopathy with calcifications and increased cerebrospinal fluid interferon-α. The relation between interferon-α and brain pathology is poorly understood. We report a patient with mutations in the disease-associated gene SAMHD1. Neuropathology showed an extensive microangiopathy with calcifications consistently associate with blood vessels. In an in vitro model of the microangiopathy, interferon-α enhanced vascular smooth muscle cell-derived calcifications. The noninfarcted white matter harbored apoptotic oligodendrocytes and increased numbers of oligodendrocyte progenitors. These findings better define the white matter pathology and provide evidence that interferon-α plays a direct pathogenetic role in the calcifying angiopathy typical of this disease.

Altered PLP1 splicing causes hypomyelination of early myelinating structures.

OBJECTIVE: The objective of this study was to investigate the genetic etiology of the X-linked disorder "Hypomyelination of Early Myelinating Structures" (HEMS). METHODS: We included 16 patients from 10 families diagnosed with HEMS by brain MRI criteria. Exome sequencing was used to search for causal mutations. In silico analysis of effects of the mutations on splicing and RNA folding was performed. In vitro gene splicing was examined in RNA from patients' fibroblasts and an immortalized immature oligodendrocyte cell line after transfection with mutant minigene splicing constructs. RESULTS: All patients had unusual hemizygous mutations of PLP1 located in exon 3B (one deletion, one missense and two silent), which is spliced out in isoform DM20, or in intron 3 (five mutations). The deletion led to truncation of PLP1, but not DM20. Four mutations were predicted to affect PLP1/DM20 alternative splicing by creating exonic splicing silencer motifs or new splice donor sites or by affecting the local RNA structure of the PLP1 splice donor site. Four deep intronic mutations were predicted to destabilize a long-distance interaction structure in the secondary PLP1 RNA fragment involved in regulating PLP1/DM20 alternative splicing. Splicing studies in fibroblasts and transfected cells confirmed a decreased PLP1/DM20 ratio. INTERPRETATION: Brain structures that normally myelinate early are poorly myelinated in HEMS, while they are the best myelinated structures in Pelizaeus-Merzbacher disease, also caused by PLP1 alterations. Our data extend the phenotypic spectrum of PLP1-related disorders indicating that normal PLP1/DM20 alternative splicing is essential for early myelination and support the need to include intron 3 in diagnostic sequencing.

Large exonic deletions in POLR3B gene cause POLR3-related leukodystrophy.

POLR3-related (or 4H) leukodystrophy is an autosomal recessive disorder caused by mutations in POLR3A or POLR3B and is characterized by neurological and non-neurological features. In a small proportion of patients, no mutation in either gene or only one mutation is found. Analysis of the POLR3B cDNA revealed a large deletion of exons 21-22 in one case and of exons 26-27 in another case. These are the first reports of long deletions causing POLR3-related leukodystrophy, suggesting that deletions and duplications in POLR3A or POLR3B should be investigated in patients with a compatible phenotype, especially if one pathogenic variant has been identified.

POLR3A and POLR3B Mutations in Unclassified Hypomyelination.

OBJECTIVE: This study aims to ascertain frequency of mutations in POLR3A or POLR3B, which are associated with 4H leukodystrophy, in a cohort of patients with unclassified hypomyelination. METHODS AND RESULTS: In a cohort of 22 patients with the magnetic resonance imaging (MRI) diagnosis of unclassified hypomyelination and without typical clinical signs, we evaluated clinical and MRI features. Developmental delay or intellectual disability, ataxia, and spasticity were frequent symptoms. POLR3A and POLR3B were sequenced. A compound heterozygote mutation in POLR3B was found in only one patient. Additional investigations allowed a definitive diagnosis in 10 patients. CONCLUSION: Mutations in POLR3A or POLR3B are rare in patients with unclassified hypomyelination, and alternative diagnoses should be considered first.

Clinical spectrum of 4H leukodystrophy caused by POLR3A and POLR3B mutations.

OBJECTIVE: To study the clinical and radiologic spectrum and genotype-phenotype correlation of 4H (hypomyelination, hypodontia, hypogonadotropic hypogonadism) leukodystrophy caused by mutations in POLR3A or POLR3B. METHODS: We performed a multinational cross-sectional observational study of the clinical, radiologic, and molecular characteristics of 105 mutation-proven cases. RESULTS: The majority of patients presented before 6 years with gross motor delay or regression. Ten percent had an onset beyond 10 years. The disease course was milder in patients with POLR3B than in patients with POLR3A mutations. Other than the typical neurologic, dental, and endocrine features, myopia was seen in almost all and short stature in 50%. Dental and hormonal findings were not invariably present. Mutations in POLR3A and POLR3B were distributed throughout the genes. Except for French Canadian patients, patients from European backgrounds were more likely to have POLR3B mutations than other populations. Most patients carried the common c.1568T>A POLR3B mutation on one allele, homozygosity for which causes a mild phenotype. Systematic MRI review revealed that the combination of hypomyelination with relative T2 hypointensity of the ventrolateral thalamus, optic radiation, globus pallidus, and dentate nucleus, cerebellar atrophy, and thinning of the corpus callosum suggests the diagnosis. CONCLUSIONS: 4H is a well-recognizable clinical entity if all features are present. Mutations in POLR3A are associated with a more severe clinical course. MRI characteristics are helpful in addressing the diagnosis, especially if patients lack the cardinal non-neurologic features.

Exonic deletions in AUTS2 cause a syndromic form of intellectual disability and suggest a critical role for the C terminus.

Genomic rearrangements involving AUTS2 (7q11.22) are associated with autism and intellectual disability (ID), although evidence for causality is limited. By combining the results of diagnostic testing of 49,684 individuals, we identified 24 microdeletions that affect at least one exon of AUTS2, as well as one translocation and one inversion each with a breakpoint within the AUTS2 locus. Comparison of 17 well-characterized individuals enabled identification of a variable syndromic phenotype including ID, autism, short stature, microcephaly, cerebral palsy, and facial dysmorphisms. The dysmorphic features were more pronounced in persons with 3'AUTS2 deletions. This part of the gene is shown to encode a C-terminal isoform (with an alternative transcription start site) expressed in the human brain. Consistent with our genetic data, suppression of auts2 in zebrafish embryos caused microcephaly that could be rescued by either the full-length or the C-terminal isoform of AUTS2. Our observations demonstrate a causal role of AUTS2 in neurocognitive disorders, establish a hitherto unappreciated syndromic phenotype at this locus, and show how transcriptional complexity can underpin human pathology. The zebrafish model provides a valuable tool for investigating the etiology of AUTS2 syndrome and facilitating gene-function analysis in the future.

Genetic subtyping of Fanconi anemia by comprehensive mutation screening.

Fanconi anemia (FA) is a recessively inherited syndrome with predisposition to bone marrow failure and malignancies. Hypersensitivity to cross-linking agents is a cellular feature used to confirm the diagnosis. The mode of inheritance is autosomal recessive (12 subtypes) as well as X-linked (one subtype). Most genetic subtypes have initially been defined as "complementation groups" by cell fusion studies. Here we report a comprehensive genetic subtyping approach for FA that is primarily based on mutation screening, supplemented by protein expression analysis and by functional assays to test for pathogenicity of unclassified variants. Of 80 FA cases analyzed, 73 (91%) were successfully subtyped. In total, 92 distinct mutations were detected, of which 56 were novel (40 in FANCA, eight in FANCC, two in FANCD1, three in FANCE, one in FANCF, and three in FANCG). All known complementation groups were represented, except D2, J, L, and M. Three patients could not be classified because proliferating cell cultures from the probands were lacking. In cell lines from the remaining four patients, immunoblotting was used to determine their capacity to monoubiquitinate FANCD2. In one case FANCD2 monoubiquitination was normal, indicating a defect downstream. In the remaining three cases monoubiquitination was not detectable, indicating a defect upstream. In the latter four patients, pathogenic mutations in a known FA gene may have been missed, or these patients might represent novel genetic subtypes. We conclude that direct mutation screening allows a molecular diagnosis of FA in the vast majority of patients, even in cases where growing cells from affected individuals are unavailable. Proliferating cell lines are required in a minority (<15%) of the patients, to allow testing for FANCD2 ubiquitination status and sequencing of FANCD2 using cDNA, to avoid interference from pseudogenes.