Elucidating the Genetic and Molecular Mechanisms of Recessive Pediatric Brain Disease

The structural organization and maturation of the brain are the result of a precisely orchestrated series of developmental processes with complex genetic regulation that occur during embryonic gestation and are critical for neuronal identity, wiring and connectivity. Mutations affecting any of the cellular processes involved in proper human brain development can lead to altered neurodevelopment and result in a number of different neurological diseases. Such diseases arising as a consequence of a biallelic mutation in a single gene are much more prevalent among offspring of consanguineous marriages, increasing the odds that a deleterious mutation will be inherited on both chromosomes. The identification of a disease-causing gene in families with inherited brain disorders is one of the most useful methods for gaining insight into human brain development, function, and pathology. This study largely focuses on identifying novel mutations in both known and novel recessive pediatric brain diseases using next-generation sequencing approaches in combination with in vitro and in vivo modeling. By taking advantage of the large number of consanguineous families in our cohort, we are not only able to explore the cause of disease directly in humans, but also able to determine genetic disease risk and identify novel disease-causing variants with a high level of certainty. A total of 88 affected individuals were identified and studied from 46 families displaying a range of SBDs including neurodegeneration (i.e. cortical and cerebellar atrophy), microlissencephaly, PCH, and Joubert syndrome, among other symptoms. A total of six disease-causing genes, ADPRHL2, TMX2, TRAPPC4, HEATR5B, HPDL, and ARMC9 were identified, four of which had never been implicated in disease prior to this study. We were further able to model disease in vivo for three out of the five genes (ADPRHL2, HEATR5B, and HPDL) using either fly or mouse models. Taken together, this study not only provides further knowledge in identifying novel causes of both previously unknown diseases as well as known diseases with unknown cause, but also provides further promise to the application of NGS in revealing the full spectrum of mutations associated with these diseases and in further delineating the relevant molecular pathways involved

Elucidating the Genetic and Molecular Mechanisms of Recessive Pediatric Brain Disease

The structural organization and maturation of the brain are the result of a precisely orchestrated series of developmental processes with complex genetic regulation that occur during embryonic gestation and are critical for neuronal identity, wiring and connectivity. Mutations affecting any of the cellular processes involved in proper human brain development can lead to altered neurodevelopment and result in a number of different neurological diseases. Such diseases arising as a consequence of a biallelic mutation in a single gene are much more prevalent among offspring of consanguineous marriages, increasing the odds that a deleterious mutation will be inherited on both chromosomes. The identification of a disease-causing gene in families with inherited brain disorders is one of the most useful methods for gaining insight into human brain development, function, and pathology. This study largely focuses on identifying novel mutations in both known and novel recessive pediatric brain diseases using next-generation sequencing approaches in combination with in vitro and in vivo modeling. By taking advantage of the large number of consanguineous families in our cohort, we are not only able to explore the cause of disease directly in humans, but also able to determine genetic disease risk and identify novel disease-causing variants with a high level of certainty. A total of 88 affected individuals were identified and studied from 46 families displaying a range of SBDs including neurodegeneration (i.e. cortical and cerebellar atrophy), microlissencephaly, PCH, and Joubert syndrome, among other symptoms. A total of six disease-causing genes, ADPRHL2, TMX2, TRAPPC4, HEATR5B, HPDL, and ARMC9 were identified, four of which had never been implicated in disease prior to this study. We were further able to model disease in vivo for three out of the five genes (ADPRHL2, HEATR5B, and HPDL) using either fly or mouse models. Taken together, this study not only provides further knowledge in identifying novel causes of both previously unknown diseases as well as known diseases with unknown cause, but also provides further promise to the application of NGS in revealing the full spectrum of mutations associated with these diseases and in further delineating the relevant molecular pathways involved

Motoneuron expression profiling identifies an association between an axonal splice variant of HDGF-related protein 3 and peripheral myelination

Disorders that disrupt myelin formation during development or in adulthood, such as multiple sclerosis and peripheral neuropathies, lead to severe pathologies, illustrating myelin's crucial role in normal neural functioning. However, although our understanding of glial biology is increasing, the signals that emanate from axons and regulate myelination remain largely unknown. To identify the core components of the myelination process, here we adopted a microarray analysis approach combined with laser-capture microdissection of spinal motoneurons during the myelinogenic phase of development. We identified neuronal genes whose expression was enriched during myelination and further investigated hepatoma-derived growth factor-related protein 3 (HRP3 or HDGFRP3). HRP3 was strongly expressed in the white matter fiber tracts of the peripheral (PNS) and central (CNS) nervous systems during myelination and remyelination in a cuprizone-induced demyelination model. The dynamic localization of HPR3 between axons and nuclei during myelination was consistent with its axonal localization during neuritogenesis. To study this phenomenon, we identified two splice variants encoded by theHRP3gene: the canonical isoform HRP3-I and a newly recognized isoform, HRP3-II. HRP3-I remained solely in the nucleus, whereas HRP3-II displayed distinct axonal localization both before and during myelination. Interestingly, HRP3-II remained in the nuclei of unmyelinated neurons and glial cells, suggesting the existence of a molecular machinery that transfers it to and retains it in the axons of neurons fated for myelination. Overexpression of HRP3-II, but not of HRP3-I, increased Schwann cell numbers and myelination in PNS neuron-glia co-cultures. However, HRP3-II overexpression in CNS co-cultures did not alter myelination.Christopher and Dana Reeve Foundation Roche Foundation Swiss National Science Foundation (SNSF) American Heart Association Paul G. Allen Frontiers Group Grant Leona M. and Harry B. Helmsley Charitable Trust Grant Grace Foundation JPB Foundation Robert and Mary Jane Engman Foundation Ray and Dagmar Dolby Family Fund Crick-Jacob Center for Theoretical and Computational Biology Neurosurgery Neuroscience Consortium Fellowshi

Recurrent homozygous damaging mutation in TMX2, encoding a protein disulfide isomerase, in four families with microlissencephaly

BackgroundProtein disulfide isomerase (PDI) proteins are part of the thioredoxin protein superfamily. PDIs are involved in the formation and rearrangement of disulfide bonds between cysteine residues during protein folding in the endoplasmic reticulum and are implicated in stress response pathways.MethodsEight children from four consanguineous families residing in distinct geographies within the Middle East and Central Asia were recruited for study. All probands showed structurally similar microcephaly with lissencephaly (microlissencephaly) brain malformations. DNA samples from each family underwent whole exome sequencing, assessment for repeat expansions and confirmatory segregation analysis.ResultsAn identical homozygous variant in TMX2 (c.500G>A), encoding thioredoxin-related transmembrane protein 2, segregated with disease in all four families. This variant changed the last coding base of exon 6, and impacted mRNA stability. All patients presented with microlissencephaly, global developmental delay, intellectual disability and epilepsy. While TMX2 is an activator of cellular C9ORF72 repeat expansion toxicity, patients showed no evidence of C9ORF72 repeat expansions.ConclusionThe TMX2 c.500G>A allele associates with recessive microlissencephaly, and patients show no evidence of C9ORF72 expansions. TMX2 is the first PDI implicated in a recessive disease, suggesting a protein isomerisation defect in microlissencephaly