The Genetics of Primary Microcephaly Jayaraman, Divya; Bae, Byoung-Il; Walsh, Christopher A
Annual review of genomics and human genetics,
08/2018, Letnik:
19, Številka:
1
Journal Article
Recenzirano
Primary microcephaly (MCPH, for "microcephaly primary hereditary") is a disorder of brain development that results in a head circumference more than 3 standard deviations below the mean for age and ...gender. It has a wide variety of causes, including toxic exposures, in utero infections, and metabolic conditions. While the genetic microcephaly syndromes are relatively rare, studying these syndromes can reveal molecular mechanisms that are critical in the regulation of neural progenitor cells, brain size, and human brain evolution. Many of the causative genes for MCPH encode centrosomal proteins involved in centriole biogenesis. However, other MCPH genes fall under different mechanistic categories, notably DNA replication and repair. Recent gene discoveries and functional studies have implicated novel cellular processes, such as cytokinesis, centromere and kinetochore function, transmembrane or intracellular transport, Wnt signaling, and autophagy, as well as the apical polarity complex. Thus, MCPH genes implicate a wide variety of molecular and cellular mechanisms in the regulation of cerebral cortical size during development.
Resolving lineage relationships between cells in an organism is a fundamental interest of developmental biology. Furthermore, investigating lineage can drive understanding of pathological states, ...including cancer, as well as understanding of developmental pathways that are amenable to manipulation by directed differentiation. Although lineage tracking through the injection of retroviral libraries has long been the state of the art, a recent explosion of methodological advances in exogenous labelling and single-cell sequencing have enabled lineage tracking at larger scales, in more detail, and in a wider range of species than was previously considered possible. In this Review, we discuss these techniques for cell lineage tracking, with attention both to those that trace lineage forwards from experimental labelling, and those that trace backwards across the life history of an organism.
During prenatal brain development, ion channels are ubiquitous across several cell types, including progenitor cells and migrating neurons but their function has not been clear. In the past, ion ...channel dysfunction has been primarily studied in the context of postnatal, differentiated neurons that fire action potentials – notably ion channels mutated in the epilepsies – yet data now support a surprising role in prenatal human brain disorders as well. Modern gene discovery approaches have identified defective ion channels in individuals with cerebral cortex malformations, which reflect abnormalities in early-to-middle stages of embryonic development (prior to ubiquitous action potentials). These human genetics studies and recent in utero animal modeling work suggest that precise control of ionic flux (calcium, sodium, and potassium) contributes to in utero developmental processes such as neural proliferation, migration, and differentiation.
Developmental channelopathy is an ion channel disease with a pathophysiological origin during the gestational period.Human genetics studies reveal that individuals with dysfunctional sodium and glutamate channels can have improperly folded cerebral cortices.The gestational brain is comprised of diverse and transient cell types, including dividing and newly differentiated cells. Some of the mechanisms involved in developmental channelopathies during gestation are different from those acting postnatally during ion channel diseases.Ion channel mutations that result in greater excitability (gain-of-function) are often associated with human cortical malformations.
A major unanswered question in neuroscience is whether there exists genomic variability between individual neurons of the brain, contributing to functional diversity or to an unexplained burden of ...neurological disease. To address this question, we developed a method to amplify genomes of single neurons from human brains. Because recent reports suggest frequent LINE-1 (L1) retrotransposition in human brains, we performed genome-wide L1 insertion profiling of 300 single neurons from cerebral cortex and caudate nucleus of three normal individuals, recovering >80% of germline insertions from single neurons. While we find somatic L1 insertions, we estimate <0.6 unique somatic insertions per neuron, and most neurons lack detectable somatic insertions, suggesting that L1 is not a major generator of neuronal diversity in cortex and caudate. We then genotyped single cortical cells to characterize the mosaicism of a somatic AKT3 mutation identified in a child with hemimegalencephaly. Single-neuron sequencing allows systematic assessment of genomic diversity in the human brain.
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▸ Whole-genome amplification and sequencing of single neurons from human brains ▸ Single-neuron genome-wide analysis of somatic L1 retrotransposition ▸ Somatic L1 insertions are infrequent in cerebral cortex and caudate nucleus ▸ Single-cell analysis of a somatic mutation causing malformation of the brain
Whole-genome sequencing of individual neurons from human brain shows that retrotransposition of the L1 element is infrequent. L1 insertions may thus generate some somatic mutations but are unlikely to account for neuronal diversity.
Genetic mosaicism is the result of the accumulation of somatic mutations in the human genome starting from the first postzygotic cell generation and continuing throughout the whole life of an ...individual. The rapid development of next-generation and single-cell sequencing technologies is now allowing the study of genetic mosaicism in normal tissues, revealing unprecedented insights into their clonal architecture and physiology. The somatic variant repertoire of an adult human neuron is the result of somatic mutations that accumulate in the brain by different mechanisms and at different rates during development and ageing. Non-pathogenic developmental mutations function as natural barcodes that once identified in deep bulk or single-cell sequencing can be used to retrospectively reconstruct human lineages. This approach has revealed novel insights into the clonal structure of the human brain, which is a mosaic of clones traceable to the early embryo that contribute differentially to the brain and distinct areas of the cortex. Some of the mutations happening during development, however, have a pathogenic effect and can contribute to some epileptic malformations of cortical development and autism spectrum disorder. In this Review, we discuss recent findings in the context of genetic mosaicism and their implications for brain development and disease.
Genetic mutations causing human disease are conventionally thought to be inherited through the germ line from one's parents and present in all somatic (body) cells, except for most cancer mutations, ...which arise somatically. Increasingly, somatic mutations are being identified in diseases other than cancer, including neurodevelopmental diseases. Somatic mutations can arise during the course of prenatal brain development and cause neurological disease-even when present at low levels of mosaicism, for example-resulting in brain malformations associated with epilepsy and intellectual disability. Novel, highly sensitive technologies will allow more accurate evaluation of somatic mutations in neurodevelopmental disorders and during normal brain development.
Cortical development depends on the active integration of cell-autonomous and extrinsic cues, but the coordination of these processes is poorly understood. Here, we show that the apical complex ...protein Pals1 and Pten have opposing roles in localizing the Igf1R to the apical, ventricular domain of cerebral cortical progenitor cells. We found that the cerebrospinal fluid (CSF), which contacts this apical domain, has an age-dependent effect on proliferation, much of which is attributable to Igf2, but that CSF contains other signaling activities as well. CSF samples from patients with glioblastoma multiforme show elevated Igf2 and stimulate stem cell proliferation in an Igf2-dependent manner. Together, our findings demonstrate that the apical complex couples intrinsic and extrinsic signaling, enabling progenitors to sense and respond appropriately to diffusible CSF-borne signals distributed widely throughout the brain. The temporal control of CSF composition may have critical relevance to normal development and neuropathological conditions.
► Embryonic CSF distributes many factors to the developing brain ► CSF Igf2 binds to progenitors and stimulates their proliferation ► Pals1 and Pten regulate Igf1 receptor expression ► Igf2 in glioblastoma CSF regulates progenitor proliferation and disease state
Genetic Changes Shaping the Human Brain Bae, Byoung-Il; Jayaraman, Divya; Walsh, Christopher A.
Developmental cell,
02/2015, Letnik:
32, Številka:
4
Journal Article
Recenzirano
Odprti dostop
The development and function of our brain are governed by a genetic blueprint, which reflects dynamic changes over the history of evolution. Recent progress in genetics and genomics, facilitated by ...next-generation sequencing and single-cell sorting, has identified numerous genomic loci that are associated with a neuroanatomical or neurobehavioral phenotype. Here, we review some of the genetic changes in both protein-coding and noncoding regions that affect brain development and evolution, as well as recent progress in brain transcriptomics. Understanding these genetic changes may provide novel insights into neurological and neuropsychiatric disorders, such as autism and schizophrenia.
De novo copy-number variants (CNVs) can cause neuropsychiatric disease, but the degree to which they occur somatically, and during development, is unknown. Single-cell whole-genome sequencing (WGS) ...in >200 single cells, including >160 neurons from three normal and two pathological human brains, sensitively identified germline trisomy of chromosome 18 but found most (≥95%) neurons in normal brain tissue to be euploid. Analysis of a patient with hemimegalencephaly (HMG) due to a somatic CNV of chromosome 1q found unexpected tetrasomy 1q in ∼20% of neurons, suggesting that CNVs in a minority of cells can cause widespread brain dysfunction. Single-cell analysis identified large (>1 Mb) clonal CNVs in lymphoblasts and in single neurons from normal human brain tissue, suggesting that some CNVs occur during neurogenesis. Many neurons contained one or more large candidate private CNVs, including one at chromosome 15q13.2-13.3, a site of duplication in neuropsychiatric conditions. Large private and clonal somatic CNVs occur in normal and diseased human brains.
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•Single-neuron, whole-genome sequencing allows copy-number variant detection•Most human single neurons are euploid at the chromosome level•Large (>1 Mb) somatic copy-number variants occur approximately once per genome•Some somatic copy-number variants are clonally inherited and some cause disease
Cai et al. performed copy-number analysis on over 200 single cells, including glia and >160 neurons and glia isolated from primary human brain tissue. Although most neurons in normal brains are euploid, large subchromosomal CNVs were commonly identified, suggesting genetic heterogeneity within normal human brains. Clonal somatic CNVs were identified from both normal and diseased human brains, suggesting that CNVs occurring during neurogenesis can have a wide range of functional impacts on human cognitive function.
Hemimegalencephaly (HMG) is a developmental brain disorder characterized by an enlarged, malformed cerebral hemisphere, typically causing epilepsy that requires surgical resection. We studied ...resected HMG tissue to test whether the condition might reflect somatic mutations affecting genes critical to brain development. We found that two out of eight HMG samples showed trisomy of chromosome 1q, which encompasses many genes, including AKT3, a gene known to regulate brain size. A third case showed a known activating mutation in AKT3 (c.49G→A, creating p.E17K) that was not present in the patient's blood cells. Remarkably, the E17K mutation in AKT3 is exactly paralogous to E17K mutations in AKT1 and AKT2 recently discovered in somatic overgrowth syndromes. We show that AKT3 is the most abundant AKT paralog in the brain during neurogenesis and that phosphorylated AKT is abundant in cortical progenitor cells. Our data suggest that somatic mutations limited to the brain could represent an important cause of complex neurogenetic disease.
► Two cases showing trisomy 1q in brain tissue were associated with hemimegalencephaly ► A third hemimegalencephaly case had a somatic mosaic-activating mutation in AKT3 ► AKT3 is the most prevalent of the three AKT paralogues during human neurogenesis ► Phosphorylated AKT is expressed in apical progenitors of the developing cortex
Poduri et al. identify trisomy on chromosome 1q, which encompasses AKT3, and a separate activating AKT3 somatic mutation in patients with the developmental brain disorder hemimegalencephaly. Results suggest that somatic mutations limited to the brain may contribute to neurodevelopmental disorders.
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