Dissecting the Genetic and Etiological Causes of Primary Microcephaly

Abstract

Autosomal recessive primary microcephaly (MCPH; "small head syndrome") is a rare, heterogeneous disease arising from the decreased production of neurons during brain development. As of August 2020, the Online Mendelian Inheritance in Man (OMIM) database lists 25 genes (involved in molecular processes such as centriole biogenesis, microtubule dynamics, spindle positioning, DNA repair, transcriptional regulation, Wnt signaling, and cell cycle checkpoints) that are implicated in causing MCPH. Many of these 25 genes were only discovered in the last 10 years following advances in exome and genome sequencing that have improved our ability to identify disease-causing variants. Despite these advances, many patients still lack a genetic diagnosis. This demonstrates a need to understand in greater detail the molecular mechanisms and genetics underlying MCPH. Here, we briefly review the molecular functions of each MCPH gene and how their loss disrupts the neurogenesis program, ultimately demonstrating that microcephaly arises from cell cycle dysregulation. We also explore the current issues in the genetic basis and clinical presentation of MCPH as additional avenues of improving gene/variant prioritization. Ultimately, we illustrate that the detailed exploration of the etiology and inheritance of MCPH improves the predictive power in identifying previously unknown MCPH candidates and diagnosing microcephalic patients.

Keywords: cell cycle; genetics; microcephaly; neurogenesis; rare disease (RD).

Figure 1

Neurogenesis in the developing neocortex. (A) Apical progenitor cells in the ventricular zone undergo symmetrical proliferative divisions, generating a pool of progenitor cells. (B) Expression of glial markers causes progenitor cells to differentiate into radial glial cells, which can subsequently undergo symmentrical divisions to generate more radial glial cells or immature neurons. Distinct cortical layers begin to form: ventricular zone, subventricular zone, intermediate zone, cortical plate, and the marginal zone. (C) Radial glial cells favor asymmetric division to generate more diverse neuron types and basal progenitors, a secondary progenitor. Radial glial cells continue to differentiate into mature neurons and basal radial glia. AP, apical progenitors; BP, basal progenitors; BRG, basal radial glia; CP, cortical plate; IN, immature neurons; IZ, intermediate zone; MN, mature neurons; MZ, marginal zone; RG, radial glia; SVZ, subventricular zone; VZ, ventricular zone.

Figure 2

MCPH-associated proteins have overlapping cellular functions that affect cell cycle progression. Aberrant activity in any of these cellular functions would create delays in the timing of the cell cycle and overall proliferation through development. Several proteins act across more than one functional pathway (i.e., centriole biogenesis and mitotic spindle orientation), further delaying the cell cycle at each functionally relevant timepoint.

Figure 3

Centrosome biogenesis is linked to the cell cycle. (G1 phase) Centrioles disengage through both separase activity and pericentriolar material (PCM) degradation. The disengaged centriole pair becomes the cilium basal body and acts as the template for ciliogenesis. (S phase) The daughter centriole becomes replication competent and centriole biogenesis is initiated by the recruitment of PLK4 which phosphorylates STIL to begin SAS6 recruitment to generate the central hub. Daughter centrioles elongate and remain attached to the mother centriole via cohesion. (G2 phase) Mother centrioles unlink and centrosome maturation begins with the development of the pericentriolar material and formation of distal appendages. (M phase) Centrosomes travel to opposite poles of the cell for spindle formation and attachment and cell division; each daughter cell contains one centrosome to repeat the cycle.

Figure 4

Microtubule dynamics orient the mitotic spindle and drive cell division. (A) Contractile ring component CIT at the midbody recruits KIF14 to the central spindle to stabilize the microtubule network. MAP11 promotes cell abscission at the midbody. (B) ASPM and NuMA at spindle poles recruit dynein–dynactin to astral microtubules to position spindles in the dividing cell. (C) The kinetochore, composed of three distinct layers (inner, outer, and corona), contains several proteins to securely attach microtubules. CENPE in the coronal layer binds microtubules through the motor domain; unbound CENPE signals through BUB1B to the APC/C to delay anaphase. The outer layer complex similarly binds the microtubule positive end; one of the components, KNL1, signals the APC/C to delay cycle if there is improper attachment at this layer. In the inner layer, the CENP complex binds the kinetochore to the condensed chromosome ensuring proper attachment for segregation. (D) Pericentriolar material scaffold is formed by CDK5RAP2 for microtubule nucleation by γ-tubulin at the centrosome. APC/C, anaphase promoting complex/cyclosome; MT, microtubules.

Figure 5

DNA dynamics are linked to the cell cycle. (G1 phase) The nuclear envelope reforms after mitosis, then chromatin is positioned in the nucleus and remodeled for transcription by ZNF335 and PHC1. In preparation for synthesis and the G1 cell cycle checkpoint, DNA repair proteins correct any damage present in the genome. (S phase) Chromosomes undergo replication and repair proteins correct any errors or DNA breaks that occurred during the synthesis process. (G2 phase) Sister chromatids are brought together and bound by cohesin complexes. Prior to mitotic entry, condensin II begins the condensation of the chromatids as the negative regulator MCPH1 is broken down. (M phase) Nuclear envelope is broken down by ANKLE2 and homologous chromosomes align at the metaphase plate. Separase disintegrates the cohesin bonds between sister chromatids so they can be segregated to opposite poles before cytokinesis divides the daughter cells.