Monday, July 3, 2017

ASPM microcephaly

Inspired by a patient

Hashmi JA, Al-Harbi KM, Ramzan K, Albalawi AM, Mehmood A, Samman MI, Basit S. A novel splice-site mutation in the ASPM gene underlies autosomal recessive primary microcephaly. Ann Saudi Med. 2016 Nov-Dec;36(6):391-396.

Autosomal recessive primary microcephaly (MCPH) is a clinically and genetically heterogeneous disorder. Patients with MCPH exhibit reduced occipito-frontal head circumference and non-progressive intellectual disability. To date, 17 genes have been known as an underlying cause of MCPH in humans. ASPM (abnormal spindle-like, microcephaly associated) is the most commonly mutated MCPH gene.
Identify the genetic defect underlying MCPH in a Saudi family.
A cross-sectional clinical genetic study of a Saudi family.
Madinah Maternity and Children Hospital and Centre for Genetics and Inherited Diseases, Taibah University.
A molecular analysis was carried out on DNA samples from 10 individuals of a Saudi family segregating MCPH. DNA was isolated from the peripheral blood of 10 individuals, including 2 patients, and whole exome sequencing was performed using the Nextera Rapid Capture kit and NextSeq500 instrument. VariantStudio was used to filter and prioritize variants.
Detection of mutation in the ASPM gene in a family segregating autoso- mal recessive primary microcephaly.
A novel homozygous splice-site variant (c.3742-1G > C) in the ASPM gene was identified. The variant is predicted to have an effect on splicing. Human Splice Finder, an in silico tool, predicted skipping of exon 16 due to this variant.
Skipping of exon 16 may change the order and number of IQ motifs in the ASPM protein leading to typical MCPH phenotype.
Single family study.

Abdel-Hamid MS, Ismail MF, Darwish HA, Effat LK, Zaki MS, Abdel-Salam GM. Molecular and phenotypic spectrum of ASPM-related primary microcephaly: Identification of eight novel mutations. Am J Med Genet A. 2016 Aug;170(8):2133-40.

Autosomal recessive primary microcephaly (MCPH) is an abnormal proliferation of neurons during brain development that leads to a small brain size but architecturally normal in most instances. Mutations in the ASPM gene have been identified to be the most prevalent. Thirty-seven patients from 30 unrelated families with a clinical diagnosis of MCPH were enrolled in this study. Screening of ASPM gene mutations was performed by targeted linkage analysis followed by direct sequencing. Thirteen protein truncating mutations of the ASPM were identified in 15 families (50%), eight of which were novel mutations. The mutations detected were eight nonsense, four frameshift, and one splice site. Two of these mutations (p.R1327* and p.R3181*) were recurrent and shared similar haplotypes suggesting founder effect. Patients with ASPM mutations had mild to severe intellectual disability and variable degrees of simplified gyral pattern and small frontal lobe. In addition, hypoplasia of corpus callosum (18 patients), mildly small cerebellar vermis (10 patients), and relatively small pons (13 patients) were found in 85.7%, 47.6%, and 61.9%, respectively. Furthermore, one patient had porencephaly and another had a small midline cyst. Epilepsy was documented in two patients (9.5%). Non-neurologic abnormalities consisted of growth retardation (four patients), and co-incidental association of oculo-cutaneous albinism (one patient). Our study expands the mutation spectrum of ASPM. Moreover, the simplified gyral pattern and small frontal lobe together with hypoplastic corpus callosum, small cerebellum and pons enable ASPM mutated patients to be distinguished.

Jayaraman D, Kodani A, Gonzalez DM, Mancias JD, Mochida GH, Vagnoni C, Johnson J, Krogan N, Harper JW, Reiter JF, Yu TW, Bae BI, Walsh CA. Microcephaly Proteins Wdr62 and Aspm Define a Mother Centriole Complex Regulating Centriole Biogenesis, Apical Complex, and Cell Fate. Neuron. 2016 Nov 23;92(4):813-828.

Mutations in several genes encoding centrosomal proteins dramatically decrease the size of the human brain. We show that Aspm (abnormal spindle-like, microcephaly-associated) and Wdr62 (WD repeat-containing protein 62) interact genetically to control brain size, with mice lacking Wdr62, Aspm, or both showing gene dose-related centriole duplication defects that parallel the severity of the microcephaly and increased ectopic basal progenitors, suggesting premature delamination from the ventricular zone. Wdr62 and Aspm localize to the proximal end of the mother centriole and interact physically, with Wdr62 required for Aspm localization, and both proteins, as well as microcephaly protein Cep63, required to localize CENPJ/CPAP/Sas-4, a final common target. Unexpectedly, Aspm and Wdr62 are required for normal apical complex localization and apical epithelial structure, providing a plausible unifying mechanism for the premature delamination and precocious differentiation of progenitors. Together, our results reveal links among centrioles, apical proteins, and cell fate, and illuminate how alterations in these interactions can dynamically regulate brain size.


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