Human Microcephaly Protein RTTN Is Required for Proper Mitotic Progression and Correct Spindle Position

Case Report: Novel biallelic moderately damaging variants in RTTN in a patient with cerebellar dysplasia

Rotatin, encoded by the RTTN gene, is a centrosomal protein with multiple, emerging functions, including left-right specification, ciliogenesis, and neuronal migration. Recessive variants in RTTN are associated with a neurodevelopmental disorder with microcephaly and malformations of cortical development known as "Microcephaly, short stature, and polymicrogyria with seizures" (MSSP, MIM #614833). Affected individuals show a wide spectrum of clinical manifestations like intellectual disability, poor/absent speech, short stature, microcephaly, and congenital malformations. Here, we report a subject showing a distinctive neuroradiological phenotype and harboring novel biallelic variants in RTTN: the c.5500A>G, p.(Asn1834Asp), (dbSNP: rs200169343, ClinVar ID:1438510) and c.19A>G, p.(Ile7Val), (dbSNP: rs201165599, ClinVar ID:1905275) variants. In particular brain magnetic resonance imaging (MRI) showed a peculiar pattern, with cerebellar hypo-dysplasia, and multiple arachnoid cysts in the lateral cerebello-medullary cisterns, in addition to left Meckel cave. Thus, we compare his phenotypic features with current literature, speculating a possible role of newly identified RTTN variants in his clinical picture, and supporting a relevant variability in this emerging condition.

Microcephaly, Short Stature, Intellectual Disability, Speech Absence and Cataract Are Associated with Novel Bi-Allelic Missense Variant in RTTN Gene: A Seckel Syndrome Case Report

The RTTN gene encodes centriole biogenesis, replication, symmetry and cohesion, basal body organization and has recently been associated with the appearance of microcephaly syndromes. RTTN-related neurological defects including microcephaly, intellectual disability, congenital dwarfism, ophthalmic manifestations, and epilepsy are mainly due to abnormal brain development pathways and loss-of-function protein mutations. We present a consanguineous Pakistani family clinically suspected of Seckel syndrome with severe microcephaly, severe intellectual disability, short stature, absence of speech, pointed nose, narrow face and bilateral cataract in two siblings residing in the suburbs of Islamabad. Forty cases of Seckel syndrome have been reported to date in the literature due to mutations in the ATR, TRAIP, RBBP8, NSMCE2, NIN, CENPJ, DNA2, CEP152 and CEP63 genes. The objective of the study was to perform a clinical diagnosis, genetic analysis, and pathophysiology of Seckel syndrome in the proband. Whole-exome sequencing discovered NM_173630.4: c.57G > T(pGlu19Asp) missense variant in exon 2 of the RTTN gene that co-segregates in the family. This novel variant, to the best of our knowledge, is pathogenic and with autosomal recessive inheritance expressed as Seckel syndrome in the affected members of the family. The present study has expanded the genetic knowledge of novel RTTN gene variants associated with Seckel syndrome and has broadened its phenotype spectrum in the Pakistani population, which comprises diverse ethnicities. We hope that our study will open new horizons for individual molecular diagnosis and therapeutics to improve the life of patients with this congenital syndrome.

Modeling Human Primary Microcephaly With hiPSC-Derived Brain Organoids Carrying CPAP-E1235V Disease-Associated Mutant Protein

The centrosome is composed of a pair of centrioles and serves as the major microtubule-organizing center (MTOC) in cells. Centrosome dysfunction has been linked to autosomal recessive primary microcephaly (MCPH), which is a rare human neurodevelopmental disorder characterized by small brain size with intellectual disability. Recently, several mouse models carrying mutated genes encoding centrosomal proteins have been generated to address the genotype-phenotype relationships in MCPH. However, several human-specific features were not observed in the mouse models during brain development. Herein, we generated isogenic hiPSCs carrying the gene encoding centrosomal CPAP-E1235V mutant protein using the CRISPR-Cas9 genome editing system, and examined the phenotypic features of wild-type and mutant hiPSCs and their derived brain organoids. Our results showed that the CPAP-E1235V mutant perturbed the recruitment of several centriolar proteins involved in centriole elongation, including CEP120, CEP295, CENTROBIN, POC5, and POC1B, onto nascent centrioles, resulting in the production of short centrioles but long cilia. Importantly, our wild-type hiPSC-derived brain organoid recapitulated many cellular events seen in the developing human brain, including neuronal differentiation and cortical spatial lamination. Interestingly, hiPSC-CPAP-E1235V-derived brain organoids induced p53-dependent neuronal cell death, resulting in the production of smaller brain organoids that mimic the microcephaly phenotype. Furthermore, we observed that the CPAP-E1235V mutation altered the spindle orientation of neuronal progenitor cells and induced premature neuronal differentiation. In summary, we have shown that the hiPSC-derived brain organoid coupled with CRISPR/Cas9 gene editing technology can recapitulate the centrosome/centriole-associated MCPH pathological features. Possible mechanisms for MCPH with centriole/centrosome dysfunction are discussed.