TMEM63C mutations cause mitochondrial morphology defects and underlie hereditary spastic paraplegia

The hereditary spastic paraplegias (HSP) are among the most genetically diverse of all Mendelian disorders. They comprise a large group of neurodegenerative diseases that may be divided into 'pure HSP' in forms of the disease primarily entailing progressive lower-limb weakness and spasticity, and 'complex HSP' when these features are accompanied by other neurological (or non-neurological) clinical signs. Here, we identified biallelic variants in the transmembrane protein 63C (TMEM63C) gene, encoding a predicted osmosensitive calcium-permeable cation channel, in individuals with hereditary spastic paraplegias associated with mild intellectual disability in some, but not all cases. Biochemical and microscopy analyses revealed that TMEM63C is an endoplasmic reticulum-localized protein, which is particularly enriched at mitochondria-endoplasmic reticulum contact sites. Functional in cellula studies indicate a role for TMEM63C in regulating both endoplasmic reticulum and mitochondrial morphologies. Together, these findings identify autosomal recessive TMEM63C variants as a cause of pure and complex HSP and add to the growing evidence of a fundamental pathomolecular role of perturbed mitochondrial-endoplasmic reticulum dynamics in motor neurone degenerative diseases.

A mutation of EPT1 (SELENOI) underlies a new disorder of Kennedy pathway phospholipid biosynthesis

Mutations in genes involved in lipid metabolism have increasingly been associated with various subtypes of hereditary spastic paraplegia, a highly heterogeneous group of neurodegenerative motor neuron disorders characterized by spastic paraparesis. Here, we report an unusual autosomal recessive neurodegenerative condition, best classified as a complicated form of hereditary spastic paraplegia, associated with mutation in the ethanolaminephosphotransferase 1 (EPT1) gene (now known as SELENOI), responsible for the final step in Kennedy pathway forming phosphatidylethanolamine from CDP-ethanolamine. Phosphatidylethanolamine is a glycerophospholipid that, together with phosphatidylcholine, constitutes more than half of the total phospholipids in eukaryotic cell membranes. We determined that the mutation defined dramatically reduces the enzymatic activity of EPT1, thereby hindering the final step in phosphatidylethanolamine synthesis. Additionally, due to central nervous system inaccessibility we undertook quantification of phosphatidylethanolamine levels and species in patient and control blood samples as an indication of liver phosphatidylethanolamine biosynthesis. Although this revealed alteration to levels of specific phosphatidylethanolamine fatty acyl species in patients, overall phosphatidylethanolamine levels were broadly unaffected indicating that in blood EPT1 inactivity may be compensated for, in part, via alternate biochemical pathways. These studies define the first human disorder arising due to defective CDP-ethanolamine biosynthesis and provide new insight into the role of Kennedy pathway components in human neurological function.

PRUNE is crucial for normal brain development and mutated in microcephaly with neurodevelopmental impairment

PRUNE is a member of the DHH (Asp-His-His) phosphoesterase protein superfamily of molecules important for cell motility, and implicated in cancer progression. Here we investigated multiple families from Oman, India, Iran and Italy with individuals affected by a new autosomal recessive neurodevelopmental and degenerative disorder in which the cardinal features include primary microcephaly and profound global developmental delay. Our genetic studies identified biallelic mutations of PRUNE1 as responsible. Our functional assays of disease-associated variant alleles revealed impaired microtubule polymerization, as well as cell migration and proliferation properties, of mutant PRUNE. Additionally, our studies also highlight a potential new role for PRUNE during microtubule polymerization, which is essential for the cytoskeletal rearrangements that occur during cellular division and proliferation. Together these studies define PRUNE as a molecule fundamental for normal human cortical development and define cellular and clinical consequences associated with PRUNE mutation.

Loss of PCLO function underlies pontocerebellar hypoplasia type III

OBJECTIVE

To identify the genetic cause of pontocerebellar hypoplasia type III (PCH3).

METHODS

We studied the original reported pedigree of PCH3 and performed genetic analysis including genome-wide single nucleotide polymorphism genotyping, linkage analysis, whole-exome sequencing, and Sanger sequencing. Human fetal brain RNA sequencing data were then analyzed for the identified candidate gene.

RESULTS

The affected individuals presented with severe global developmental delay and seizures starting in the first year of life. Brain MRI of an affected individual showed diffuse atrophy of the cerebrum, cerebellum, and brainstem. Genome-wide single nucleotide polymorphism analysis confirmed the linkage to chromosome 7q we previously reported, and showed no other genomic areas of linkage. Whole-exome sequencing of 2 affected individuals identified a shared homozygous, nonsense variant in the PCLO (piccolo) gene. This variant segregated with the disease phenotype in the pedigree was rare in the population and was predicted to eliminate the PDZ and C2 domains in the C-terminus of the protein. RNA sequencing data of human fetal brain showed that PCLO was moderately expressed in the developing cerebral cortex.

CONCLUSIONS

Here, we show that a homozygous, nonsense PCLO mutation underlies the autosomal recessive neurodegenerative disorder, PCH3. PCLO is a component of the presynaptic cytoskeletal matrix, and is thought to be involved in regulation of presynaptic proteins and synaptic vesicles. Our findings suggest that PCLO is crucial for the development and survival of a wide range of neuronal types in the human brain.

Disclosing the origin and diversity of Omani cattle

Among all livestock species, cattle have a prominent status as they have contributed greatly to the economy, nutrition and culture from the beginning of farming societies until the present time. The origins and diversity of local cattle breeds have been widely assessed. However, there are still some regions for which very little of their local genetic resources is known. The present work aimed to estimate the genetic diversity and the origins of Omani cattle. Located in the south-eastern corner of the Arabian Peninsula, close to the Near East, East Africa and the Indian subcontinent, the Sultanate of Oman occupies a key position, which may enable understanding cattle dispersal around the Indian Ocean. To disclose the origin of this cattle population, we used a set of 11 polymorphic microsatellites and 113 samples representing the European, African and Indian ancestry to compare with cattle from Oman. This study found a very heterogenic population with a markedly Bos indicus ancestry and with some degree of admixture with Bos taurus of African and Near East origin.

A PCR-RFLP test for simultaneous detection of two single-nucleotide insertions in the Connexin-26 gene promoter

Comparisons of Connexin-26 (GJB2) gene sequences available in the GenBank data base indicate the presence of a polymorphism in the promoter, but no easy method is available for the detection of this polymorphism. We have developed a PCR-RFLP test for simultaneous detection of two single nucleotide insertions (G and A) in the GJB2 promoter. The test is based on amplification of a 146-bp DNA fragment, which was digested with Mae I to detect the G insertion in the promoter. A similar digestion with Hinf I detects the A insertion. The test was validated using direct DNA sequencing of amplified DNA from 33 samples. After validation, we have used it to investigate DNA samples from 160 control subjects and 51 unrelated patients with nonsyndromic autosomal recessive deafness. All of the samples analyzed using the PCR test and DNA sequencing were found to contain both the G and A insertions in the GJB2 gene promoter. This PCR test will be useful in studying the prevalence of these two insertions in other populations.

Absence of deafness-associated connexin-26 (GJB2) gene mutations in the Omani population

We have investigated the prevalence of mutations in the connexin 26 (GJB2) gene in Omani population using both PCR-RFLP and direct DNA sequencing methods. Two common GJB2 gene mutations (35delG and 167delT) were screened in 280 healthy controls and 95 deaf patients using two different PCR-RFLP methods. To investigate other GJB2 mutations, we have amplified and sequenced DNA from 51 unrelated deaf patients and 17 control subjects. None of the samples studied, either by RFLP or sequencing, revealed any deafness-associated mutations in the coding region of the GJB2 gene. These findings disagree with many reports on the GJB2 gene, describing various mutations as the cause of congenital recessive deafness. Although, an amino acid substitution (S86T) was identified by sequencing, we conclude that this change could not be associated with deafness since it was present in all the control and patient samples sequenced.

Diversity in expression of glucose-6-phosphate dehydrogenase deficiency in females

The aims of this study were to determine the prevalence of glucose-6-phosphate dehydrogenase (G6PD) deficiency in the United Arab Emirates (UAE), to describe the different mutations in the population, to determine its prevalence, and to study inheritance patterns in families of G6PD-deficient individuals. All infants born at Tawam Hospital, Al-Ain, UAE from January 1994 to September 1996 were screened at birth for their G6PD status. In addition, those attending well-baby clinics during the period were also screened for the disorder. Families of 40 known G6PD-deficient individuals, selected randomly from the records of three hospitals in the country, were assessed for G6PD deficiency. Where appropriate, this was followed by definition of G6PD mutations. Of 8198 infants, 746 (9.1%), comprising 15% of males and 5% of females tested, were found to be G6PD deficient. A total of 27 families were further assessed: of these, all but one family had the nt563 Mediterranean mutation. In one family, two individuals had the nt202 African mutation. The high manifestation of G6PD deficiency in women may be due to the preferential expression of the G6PD-deficient gene and X-inactivation of the normal gene, and/or to the presence of an 'enhancer' gene that makes the expression of the G6PD deficiency more likely. The high level of consanguinity which, theoretically, should result in a high proportion of homozygotes and consequently a higher proportion of females with the deficiency, was not found to be a significant factor.