Defect in phosphoinositide signalling through a homozygous variant in PLCB3 causes a new form of spondylometaphyseal dysplasia with corneal dystrophy.

BACKGROUND: Bone dysplasias are a large group of disorders affecting the growth and structure of the skeletal system. METHODS: In the present study, we report the clinical and molecular delineation of a new form of syndromic autosomal recessive spondylometaphyseal dysplasia (SMD) in two Emirati first cousins. They displayed postnatal growth deficiency causing profound limb shortening with proximal and distal segments involvement, narrow chest, radiological abnormalities involving the spine, pelvis and metaphyses, corneal clouding and intellectual disability. Whole genome homozygosity mapping localised the genetic cause to 11q12.1-q13.1, a region spanning 19.32 Mb with ~490 genes. Using whole exome sequencing, we identified four novel homozygous variants within the shared block of homozygosity. Pathogenic variants in genes involved in phospholipid metabolism, such as PLCB4 and PCYT1A, are known to cause bone dysplasia with or without eye anomalies, which led us to select PLCB3 as a strong candidate. This gene encodes phospholipase C ß 3, an enzyme that converts phosphatidylinositol 4,5 bisphosphate (PIP2) to inositol 1,4,5 triphosphate (IP3) and diacylglycerol. RESULTS: The identified variant (c.2632G>T) substitutes a serine for a highly conserved alanine within the Ha2' element of the proximal C-terminal domain. This disrupts binding of the Ha2' element to the catalytic core and destabilises PLCB3. Here we show that this hypomorphic variant leads to elevated levels of PIP2 in patient fibroblasts, causing disorganisation of the F-actin cytoskeleton. CONCLUSIONS: Our results connect a homozygous loss of function variant in PLCB3 with a new SMD associated with corneal dystrophy and developmental delay (SMDCD).

Forebrain cellular bioenergetics in neonatal mice.

BACKGROUND: Hypoglycemia occurs frequently in the neonate and may result in neurologic dysfunction. Its impact on the kinetics of cellular respiration and bioenergetics in the neonatal brain remains to be explored. AIMS: Develop murine model to investigate the effects of hypoglycemia on neonatal brain bioenergetics. STUDY DESIGN: Forebrain fragments were excised from euthanized BALB/c pups aged <24 hours to 14 days. We measured cellular respiration (µM O2 min-1.mg-1) in phosphate-buffered saline with and without glucose, using phosphorescence oxygen analyzer, as well as cellular adenosine triphosphate (ATP, nmol.mg-1) using the luciferin-luciferase system. RESULTS: In the presence of glucose, although cellular respiration was 11% lower in pups ≤3 days compared to those 3- 14 days old (0.48 vs. 0.54), that difference was not statistically significant (p = 0.14). Respiration driven by endogenous metabolic fuels (without added glucose) was 16% lower in pups ≤3 days compared to those 3- 14 days (0.35 vs. 0.42, p = 0.03), confirming their increased dependency on exogenous glucose. Although cellular ATP was similar between the two age groups (14.9 vs. 11.2, p = 0.32), the ATP content was more severely depleted without added glucose in the younger pups, especially in the presence of the cytochrome c oxidase inhibitor cyanide. The first-order rate constant of cellular ATP decay (hydrolysis) was 44% lower in 2-day-old pups compared to 14-day-old mice (0.43 vs. 0.77 min-1, p = 0.03). CONCLUSIONS: Forebrain cellular respiration and ATP consumption are lower in young pups than older mice. In the absence of glucose, the support for these processes is reduced in young pups, explaining their brain hypersensitivity to hypoglycemia.

Mitochondrial oxygen consumption by the foreskin and its fibroblast-rich culture

This study investigated the feasibility of using a phosphorescence oxygen analyser to measure cellular respiration [mitochondrial O[2] consumption] in foreskin samples and their fibroblast-rich cultures. Foreskin specimens from normal infants were collected immediately after circumcision and processed for measuring cellular respiration and for culture. Cellular mitochondrial O[2] consumption was determined as a function of time from the phosphorescence decay of the Pd [II] meso-tetra-[4-sulfonatophenyl]-tetrabenzoporphyrin. In sealed vials containing a foreskin specimen and glucose, O[2] concentration decreased linearly with time, confirming the zero-order kinetics of O[2] consumption by cytochrome oxidase. Cyanide inhibited O[2] consumption, confirming that the oxidation occurred mainly in the mitochondrial respiratory chain. The rate of foreskin respiration [mean +/- SD] was 0.074 +/- 0.02 microM O[2] min[-1] mg[-1] [n = 23]. The corresponding rate for fibroblast-rich cultures was 9.84 +/- 2.43 microM O[2] min[-1] per 10[7] cells [n = 15]. Fibroblast respiration was significantly lower in a male infant with dihydrolipoamide dehydrogenase gene mutations, but normalised with the addition of thiamine or carnitine. The foreskin and its fibroblast-rich culture are suitable for assessment of cellular respiration. However, the clinical utility of foreskin specimens to detect disorders of impaired cellular bioenergetics requires further investigation

Subtelomeric chromosome rearrangements in children with unexplained mental retardation in the population of the United Arab Emirates

Cryptic subtelomeric chromosome anomalies have been recognized as a significant cause of mental retardation. Their frequency in mental retardation in the UAE population has not been determined. The aim of the study was to determine the frequency of such abnormalities in mental retardation in the UAE population. We screened 105 patients with idiopathic mental retardation who had normal G banded karyotype at 400-500 band level and where no recognizable cause for their mental retardation was identified. We detected 3 [2.8%] subtelomeric deletion/duplication that are considered disease causing. One of these anomalies was familial. We also found 2 [1.9%] small rearrangements which are probably not related to the patient s phenotype. Our results are comparable to other studies and suggest that telomere screening should be included in the list of investigation of mentally retarded children in the UAE