De novo and biallelic DEAF1 variants cause a phenotypic spectrum.

PURPOSE: To investigate the effect of different DEAF1 variants on the phenotype of patients with autosomal dominant and recessive inheritance patterns and on DEAF1 activity in vitro. METHODS: We assembled a cohort of 23 patients with de novo and biallelic DEAF1 variants, described the genotype-phenotype correlation, and investigated the differential effect of de novo and recessive variants on transcription assays using DEAF1 and Eif4g3 promoter luciferase constructs. RESULTS: The proportion of the most prevalent phenotypic features, including intellectual disability, speech delay, motor delay, autism, sleep disturbances, and a high pain threshold, were not significantly different in patients with biallelic and pathogenic de novo DEAF1 variants. However, microcephaly was exclusively observed in patients with recessive variants (p < 0.0001). CONCLUSION: We propose that different variants in the DEAF1 gene result in a phenotypic spectrum centered around neurodevelopmental delay. While a pathogenic de novo dominant variant would also incapacitate the product of the wild-type allele and result in a dominant-negative effect, a combination of two recessive variants would result in a partial loss of function. Because the clinical picture can be nonspecific, detailed phenotype information, segregation, and functional analysis are fundamental to determine the pathogenicity of novel variants and to improve the care of these patients

LRP5, Bone Mass Polymorphisms and Skeletal Disorders

The formation and maintenance of the gross structure and microarchitecture of the human skeleton require the concerted functioning of a plethora of morphogenic signaling processes. Through recent discoveries in the field of genetics, numerous genotypic variants have been implicated in pathologic skeletal phenotypes and disorders arising from the disturbance of one or more of these processes. For example, total loss-of-function variants of LRP5 were found to be the cause of osteoporosis-pseudoglioma syndrome (OPPG). LRP5 encodes for the low-density lipoprotein receptor-related protein 5, a co-receptor in the canonical WNT–β-catenin signaling pathway and a crucial protein involved in the formation and maintenance of homeostasis of the human skeleton. Beyond OPPG, other partial loss-of-function variants of LRP5 have been found to be associated with other low bone mass phenotypes and disorders, while LRP5 gain-of-function variants have been implicated in high bone mass phenotypes. This review introduces the roles that LRP5 plays in skeletal morphogenesis and discusses some of the structural consequences that result from abnormalities in LRP5. A greater understanding of how the LRP5 receptor functions in bone and other body tissues could provide insights into a variety of pathologies and their potential treatments, from osteoporosis and a variety of skeletal abnormalities to congenital disorders that can lead to lifelong disabilities

The effect of rapamycin on DNA synthesis in multiple tissues from late gestation fetal and postnatal rats

Rapamycin is a potent antiproliferative agent that arrests cells in the G1 phase of the cell cycle through a variety of mechanisms involving the inhibition of the mammalian target of rapamycin (mTOR) pathway. The majority of normal cells in culture are sensitive to the cytostatic effects of rapamycin, whereas the growth of many malignant cells and tumors is rapamycin resistant. We had shown previously that hepatic DNA synthesis in the late gestation rat fetus is rapamycin resistant even though signaling through the mTOR/S6 kinase (S6K) pathway is attenuated. On the basis of this finding, we went on to characterize the response to rapamycin in a spectrum of tissues during late gestation and the early postnatal period in the rat. We found that rapamycin had no effect on DNA synthesis in major organs such as heart, intestine, and kidney in the fetal and early postnatal rat despite a marked attenuation in the phosphorylation of ribosomal protein S6. In contrast, the proliferation of mature hepatocytes during liver regeneration was highly sensitive to rapamycin. These data indicate that basal cellular proliferation in a wide variety of tissues is rapamycin resistant and occurs independently of mTOR/S6K signaling. Furthermore, the well-characterized effects of rapamycin in tissue culture systems are not recapitulated in the asynchronous cell proliferation that accompanies normal growth and tissue remodeling

Leucine restriction inhibits chondrocyte proliferation and differentiation through mechanisms both dependent and independent of mTOR signaling

Linear growth in children is sensitive to nutritional status. Amino acids, in particular leucine, have been shown to regulate cell growth, proliferation, and differentiation through the mammalian target of rapamycin (mTOR), a nutrient-sensing protein kinase. Having recently demonstrated a role for mTOR in chondrogenesis, we hypothesized that leucine restriction, acting through mTOR, would inhibit growth plate chondrocyte proliferation and differentiation. The effect of leucine restriction was compared with that of the specific mTOR inhibitor, rapamycin. Leucine restriction produced a dose-dependent inhibition of fetal rat metatarsal explant growth. This was accounted by reduced cell proliferation and hypertrophy but not apoptosis. mTOR activity, as reflected by ribosomal protein S6 phosphorylation, was only partially inhibited by leucine restriction, whereas rapamycin abolished S6 phosphorylation. In chondrogenic ATDC5 cells, leucine restriction inhibited cell number, proteoglycan accumulation, and collagen X expression despite minimal inhibition of mTOR. Microarray analysis demonstrated that the effect of leucine restriction on ATDC5 cell gene expression differed from that of rapamycin. Out of 1,571 genes affected by leucine restriction and 535 genes affected by rapamycin, only 176 genes were affected by both. These findings indicate that the decreased chondrocyte growth and differentiation associated with leucine restriction is only partly attributable to inhibition of mTOR signaling. Thus nutrient restriction appears to directly modulate bone growth through unidentified mTOR-independent mechanisms in addition to the well-characterized mTOR nutrient-sensing pathway