Novel pathogenic variants in GBE1 causing fetal akinesia deformation sequence and severe neuromuscular form of glycogen storage disease type IV.

Glycogen storage disease IV (GSD IV), caused by a defect in GBE1, is a clinically heterogeneous disorder. A classical hepatic form and a neuromuscular form have been described. The severe neuromuscular form presents as a fetal akinesia deformation sequence or a congenital subtype. We ascertained three unrelated families with fetuses/neonates who presented with fetal akinesia deformation sequence to our clinic for genetic counseling. We performed a detailed clinical evaluation, exome sequencing, and histopathology examination of two fetuses and two neonates from three unrelated families presenting with these perinatally lethal neuromuscular forms of GSD IV. Exome sequencing in the affected fetuses/neonates identified four novel pathogenic variants (c.1459G>T, c.144-1G>A, c.1680C>G, and c.1843G>C) in GBE1 (NM_000158). Histopathology examination of tissues from the affected fetuses/neonate was consistent with the diagnosis. Here, we add three more families with the severe perinatally lethal neuromuscular forms of GSD IV to the GBE1 mutation spectrum.

Disentangling mechanisms involved in collagen pyridinoline cross-linking: The immunophilin FKBP65 is critical for dimerization of lysyl hydroxylase 2.

Collagens are subjected to extensive posttranslational modifications, such as lysine hydroxylation. Bruck syndrome (BS) is a connective tissue disorder characterized at the molecular level by a loss of telopeptide lysine hydroxylation, resulting in reduced collagen pyridinoline cross-linking. BS results from mutations in the genes coding for lysyl hydroxylase (LH) 2 or peptidyl-prolyl cis-trans isomerase (PPIase) FKBP65. Given that the immunophilin FKBP65 does not exhibit LH activity, it is likely that LH2 activity is somehow dependent on FKPB65. In this report, we provide insights regarding the interplay between LH2 and FKBP65. We found that FKBP65 forms complexes with LH2 splice variants LH2A and LH2B but not with LH1 and LH3. Ablating the catalytic activity of FKBP65 or LH2 did not affect complex formation. Both depletion of FKBP65 and inhibition of FKBP65 PPIase activity reduced the dimeric (active) form of LH2 but did not affect the binding of monomeric (inactive) LH2 to procollagen Iα1. Furthermore, we show that LH2A and LH2B cannot form heterodimers with each other but are able to form heterodimers with LH1 and LH3. Collectively, our results indicate that FKBP65 is linked to pyridinoline cross-linking by specifically mediating the dimerization of LH2. Moreover, FKBP65 does not interact with LH1 and LH3, explaining why in BS triple-helical hydroxylysines are not affected. Our results provide a mechanistic link between FKBP65 and the loss of pyridinolines and may hold the key to future treatments for diseases related to collagen cross-linking anomalies, such as fibrosis and cancer.

Germline recessive mutations in PI4KA are associated with perisylvian polymicrogyria, cerebellar hypoplasia and arthrogryposis.

Polymicrogyria (PMG) is a structural brain abnormality involving the cerebral cortex that results from impaired neuronal migration and although several genes have been implicated, many cases remain unsolved. In this study, exome sequencing in a family where three fetuses had all been diagnosed with PMG and cerebellar hypoplasia allowed us to identify regions of the genome for which both chromosomes were shared identical-by-descent, reducing the search space for causative variants to 8.6% of the genome. In these regions, the only plausibly pathogenic mutations were compound heterozygous variants in PI4KA, which Sanger sequencing confirmed segregated consistent with autosomal recessive inheritance. The paternally transmitted variant predicted a premature stop mutation (c.2386C>T; p.R796X), whereas the maternally transmitted variant predicted a missense substitution (c.5560G>A; p.D1854N) at a conserved residue within the catalytic domain. Functional studies using expressed wild-type or mutant PI4KA enzyme confirmed the importance of p.D1854 for kinase activity. Our results emphasize the importance of phosphoinositide signalling in early brain development.

ARC syndrome with high GGT cholestasis caused by VPS33B mutations.

Arthrogryposis, renal dysfunction and cholestasis (ARC) syndrome (OMIM 208085) is an autosomal recessive disorder that is caused by mutations in 2 interacting genes VPS33B and VIPAS39. Mutations in VPS33B gene account for most cases of ARC. As low or normal gamma-glutamyl transpeptidase (GGT) activity has been described in all patients with ARC syndrome identified so far, ARC syndrome is a possible diagnosis for low GGT cholestasis. Here we describe a Chinese patient with neonatal cholestasis and a high GGT level in three consecutive tests. She had other typical manifestations of ARC syndrome, including arthrogryposis multiplex congenita, renal involvement and ichthyosis. Genetic study of the VPS33B gene further confirmed the diagnosis by identification of compound heterozygosity of two known disease-causing mutations, c.403+2T > A and c.1509-1510insG. The mechanism of high GGT in this patient is unclear. Nevertheless, this case indicates that ARC syndrome cannot be excluded from the differential diagnosis of neonatal cholestasis even if high GGT activity is found.

Lethal congenital contractural syndrome type 2 (LCCS2) is caused by a mutation in ERBB3 (Her3), a modulator of the phosphatidylinositol-3-kinase/Akt pathway.

Lethal congenital contractural syndrome type 2 (LCCS2) is an autosomal recessive neurogenic form of arthrogryposis that is associated with atrophy of the anterior horn of the spinal cord. We previously mapped LCCS2 to 6.4 Mb on chromosome 12q13 and have now narrowed the locus to 4.6 Mb. We show that the disease is caused by aberrant splicing of ERBB3, which leads to a predicted truncated protein. ERBB3 (Her3), an activator of the phosphatidylinositol-3-kinase/Akt pathway--regulating cell survival and vesicle trafficking--is essential for the generation of precursors of Schwann cells that normally accompany peripheral axons of motor neurons. Gain-of-function mutations in members of the epidermal growth-factor tyrosine kinase-receptor family have been associated with predilection to cancer. This is the first report of a human phenotype resulting from loss of function of a member of this group.

Mitochondrial complex I deficiency in a female with multiplex arthrogryposis congenita.

A 10-year-old female with arthrogryposis multiplex congenita is presented. Clinical, neurophysiologic, and histologic findings suggested a mild myopathy. The analysis of enzymatic activity in the homogenate and of mitochondrial function in saponin-permeabilized fibers from the muscle biopsy revealed an approximately twofold-decreased specific activity of the NADH:CoQ oxidoreductase (complex I of the mitochondrial respiratory chain) that was compensated for by an increased number of mitochondria. The complex I deficiency was also detected in cultivated skin fibroblasts of the patient. The observed defect of mitochondrial oxidative phosphorylation in arthrogryposis multiplex congenita may be of pathogenetic relevance.

Arthrogryposis multiplex congenita. 2. Muscle pathology and pathogenesis.

Pathological findings are reported on 34 specimens from 16 cases of arthrogryposis multiplex congenita (AMC), including initial observations on paraffin sections from 28 muscles, and subsequent observations on six additional specimens from three of these cases studied both histologically and histochemically. Thirteen of the 34 specimens (from 11 cases) were histologically normal, probably on account of an unaffected muscle being sampled. The most constant pathological feature in the remaining specimens was a disorganization of the muscle fibres and fascicles by severe fibrosis; only three specimens (from two cases) did not show this. Very thin faintly striated muscle fibres embedded in this matrix were encountered in 10 specimens from nine cases. An attempt at grouping of these atrophic or ill-developed fibres was noticed in four specimens; but this may not be denervation atrophy. Two specimens (from two cases) showed `myopathic' features. Repeat biopsy after two to three years was carried out on two affected muscles each from three patients. Case 3 showed well preserved but uniformly small fibres. Case 4 showed extremely few and scattered small rounded fibres. Case 14 showed pronounced variation of fibre size in both, with both atrophic and hypertrophied fibres. Normal nerves and spindles were seen in all these six specimens irrespective of the state of the muscle, and excessive fibro-fatty tissue in cases 4 and 14. Histochemical examination for oxidative enzymes, ATPase, and phosphorylase in these six specimens revealed a normal checkerboard pattern and ratio of type I and type II fibres, in case 3 only. The muscles of case 4 showed a preponderance of type I fibres. One specimen from case 14, showed the same fibres reacting for both oxidative enzymes and phosphorylase, suggesting a lack of development of fibres. The intrafusal fibres were mainly of type I in all. Two possible pathogenetic mechanisms operating in early embryonic life, which may lead to the characteristic changes of AMC, are discussed: (1) a defect in the development of the muscle whereby the full recruitment of myoblasts from the mesenchyme of the limb-bud does not take place and muscles do not form adequately; (2) a lack of innervation of the muscles on account of arrested growth of anterior horn cells. The combined operation of both these mechanisms is also considered. Fibrous tissue replaces the muscle tissue that is lacking, and contractures and deformities ensue. The evidence gathered on our material, such as the very thin smooth muscle fibres, the large numbers of well-formed nerves and spindles especially in the repeat biopsies, and the above-mentioned histochemical feature, would appear to favour the hypothesis of ill-developed muscles in the production of AMC in the majority; in the rest denervation playing either a major or concurrent role.