Sunday, August 18, 2019

A Dobyns potpourri

Holt RJ, Young RM, Crespo B, Ceroni F, Curry CJ, Bellacchio E, Bax DA, Ciolfi A, Simon M, Fagerberg CR, van Binsbergen E, De Luca A, Memo L, Dobyns WB, Mohammed AA, Clokie SJH, Zazo Seco C, Jiang YH, Sørensen KP, Andersen H, Sullivan J, Powis Z, Chassevent A, Smith-Hicks C, Petrovski S, Antoniadi T, Shashi V, Gelb BD, Wilson SW, Gerrelli D, Tartaglia M, Chassaing N, Calvas P, Ragge NK. De Novo Missense Variants in FBXW11 Cause Diverse Developmental Phenotypes Including Brain, Eye, and Digit Anomalies. Am J Hum Genet. 2019
Jul 23. pii: S0002-9297(19)30268-X. doi: 10.1016/j.ajhg.2019.07.005. [Epub ahead of print]

The identification of genetic variants implicated in human developmental disorders has been revolutionized by second-generation sequencing combined with international pooling of cases. Here, we describe seven individuals who have diverse yet overlapping developmental anomalies, and who all have de novo missense FBXW11 variants identified by whole exome or whole genome sequencing and not reported in the gnomAD database. Their phenotypes include striking neurodevelopmental, digital, jaw, and eye anomalies, and in one individual, features resembling Noonan syndrome, a condition caused by dysregulated RAS signaling. FBXW11 encodes an F-box protein, part of the Skp1-cullin-F-box (SCF) ubiquitin ligase complex, involved in ubiquitination and proteasomal degradation and thus fundamental to many protein regulatory processes. FBXW11 targets include β-catenin and GLI transcription factors, key mediators of Wnt and Hh signaling, respectively, critical to digital, neurological, and eye development. Structural analyses indicate affected residues cluster at the surface of the loops of the substrate-binding domain of FBXW11, and the variants are predicted to destabilize the protein and/or its interactions. In situ hybridization studies on human and zebrafish embryonic tissues demonstrate FBXW11 is expressed in the developing eye, brain, mandibular processes, and limb buds or pectoral fins. Knockdown of the zebrafish FBXW11 orthologs fbxw11a and fbxw11b resulted in embryos with smaller, misshapen, and underdeveloped eyes and abnormal jaw and pectoral fin development. Our findings support the role of FBXW11 in multiple developmental processes, including those involving the brain, eye, digits, and jaw.

Kanca O, Andrews JC, Lee PT, Patel C, Braddock SR, Slavotinek AM, Cohen JS, Gubbels CS, Aldinger KA, Williams J, Indaram M, Fatemi A, Yu TW, Agrawal PB, Vezina G, Simons C, Crawford J, Lau CC; Undiagnosed Diseases Network, Chung WK, Markello TC, Dobyns WB, Adams DR, Gahl WA, Wangler MF, Yamamoto S, Bellen HJ, Malicdan MCV. De Novo Variants in WDR37 Are Associated with Epilepsy, Colobomas, Dysmorphism, Developmental Delay, Intellectual Disability, and Cerebellar Hypoplasia. Am J Hum Genet. 2019 Aug 1;105(2):413-424.

WD40 repeat-containing proteins form a large family of proteins present in all eukaryotes. Here, we identified five pediatric probands with de novo variants in WDR37, which encodes a member of the WD40 repeat protein family. Two probands shared one variant and the others have variants in nearby amino acids outside the WD40 repeats. The probands exhibited shared phenotypes of epilepsy, colobomas, facial dysmorphology reminiscent of CHARGE syndrome, developmental delay and intellectual disability, and cerebellar hypoplasia. The WDR37 protein is highly conserved in vertebrate and invertebrate model organisms and is currently not associated with a human disease. We generated a null allele of the single Drosophila ortholog to gain functional insights and replaced the coding region of the fly gene CG12333/wdr37 with GAL4. These flies are homozygous viable but display severe bang sensitivity, a phenotype associated with seizures in flies. Additionally, the mutant flies fall when climbing the walls of the vials, suggesting a defect in grip strength, and repeat the cycle of climbing and falling. Similar to wall clinging defect, mutant males often lose grip of the female abdomen during copulation. These phenotypes are rescued by using the GAL4 in the CG12333/wdr37 locus to drive the UAS-human reference WDR37 cDNA. The two variants found in three human subjects failed to rescue these phenotypes, suggesting that these alleles severely affect the function of this protein. Taken together, our data suggest that variants in WDR37 underlie a novel syndromic neurological disorder.

Monteiro FP, Curry CJ, Hevner R, Elliott S, Fisher JH, Turocy J, Dobyns WB, Costa LA, Freitas E, Kitajima JP, Kok F. Biallelic loss of function variants in ATP1A2 cause hydrops fetalis, microcephaly, arthrogryposis and extensive cortical malformations. Eur J Med Genet. 2019 Jan 25. pii: S1769-7212(18)30802-4. doi:10.1016/j.ejmg.2019.01.014. [Epub ahead of print]

The Na+/K+- ATPase acts as an ion pump maintaining the essential plasma membrane potential in all mammalian cell types, and is essential for many cellular functions. There are four α isoforms (α1, α2, α3 and α4) with distinct expression patterns, kinetic properties and substrate affinity. The α2-isoform is encoded by ATP1A2 and evidence supports its utmost importance in Cl- homeostasis in neurons, and in the function of respiratory neurons at birth. Monallelic pathogenic variants in ATP1A2 are associated with familial hemiplegic migraine type 2 (FHM2) and on rare occasions with alternating hemiplegia of childhood 1 (AHC1). To date, no instances of biallelic loss of function variants have been reported in humans. However, Atp1a2 homozygous loss of function knockout mice (α2-/- mice) show severe motor deficits, with lack of spontaneous movements, and are perinatally lethal due to absent respiratory activity. In this report we describe three newborns from two unrelated families, who died neonatally, presenting in utero with an unusual form of fetal hydrops, seizures and polyhydramnios. At birth they had multiple joint contractures (e.g. arthrogryposis), microcephaly, malformations of cortical development, dysmorphic features and severe respiratory insufficiency. Biallelic loss of function variants in ATP1A2, predicted to be pathogenic were found on whole exome sequencing. We propose that this is a distinctive new syndrome caused by complete absence of Na+/K+- ATPase α2-isoform expression.

1 comment:

  1. Dobyns WB, Mirzaa GM. Megalencephaly syndromes associated with mutations of core components of the PI3K-AKT-MTOR pathway: PIK3CA, PIK3R2, AKT3, and MTOR. Am J Med Genet C Semin Med Genet. 2019 Aug 23. doi: 10.1002/ajmg.c.31736. [Epub ahead of print]

    Megalencephaly (MEG) is a developmental abnormality of brain growth characterized by early onset, often progressive, brain overgrowth. Focal forms of megalencephaly associated with cortical dysplasia, such as hemimegalencephaly and focal cortical dysplasia, are common causes of focal intractable epilepsy in children. The increasing use of high throughput sequencing methods, including high depth sequencing to more accurately detect and quantify mosaic mutations, has allowed us to identify the molecular etiologies of many MEG syndromes, including most notably the PI3K-AKT-MTOR related MEG disorders. Thorough molecular and clinical characterization of affected individuals further allow us to derive preliminary genotype-phenotype correlations depending on the gene, mutation, level of mosaicism, and tissue distribution. Our review of published data on these disorders so far shows that mildly activating variants (that are typically constitutional or germline) are associated with diffuse megalencephaly with intellectual disability and/or autism spectrum disorder; moderately activating variants (that are typically high-level mosaic) are associated with megalencephaly with pigmentary abnormalities of the skin; and strongly activating variants (that are usually very low-level mosaic) are associated with focal brain malformations including hemimegalencephaly and focal cortical dysplasia. Accurate molecular diagnosis of these disorders is undoubtedly crucial to more optimally treat children with these disorders using PI3K-AKT-MTOR pathway inhibitors.