We describe a novel nephrocerebellar syndrome on the Galloway-Mowat syndrome spectrum among 30 children (ages 1.0 to 28years) from diverse Amish demes. Children with nephrocerebellar syndrome had progressive microcephaly, visual impairment,stagnant psychomotor development, abnormal extrapyramidal movements and nephrosis. Fourteen died between ages 2.7 and 28years, typically from renal failure. Post-mortem studies revealed (i) micrencephaly without polymicrogyria or heterotopia;(ii) atrophic cerebellar hemispheres with stunted folia, profound granule cell depletion, Bergmann gliosis, and signs of Purkinjecell deafferentation; (iii) selective striatal cholinergic interneuron loss; and (iv) optic atrophy with delamination of the lateralgeniculate nuclei. Renal tissue showed focal and segmental glomerulosclerosis and extensive effacement and microvillus transformation of podocyte foot processes. Nephrocerebellar syndrome mapped to 700 kb on chromosome 15, which contained a singlenovel homozygous frameshift variant (WDR73 c.888delT; p.Phe296Leufs*26). WDR73 protein is expressed in human cerebralcortex, hippocampus, and cultured embryonic kidney cells. It is concentrated at mitotic microtubules and interacts with a-, b-, and-tubulin, heat shock proteins 70 and 90 (HSP-70; HSP-90), and the carbamoyl phosphate synthetase 2/aspartate transcarbamylase/dihydroorotase multi-enzyme complex. Recombinant WDR73 p.Phe296Leufs*26 and p.Arg256Profs*18 proteins are truncated, unstable, and show increased interaction with a- and b-tubulin and HSP-70/HSP-90. Fibroblasts from patients homozygousfor WDR73 p.Phe296Leufs*26 proliferate poorly in primary culture and senesce early. Our data suggest that in humans, WDR73interacts with mitotic microtubules to regulate cell cycle progression, proliferation and survival in brain and kidney. We extend theGalloway-Mowat syndrome spectrum with the first description of diencephalic and striatal neuropathology.

We review clinical presentation, disease course, treatment, and outcomes of a genetically homogenous population of HSD3B2-deficient patients.

Intellectual disability (ID) affects approximately 1%–3% of humans with a gender bias toward males. Previous studies have identifiedmutations in more than 100 genes on the X chromosome in males with ID, but there is less evidence for de novo mutations on theX chromosome causing ID in females. In this study we present 35 unique deleterious de novo mutations in DDX3X identified by wholeexome sequencing in 38 females with ID and various other features including hypotonia, movement disorders, behavior problems,corpus callosum hypoplasia, and epilepsy. Based on our findings, mutations in DDX3X are one of the more common causes of ID,accounting for 1%–3% of unexplained ID in females. Although no de novo DDX3X mutations were identified in males, we present threefamilies with segregating missense mutations in DDX3X, suggestive of an X-linked recessive inheritance pattern. In these families, allmales with the DDX3X variant had ID, whereas carrier females were unaffected. To explore the pathogenic mechanisms accountingfor the differences in disease transmission and phenotype between affected females and affected males with DDX3X missense variants,we used canonical Wnt defects in zebrafish as a surrogate measure of DDX3X function in vivo. We demonstrate a consistent loss-of-function effect of all tested de novo mutations on the Wnt pathway, and we further show a differential effect by gender. The differentialactivity possibly reflects a dose-dependent effect of DDX3X expression in the context of functional mosaic females versus one-copymales, which reflects the complex biological nature of DDX3X mutations.

Genetic testing is routinely used for second-tier confirmation of newborn sequencing results to rule out false positives and to confirm diagnoses in newborns undergoing inpatient and outpatient care. We developed a targeted next-generation sequencing panel coupled with a variant processing pipeline and demonstrated utility and performance benchmarks across multiple newborn disease presentations in a retrospective clinical study.

GM3 synthase (ST3GAL5) is the first biosynthetic enzyme of a- and b-series gangliosides. Patients with GM3 synthase deficiency suffer severe neurological disability and deafness. Eight children (ages 4.1 ± 2.3 years) homozygous for ST3GAL5 c.694C>T had no detectable GM3 (a-series) or GD3 (b-series) in plasma. Their auditory function was characterized by the absence of middle ear muscle reflexes, distortion product otoacoustic emissions and cochlear microphonics, as well as abnormal auditory brainstem responses and cortical auditory-evoked potentials. In St3gal5−/−mice, stereocilia of outer hair cells showed signs of degeneration as early as postnatal Day 3 (P3); thereafter, blebs devoid of actin or tubulin appeared at the region of vestigial kinocilia, suggesting impaired vesicular trafficking. Stereocilia of St3gal5−/− inner hair cells were fused by P17, and protein tyrosine phosphatase receptor Q, normally linked to myosin VI at the tapered base of stereocilia, was maldistributed along the cell membrane. B4galnt1−/− (GM2 synthase-deficient) mice expressing only GM3 and GD3 gangliosides had normal auditory structure and function. Thus, GM3-dependent membrane microdomains might be essential for the proper organization and maintenance of stereocilia in auditory hair cells.

Cartilage hair hypoplasia (CHH) is a rare metaphyseal chondrodysplasia characterized by short stature and short limbs, found primarily in Amish and Finnish populations. Cartilage hair hypoplasia is caused by mutations in the RMRP gene located on chromosome 9p13.3. The disorder has several characteristic orthopaedic manifestations, including joint laxity, limited elbow extension, ankle varus, and genu varum. Immunodeficiency is of concern in most cases. Although patients exhibit orthopaedic problems, the orthopaedic literature on CHH patients is scant at best. The objective of this study was to characterize the orthopaedic manifestations of CHH based on the authors’ unique access to the largest collection of CHH patients ever reported.

CODAS syndrome is a multi-system developmental disorder characterized by cerebral, ocular, dental, auricular, and skeletal anomalies. Using whole-exome and Sanger sequencing, we identified four LONP1 mutations inherited as homozygous or compound-heterozygous combinations among ten individuals with CODAS syndrome. The individuals come from three different ancestral backgrounds (AmishSwiss from United States, n ¼ 8; Mennonite-German from Canada, n ¼ 1; mixed European from Canada, n ¼ 1). LONP1 encodes Lon protease, a homohexameric enzyme that mediates protein quality control, respiratory-complex assembly, gene expression, and stress responses in mitochondria. All four pathogenic amino acid substitutions cluster within the AAAþ domain at residues near the ATP-binding pocket. In biochemical assays, pathogenic Lon proteins show substrate-specific defects in ATP-dependent proteolysis. When expressed recombinantly in cells, all altered Lon proteins localize to mitochondria. The Old Order Amish Lon variant (LONP1 c.2161C>G[p.Arg721Gly]) homo-oligomerizes poorly in vitro. Lymphoblastoid cell lines generated from affected children have (1) swollen mitochondria with electron-dense inclusions and abnormal inner-membrane morphology; (2) aggregated MT-CO2, the mtDNA-encoded subunit II of cytochrome c oxidase; and (3) reduced spare respiratory capacity, leading to impaired mitochondrial proteostasis and function. CODAS syndrome is a distinct, autosomal-recessive, developmental disorder associated with dysfunction of the mitochondrial Lon protease.