Biallelic mutations in nucleoporin NUP88 cause lethal fetal akinesia deformation sequence.

Nucleoporins build the nuclear pore complex (NPC), which, as sole gate for nuclear-cytoplasmic exchange, is of outmost importance for normal cell function. Defects in the process of nucleocytoplasmic transport or in its machinery have been frequently described in human diseases, such as cancer and neurodegenerative disorders, but only in a few cases of developmental disorders. Here we report biallelic mutations in the nucleoporin NUP88 as a novel cause of lethal fetal akinesia deformation sequence (FADS) in two families. FADS comprises a spectrum of clinically and genetically heterogeneous disorders with congenital malformations related to impaired fetal movement. We show that genetic disruption of nup88 in zebrafish results in pleiotropic developmental defects reminiscent of those seen in affected human fetuses, including locomotor defects as well as defects at neuromuscular junctions. Phenotypic alterations become visible at distinct developmental stages, both in affected human fetuses and in zebrafish, whereas early stages of development are apparently normal. The zebrafish phenotypes caused by nup88 deficiency are rescued by expressing wild-type Nup88 but not the disease-linked mutant forms of Nup88. Furthermore, using human and mouse cell lines as well as immunohistochemistry on fetal muscle tissue, we demonstrate that NUP88 depletion affects rapsyn, a key regulator of the muscle nicotinic acetylcholine receptor at the neuromuscular junction. Together, our studies provide the first characterization of NUP88 in vertebrate development, expand our understanding of the molecular events causing FADS, and suggest that variants in NUP88 should be investigated in cases of FADS.

Genetics of neuromuscular fetal akinesia in the genomics era.

Fetal hypokinesia or akinesia encompasses a broad spectrum of disorders, united by impaired movement in utero. Often, the underlying aetiology is genetic in origin, affecting part of the neuromuscular system. The affordable and high-throughput nature of next-generation DNA sequencing has led to an explosion in disease gene discovery across rare diseases, including fetal akinesias. A genetic diagnosis has clinical utility as it may affect management and prognosis and informs recurrence risk, facilitating family planning decisions. More broadly, knowledge of disease genes increasingly allows population-based preconception carrier screening, which has reduced the incidence of recessive diseases in several populations. Despite gains in knowledge of the genetics of fetal akinesia, many families lack a genetic diagnosis. In this review, we describe the developments in Mendelian genetics of neuromuscular fetal akinesia in the genomics era. We examine genetic diagnoses with neuromuscular causes, specifically including the lower motor neuron, peripheral nerve, neuromuscular junction and muscle.