A Pathologist’s Approach to Nonimmune Hydrops

Prenatal mucopolysaccharidosis VII: A novel pathogenic variant identified in GUSB gene

Clinical exome sequencing is a powerful approach to overcome the wide clinical and genetic heterogeneity of mucopolysaccharidosis. These data could be useful for prenatal diagnosis of MPS VII, genetic counseling, and preimplantation genetic testing.

Severe biallelic loss-of-function mutations in nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) in two fetuses with fetal akinesia deformation sequence

The three nicotinamide mononucleotide adenylyltransferase (NMNAT) family members synthesize the electron carrier nicotinamide adenine dinucleotide (NAD⁺) and are essential for cellular metabolism. In mammalian axons, NMNAT activity appears to be required for axon survival and is predominantly provided by NMNAT2. NMNAT2 has recently been shown to also function as a chaperone to aid in the refolding of misfolded proteins. Nmnat2 deficiency in mice, or in its ortholog dNmnat in Drosophila, results in axon outgrowth and survival defects. Peripheral nerve axons in NMNAT2-deficient mice fail to extend and innervate targets, and skeletal muscle is severely underdeveloped. In addition, removing NMNAT2 from established axons initiates axon death by Wallerian degeneration. We report here on two stillborn siblings with fetal akinesia deformation sequence (FADS), severely reduced skeletal muscle mass and hydrops fetalis. Clinical exome sequencing identified compound heterozygous NMNAT2 variant alleles in both cases. Both protein variants are incapable of supporting axon survival in mouse primary neuron cultures when overexpressed. In vitro assays demonstrate altered protein stability and/or defects in NAD⁺ synthesis and chaperone functions. Thus, both patient NMNAT2 alleles are null or severely hypo-morphic. These data indicate a previously unknown role for NMNAT2 in human neurological development and provide the first direct molecular evidence to support the involvement of Wallerian degeneration in a human axonal disorder. SIGNIFICANCE: Nicotinamide Mononucleotide Adenylyltransferase 2 (NMNAT2) both synthesizes the electron carrier Nicotinamide Adenine Dinucleotide (NAD⁺) and acts a protein chaperone. NMNAT2 has emerged as a major neuron survival factor. Overexpression of NMNAT2 protects neurons from Wallerian degeneration after injury and declining levels of NMNAT2 have been implicated in neurodegeneration. While the role of NMNAT2 in neurodegeneration has been extensively studied, the role of NMNAT2 in human development remains unclear. In this work, we present the first human variants in NMNAT2 identified in two fetuses with severe skeletal muscle hypoplasia and fetal akinesia. Functional studies in vitro showed that the mutations impair both NMNAT2 NAD⁺ synthase and chaperone functions. This work identifies the critical role of NMNAT2 in human development.

Lysosomal Storage Disorders in Nonimmune Hydrops Fetalis (NIHF): An Indian Experience

Lysosomal storage disorders (LSD) are rare inherited neurovisceral inborn errors of metabolism which may present as nonimmune hydrops fetalis (NIHF) during pregnancy. Although causes of NIHF are highly diverse, LSDs are one of the underlying causes of NIHF. The aim of this study was to elucidate most frequent causes of LSDs presenting as NIHF in Indian population. Several fetal tissues were investigated for enzymatic diagnosis of LSDs using modified fluorometric assays in the current prospective study carried out at our national tertiary center from 2006 through 2016. Other general causes of NIHF were ruled out. Twenty-one percent (7/33) of cases were confirmed to have LSDs. Two patients were diagnosed with Hurler syndrome; two had Sly syndrome and one each of Niemann-Pick disease type A/B, Gaucher's disease, and mucolipidosis. Four of eleven cases (36%) with recurrent NIHF were found to have LSDs. In spite of extreme rarity of LSDs, they should be considered as a potential cause of NIHF, especially with recurrent NIHF. Specific investigations of LSD leading to definitive diagnosis may aid the clinician in providing accurate genetic counseling and prenatal diagnosis to the patients and help in subsequent pregnancies to the families. Furthermore, early intervention and management with enzyme replacement therapy may be planned for the lysosomal storage disorders where available.