An unbiased approach de-livers unexpected insight into torsin biology

An Effort to Identify Genetic Determinants in Siblings With Wilson Disease Manifesting Striking Clinical Heterogeneity: An Exome Profiling Study of Two Indian Families

BACKGROUND

Wilson disease (WD) is a rare autosomal recessive disorder of copper metabolism caused due to mutations in the copper transporter ATP7B. There is often a striking variability of clinical manifestations among patients with ATP7B mutations, including in siblings. This phenomenon may be caused by individual differences in copper accumulation in hepatocytes and intolerance to copper toxicity as governed by genetic variations in copper metabolism genes acting as modifier loci to the disease.

OBJECTIVE

To elucidate the genetic basis of striking clinical heterogeneity among two siblings of two families with WD.

METHODS

The disease diagnosis and subsequent clinical examinations were performed by expert clinicians. The younger siblings in both families presented with early neurological manifestations at a younger age than their older siblings. Interestingly, only the younger siblings were reported to have had hepatic manifestations. Exome sequencing of all the four individuals was performed to understand their heterogeneous phenotypic outcomes.

RESULTS

Genetic screening revealed no difference in the ATP7B variant spectrum between the siblings of each family. However, the siblings of both the families were found to harbor mutually exclusive pathogenic variants in suspected modifier genes implicated in copper metabolism and/or other neurological and hepatic disorders having overlapping symptoms with WD, viz., CFTR, PPARG, ABCB11, ATP7A, CYP2D6, mTOR, TOR1A, and CP, which can potentially explain their differential clinical phenotypes.

CONCLUSION

Clinical heterogeneity between siblings with WD with the same ATP7B mutation profile may be attributed to the presence of different pathogenic variants in potential modifier genes.

Effect of Gabapentin in a Neuropathic Pain Model in Mice Overexpressing Human Wild-Type or Human Mutated Torsin A

BACKGROUND

DYT1 dystonia is the most common form of early-onset inherited dystonia, which is caused by mutation of torsin A (TA) belonging to the "ATPases associated with a variety of cellular activities" (AAA + ATPase). Dystonia is often accompanied by pain, and neuropathic pain can be associated to peripherally induced movement disorder and dystonia. However, no evidence exists on the effect of gabapentin in mice subjected to neuropathic pain model overexpressing human normal or mutated TA.

METHODS

Mice subjected to L5 spinal nerve ligation (SNL) develop mechanical allodynia and upregulation of the α2δ-1 L-type calcium channel subunit, forming a validated experimental model of neuropathic pain. Under these experimental conditions, TA is expressed in dorsal horn neurons and astrocytes and colocalizes with α2δ-1. Similar to this subunit, TA is overexpressed in dorsal horn 7 days after SNL. This model has been used to investigate (1) basal mechanical sensitivity; (2) neuropathic pain phases; and (3) the effect of gabapentin, an α2δ-1 ligand used against neuropathic pain, in non-transgenic (NT) C57BL/6 mice and in mice overexpressing human wild-type (hWT) or mutant (hMT) TA.

RESULTS

In comparison to non-transgenic mice, the threshold for mechanical sensitivity in hWT or hMT does not differ (Kruskal-Wallis test = 1.478; p = 0.4777, although, in the latter animals, neuropathic pain recovery phase is delayed. Interestingly, gabapentin (100 mg/Kg) reduces allodynia at its peak (occurring between post-operative day 7 and day 10) but not in the phase of recovery.

CONCLUSIONS

These data lend support to the investigation on the role of TA in the molecular machinery engaged during neuropathic pain.

The Role of Torsin AAA+ Proteins in Preserving Nuclear Envelope Integrity and Safeguarding Against Disease

Torsin ATPases are members of the AAA+ (ATPases associated with various cellular activities) superfamily of proteins, which participate in essential cellular processes. While AAA+ proteins are ubiquitously expressed and demonstrate distinct subcellular localizations, Torsins are the only AAA+ to reside within the nuclear envelope (NE) and endoplasmic reticulum (ER) network. Moreover, due to the absence of integral catalytic features, Torsins require the NE- and ER-specific regulatory cofactors, lamina-associated polypeptide 1 (LAP1) and luminal domain like LAP1 (LULL1), to efficiently trigger their atypical mode of ATP hydrolysis. Despite their implication in an ever-growing list of diverse processes, the specific contributions of Torsin/cofactor assemblies in maintaining normal cellular physiology remain largely enigmatic. Resolving gaps in the functional and mechanistic principles of Torsins and their cofactors are of considerable medical importance, as aberrant Torsin behavior is the principal cause of the movement disorder DYT1 early-onset dystonia. In this review, we examine recent findings regarding the phenotypic consequences of compromised Torsin and cofactor activities. In particular, we focus on the molecular features underlying NE defects and the contributions of Torsins to nuclear pore complex biogenesis, as well as the growing implications of Torsins in cellular lipid metabolism. Additionally, we discuss how understanding Torsins may facilitate the study of essential but poorly understood processes at the NE and ER, and aid in the development of therapeutic strategies for dystonia.