Page 88 - GPD-4-2

Gene & Protein in Disease Regulatory elements of ATP7B in WD

transporting P-type adenosine triphosphatase (ATPase). machinery, alter chromatin structure, and ensure finely
3
ATP7B encodes a protein with several membrane- tuned transcriptional regulation.
spanning domains, an ATPase consensus sequence, a The advent of the Encyclopedia of DNA Elements
hinge domain, a phosphorylation site, and at least two (ENCODE) project has led to a paradigm shift in
putative copper-binding sites. This monomeric protein understanding the causality of genetic diseases. Variants in
functions as a copper-transporting ATPase, exporting cis-regulatory elements (proximal or distal) are increasingly
copper from cells, such as from the liver into the bile. implicated in diseases such as cancer. 22,23 Moreover,
Over 600 pathogenic mutations have been identified in alterations in non-coding cis-regulatory elements are
ATP7B, either as homozygous or compound heterozygous increasingly recognized as the cause of monogenic diseases,
variants. The frequency of these mutations varies among which were once thought to be caused by mutations within
4-7
different ethnicities. WD patients often present with a coding regions. Mutations or alterations in enhancers,
8
variable age of onset and a differential spectrum of hepatic promoters, silencers, and insulators can perturb normal
and neurological symptoms. 1,9,10 Even affected siblings gene expression and contribute to disease pathogenesis.
within the same family can exhibit different clinical For instance, mutations in enhancers of the SOX9 gene
phenotypes. 11 have been implicated in campomelic dysplasia, a severe
24
Other inherited disorders, such as MEDNIK skeletal disorder. Similarly, mutations within the promoter
syndrome, 12 Huppke–Brendel syndrome, MDR3 of the HBB gene, encoding β-globin, result in reduced
13
deficiency, aceruloplasminemia, congenital defects gene expression and are associated with certain forms of
14
15
25
in glycosylation, manganese retention disorder, and β-thalassemia. In addition, dysregulation of PAX6 due
16
childhood cirrhosis share striking clinical features with to mutations in its silencer elements is linked to aniridia,
26
WD. Hence, molecular genetic analysis is crucial a congenital eye disorder. These findings underscore
17
to mitigate diagnostic challenges. However, despite the importance of cis-regulatory elements in maintaining
comprehensive genetic analyses, including sequencing of gene expression fidelity and highlight their role as key
exons and splice-sites and multiplex ligation-dependent contributors to the etiology of monogenic diseases.
probe amplification studies to identify heterozygous Extensive genetic evaluation of WD patients from the
indels, approximately 10 – 15% of WD patients worldwide eastern and western parts of India (including unpublished
lack mutations in both alleles of the ATP7B gene or have data from our lab) has identified a plethora of mutations
only a single mutated allele. 18,19 A recent review on the in the ATP7B gene, 18,27-30 with p.Cys271* emerging as the
missing heritability of WD has discussed the potential most prevalent mutation. However, consistent with global
29
locations of these uncharacterized mutations, including reports, some patients have either no detectable mutation or
20
cis-regulatory element(s) of ATP7B, where a disruptive only a single mutated allele. Hence, we propose that mutations
mutation can significantly affect gene expression. in the cis-regulatory elements of ATP7B may represent the
Alternatively, the disease phenotype might arise from the uncharacterized mutations that could partially explain the
cumulative effect of hypofunctional alleles in the other key missing heritability of WD. While a few studies have screened
genes of the copper metabolism pathway. In addition, the the promoter of ATP7B, 18,31-33 to the best of our knowledge,
generation of reactive oxygen species and cuproptosis (a none has looked into the distal cis-elements comprehensively.
copper-dependent cell death) has been recently proposed Notably, Sardinian patients have been reported to have a
as a molecular basis for WD endophenotypes. Increased 15-bp deletion in the promoter region of the ATP7B gene. 31
iron deposits in the liver of WD patients may also lead Wan et al. reported two variants upstream of ATP7B that
34
to iron-related ferroptosis, contributing to phospholipid significantly reduced promoter activity. Mukherjee et al.
34
18
21
peroxidation within subcellular organelle membranes. also reported promoter variants in the Indian WD patients
Thus, hypofunctional alleles in genes related to the altering the expression of ATP7B. At the same time, Chen
18
cuproptosis/ferroptosis pathway may also impact the WD et al. showed that promoter mutations of ATP7B disrupted
35
phenotype. the binding of metal regulatory transcription factors, leading
35
Recent advances in genomics, microscopy to diminished gene transcription. These studies suggest that
methodologies, and genome editing tools have unveiled transcriptional variation of ATP7B could be causal to WD,
the crucial role of enhancer sequences in gene expression. as insufficient functional ATP7B proteins can disrupt cellular
These enhancers interact with promoter sequences by copper homeostasis.
forming chromatin loops, enabling precise spatial and Thus, our study aimed to identify the uncharacterized
temporal control of gene expression. Through these three- mutations in the cis-regulatory elements of ATP7B in a
dimensional interactions, enhancers recruit transcriptional subset of WD patients with missing heritability, those

Volume 4 Issue 2 (2025) 2 doi: 10.36922/gpd.7503