Characteristics of MPXV
MPXV has the typical rectangular shape and size of other poxviruses such
as Smallpox, with a size of approximately 200 × 250 nm [10]. It has
a dumbbell-shaped core with lateral bodies and the surface membranes
show surface tubules or filaments [11] (Fig. 2). MPXV is an
enveloped double-stranded DNA virus with approximately 197kb of genomic
DNA encoding up to 280 genes and contains about 190 nonoverlapping open
reading frames (ORFs, >180 nucleotides) [12,13]. As
shown in Fig. 3, the genome of MPXV consists of a large conserved
central genomic region with variable regions containing inverted
terminal repeats (ITRs) at both ends [14]. Except for four ORFs
located in the ITRs, all ORFs are present in the central region of the
genome[15]. The non-conserved genes of MPXV, located in the variable
regions at both ends, are most host range and virulence genes, which are
mainly involved in MPXV immune escape [16,17]. In contrast, the
genes in the central genomic region of MPXV are relatively conserved and
encode structural proteins and essential enzymes [12]. The
genome-wide analysis confirmed that there are two clades of MPXV: the
Central African (Congo Basin) clade and the West African clade [18].
However, to avoid discriminatory geographical identification, the WHO
announced the renaming of the MPXV clades on August 12, 2022, with new
names consisting of Roman numerals and lowercase English letters. The
experts convened by WHO referred to the previous Central African clade
as Clade I and the previous West African clade as Clade II. Clade II
includes two subclades, IIa and IIb, with IIb being the main group of
virus variants transmitted in the 2022 Mpox outbreak. [19,20]. Clade
I and Clade II are genetically different and have different toxicity and
case fatality rate, 10.6% and 3.6%, respectively [18,21–23]. The
genomic comparison revealed that the nucleotide divergence between the
two clades was 0.55-0.56%. The Clade I had 173 unique functional genes,
while the Clade II had 171. Of the 56 virulence genes, 53 viral genes
were shared by the two clades. The most striking differences between
these two clades are in the gene orthogonal of BR-203, BR-209, and
COP-C3L [21].