Discussion
MODY9 is a rare type of MODY caused by a PAX4 gene mutation. In
vertebrates, the PAX family consists of nine members, which can be
divided into four subgroups according to the specific composition of the
structural domain. Among them, the subfamily IV (PAX4 and PAX6) is
composed of paired and homologous domains [12]. PAX4 gene is located
on 7q32.1 and contains 10 exons and 9 introns with a molecular weight of
7,851bp. Its encoding product is the paired box protein PAX-4. PAX-4
protein contains a paired domain (PD) and a homeobox domain (HD), both
of which are potential DNA-binding domains (DBDS)
[13]. PAX4 protein is widely distributed, mainly expressed in islet
pancreatic cells, and plays an important regulatory role in the
production of islet progenitor cells and the differentiation of isletβ -cells and islet δ -cells [11,14]. It can inhibit the
expression of α -cell genes and promote the connection of
endocrine cells with β- cells and δ -cells. As a key
regulator of mammalian pancreatic development, PAX4 protein first
appeared in endocrine progenitor cells at 9.5 days of embryos and then
is selectively expressed in β -cells [15]. In β -cells,
PAX4 protein can maintain the expression of pancreatic and duodenal
homeobox 1, Pdx1 and NK6 homeobox 1, Nkx 6.1, both of which are
important regulators of pancreatic β -cell development [16].
In animal experiments, heterozygous PAX4 knockout mice had almost no
mature β -cells and δ -cells, and a large number of abnormalα -cells were found. All of which could lead to an increase in
blood glucose levels, suggesting that PAX4 is a key regulator in the
differentiation process of precursor cells into different islet cell
[17]. Other research data showed that PAX4 homogenous variation can
cause the destruction of PAX4 protein’s targeting ability, resulting in
an reduced inhibitory function and an increase in α -cells, which
promotes glucagon production and leads to increased blood sugar
[18,19].
In 2007, Nattachet Plengvidhya et al. studied the pathogenic genes of 46
MODY probands (all Thai origin) applying DNA direct sequencing
techniques [10]. They reported a missense mutation of the PAX4 gene
with a variation of R164W and a shear mutation of PAX4 gene responsible
for a IVS7-1variation in two diabetic families, respectively, being the
first two reports on PAX4 mutation in relation to MODY and was defined
as MODY9 [10]. Nattachet Plengvidhya et al. found that 4 members of
the diabetic family with the R164W variation carried the same mutation.
Three of which suffered from diabetes. The onset age of the proband was
20, while in the other two cases it was 29 and 50. No ketosis, nor
ketoacidosis were detected at the onset of the disease. Another family
member developed impaired glucose tolerance at age 14, which did not
progress to diabetes. Patients in this group were treated by diet
control or oral hypoglycemic drugs, and their blood glucose level were
well controlled. 6 members of the diabetic family with IVS7-1 variation
presented with diabetes, of which only the proband underwent gene
sequencing analysis and was found to carry the mutation. Of the 6
members with diabetes, 1 developed retinopathy and kidney disease 10
years after diagnosis of diabetes, and 3 died of end-stage renal failure
at the age of 52-53 years [10].
MODY9 is rarely seen in clinical practice, and only a few families have
been reported so far. In 2011, Wakako Jo et al reported a Japanese
family with MODY9 [20]. There were 2 patients with DM in this
family, all of whom carried a deletion mutation in the third exon region
of PAX4 C374-412del39. The proband’s disease onset was at the age of 15
with ketosis and had been treated with insulin since diagnosis. Another
patient in the family, the father of the proband, was diagnosed at the
age of 30 with mild diabetes, which was well controlled through diet
management only. In 2018, Serena Pezzilli et al. reported a missense
mutation of PAX4 c.593C>T (A198V) in a MODY9 Italian family
with 4 diabetic patients. Among them, the proband and the sister carried
Ala198Val missense mutation, while the proband’s mother and aunt did not
undergo gene sequencing. The 4 members were diagnosed as early as 20
years old and as late as 60 years old. Three of them were treated with
oral medication and their blood glucose was well controlled, while the
other one received insulin treatment [21].
Three more missense mutations of the PAX gene were found in subsequent
studies. The missense mutation of c.377A>G (p. D126G) and
c.55C>T (p.R19W) was reported by Natalia Zubkova et al.
[22]. C.92G>T (p.r.31l) missense mutation was reported
by Aaron Chapla et al. in India [23]. So far, the inheritance
pattern of MODY9 mutation has been mainly autosomal dominant.
In this study, we report a disease that started in infancy and was
characterized by an autosomal dominant mutation, which was consistent
with the characteristics of MODY. C.487C>T missense
mutation was found in the 7th exon region of the PAX4
gene in children, leading to a variation of p.R163W in the amino acid
sequence. The R163W variation is located in the homologous domain of the
PAX4 gene, which is responsible for the binding of PAX4 protein to the
target DNA sequence and is also associated with conserved amino acid
residues between species. R163W variation causes the replacement of
polar amino acids by non-polar amino acids, which is suspected to impair
the inhibitory effect of PAX4 protein on insulin and glucagon promoters
[24], leading to hyperglycaemia and diabetes. This mutation has not
been reported in previously published papers. Genetic analysis of the
family members showed that the father of the patient carried the same
heterozygous mutation, while the mother presented with a normal
genotype, suggesting that the mutation was autosomal dominant, and the
inheritance pattern was consistent with previous literature reports.
The estimated frequency of C.487C>T missense mutation in
the PAX4 gene in the normal population database is 0.00010, which is a
low-frequency mutation. According to clinical manifestations and
sequencing results, the patient was diagnosed with MODY9 monogenic
diabetes. The father of the proband had no diabetes at that time, which
may be related to epigenetic factors or incomplete genetic penetrance.
Previous literature showed that the onset age of MODY9 patients ranged
from 14 to 50 years. The clinical manifestations of patients with the
same mutation were also significantly different. Mild cases may only
present with elevated fasting glucose levels or impaired glucose
tolerance, while some cases manifest with typical symptoms of diabetes
such as polydipsia, polyuria and emaciation, only few cases occur with
ketoacidosis. In this study, the onset of illness occurred during early
childhood, characterized by typical diabetic symptoms accompanied by
ketoacidosis. In which case was a MODY9 with early onset and severe
clinical symptoms. Islet autoantibody was negative, and fasting insulin
and C-peptide levels were significantly reduced, which was consistent
with the clinical characteristics of MODY9 reported in previous
literature.
MODY9 is generally treated the same way as other types of MODY, mainly
with diabetic diet, exercise and insulin therapy. Some patients are
responsive to oral hypoglycemic agents. In some patients, treatment has
to adjusted according to the progression of the disease. On the latest
follow-up, the patient had been diagnosed with diabetes for 17 months.
He was treated with basic and mealtime insulin injections and his blood
sugar levels were in normal range ever since.
The clinical outcome of MODY9 patients varies greatly. In mild cases,
the only symptom may be impaired glucose tolerance, without progression
to diabetes, and in severe cases, renal or retinal complications may
occur, or even death from end-stage renal failure.
In summary, in Chinese children, the heterozygous mutation of
c.487C>T in the PAX4 gene can lead to the occurrence of
MODY9. MODY9 patients differ in onset ages and clinical symptoms, making
it difficult to distinguish them from other MODY subtypes. Here, we
conclude that gene sequencing analysis is of great significance in the
diagnosis and classification of MODY, and an accurate genetic diagnosis
will enable patients to receive more specialized treatment.