A novel PHKA2 mutation in a Chinese child with glycogen storage disease type IXa: a case report and literature review

Glycogen storage diseases (GSDs) are inborn errors of glycogen metabolism characterized by an interruption of glycogen breakdown and/or accumulation of a harmful glycogen [1]. GSD type IX (GSD IX) is caused by a defect in phosphorylase b kinase (PhK), which activates glycogen phosphorylase and thus plays a key role in regulating the mobilization of glycogen to glucose [1]. PhK is composed of four different subunits: α, β, γ and δ. The subunits possess tissue-specific isoforms encoded by different genes [2]. The liver-specific isoforms of the α-, β- and γ-subunits are encoded by PHKA2, PHKB and PHKG2, respectively [2]. GSD IXa (MIM: 306000), due to a defect in the liver-particular α isoform (PHKA2), shows X-linked recessive inheritance characteristics [3]. The prevalence of GSD type IX is 1 in 100,000 [4], and GSD type IX accounts for approximately 25% of all types of GSD [4], while GSD IXa accounts for approximately 75% of total GSD IX cases. GSD IXa is characterized by hepatomegaly, chronic liver disease, hypoglycaemia, hyperketosis, hyperlipidemia, growth retardation and delayed motor development [5]. The diagnosis of GSD IX is complicated due to its rarity and the overlap of clinical phenotypes with other congenital diseases. For the technical constraints of the PhK measurement, molecular diagnosis is the key method for providing a definitive diagnosis of GSD IXa and is an effective way to avoid invasive procedures. Almost 107 mutations have been reported in the PHKA2 gene to date. Here we report a novel mutation in the PHKA2 gene and review the literature concerning the phenotypic and genotypic spectra of Chinese GSD IXa.

A boy aged 2 years and 8 months with a history of episodic fatigue and weakness was admitted for the evaluation of symptomatic hypoglycemic episodes. The patient was the first child of unrelated Chinese parents. The boy was born by a full-term normal vaginal delivery after 40 weeks of gestation, with a birth weight of 3.3 kg and a birth length of 49 cm. His pre- and postnatal periods and developmental milestones were normal, and his parents and younger sister were healthy. The symptomatic hypoglycemic episodes were first noted when he was 2 years old. Physical examination showed a non-dysmorphic boy with a height of 97 cm (0SD~ + 1SD), a weight of 15.5 kg (0SD~ + 1SD), and no palpable liver enlargement. He displayed normal muscle strength according to the MRC scale.

The patient’s biochemical characteristics are listed in Table 1. Urine ketone bodies were positive. His liver size was normal on ultrasonography with no liver hyperechogenicity or hypoechogenicity. His electroencephalography, social life ability and intelligence development were normal. His Peabody developmental motor scale presented a motor delay [Gross Motor Quotient (78); Fine Motor Quotient (87); Total Motor Quotient (83)] [6].

Consent and Ethics:

Written informed consent was provided by the parents. This study was approved by the ethics committee of Peking Union Medical College Hospital, China.

The intricacy of the enzyme structure results in heterogeneous clinical phenotypes. Overall, hepatomegaly, elevated liver transaminase levels, growth retardation, hypercholesterolemia, hypertriglyceridemia and hypoglycaemia can be present. Rare manifestations include elevated uric acid and lactic acid levels [5]. These clinical symptoms and biochemical manifestations ameliorate with age and even disappear in adulthood, with a spontaneous remission tendency [10]. The present patient manifested with symptomatic hypoglycemic episodes and motor delay at the age of 2 years old. However, common clinical manifestations, such as hepatomegaly and elevated liver transaminase levels, were not present. By contrast, Jiangwei Zhang et al. [8] reported a study including 17 GSD IXa patients, in which 16/17 patients displayed increased liver transaminase levels and 15/17 patients yielded liver enlargement. Importantly, by reviewing the previous literature, we found that patients who displayed severe hypoglycaemia were diagnosed as GSD IXa at an earlier age. Thus, hypoglycaemia may be one of the early symptoms of GSD IXa.

Through the literature review of 21 Chinese GSD IXa patients [7‐9], elevated liver transaminase levels and liver enlargement were the relevant symptoms of GSD IXa. Delayed motor development was rare, to our knowledge; this is the second Chinese GSD IXa patient reported with delayed motor development. Importantly, the diagnosis of GSD IXa takes an average of 6 years, indicating that there are still challenges for the early diagnosis of GSD IXa, especially in regions where the enzymatic assay is impracticable. Certainly, genetic diagnosis provides benefits in the field.

GSD IXa is caused by mutations of the PHKA2 gene located in Xp22.2–22.1. PHKA2 contains 33 exons and encodes the liver-α subunit [11]. Until now, approximately 151 patients with 107 mutations of the PHKA2 gene have been recorded by HGMD, including 7 splicing, 9 insertions, 29 deletion, 11 nonsense and 51 missense mutations. Currently, only sixteen mutations have been confirmed in Chinese patients [7‐9]. In this case, we detected one novel missense mutation, which was not recorded in the NCBI dbSNP database, at an ultra-low frequency (0.0008) in the DYDF database. The mutation (p.G991A) resulted in a changeable protein structure and affected the nearbysplice site, which indicates that the mutation possibly causes skipping of the following exons. Therefore, we considered that this mutation may lead to the expression of an abnormal PhK α-subunit protein.

Previous studies have reported that patients with GSD IXa showed normal growth and development and normal liver transaminase levels despite their lack of treatment [12]. The patient in our study was treated with corn starch, which avoided recurrence of hypoglycaemia in the following year. Close monitoring is critical. Hence, the study may be useful for the future study of GSD IXa and provides further information about the phenotypic characteristics of Chinese GSD IXa patients.