Pseudohypertriglyceridemia as a clue: clinical and genetic spectrum of glycerol kinase deficiency in three pediatric cases

Introduction

Glycerol kinase deficiency (GKD) is a rare X-linked metabolic disorder resulting from pathogenic variants in the GK gene located on chromosome Xp21.2. Biochemically, it is characterized by elevated levels of glycerol in the blood (hyperglycerolemia) and urine (glyceroluria). This can lead to pseudohypertriglyceridemia, as free glycerol is incorporated into enzymatic assays for triglyceride measurement. Consequently, patients may be misdiagnosed with hypertriglyceridemia and subjected to unnecessary lipid-lowering treatments [1].

Clinically, GKD manifests as a broad phenotypic spectrum and is categorized into complex and isolated forms. Complex GKD arises from large chromosomal deletions encompassing the GK gene and neighboring genes, such as DMD and NR0B1. This condition manifests during infancy, with clinical features including muscular dystrophy, adrenal insufficiency, developmental delay, and multiorgan involvement. Conversely, isolated GKD is characterized by the presence of small mutations or deletions that are restricted to the GK gene. The manifestation of this condition can be observed in two distinct clinical presentations. The first presentation is characterized by symptoms such as hypoglycemia, vomiting, and metabolic acidosis, and is typically observed in younger patients. The second presentation, which manifests in adults, is usually asymptomatic and identified incidentally during investigations for persistent hypertriglyceridemia [1].

Laboratory findings that may be indicative of this condition include elevated triglycerides that are unresponsive to treatment, non-lipemic serum, normal or high HDL, elevated plasma glycerol, and glyceroluria. Genetic analysis is imperative for the confirmation of results [1], 2].

Given its heterogeneous clinical presentation and frequent misdiagnosis, GKD should be considered in patients with unexplained and treatment-resistant hypertriglyceridemia. This case series describes three patients diagnosed with GKD, emphasizing pseudohypertriglyceridemia as the underlying etiology.

Case reports

Case 1

An 11-year-old male was referred for evaluation of persistent hypertriglyceridemia, initially identified at the age of 11. He was born at term to consanguineous parents and had a history of specific learning disorder, for which he was recently prescribed methylphenidate. He was non-obese, with a weight of 26 kg (25th–50th percentile), height of 128 cm (50th percentile), and a body mass index (BMI) of 15.5 kg/m² (25th–50th percentile). Physical examination was unremarkable, except for a grade 2/6 systolic murmur.

Laboratory investigations revealed normal findings in complete blood count, aminotransferase levels, serum electrolytes, renal function tests, amylase, lipase, and creatine kinase (CK) levels. However, serum triglyceride (TG) levels were elevated. The patient demonstrated hypertriglyceridemia with a mildly elevated high-density lipoprotein (HDL) level, while total cholesterol and low-density lipoprotein (LDL) levels remained within normal limits. Lipid profile results were as follows: TG: 393 mg/dL (0–150 mg/dL), LDL: 49 mg/dL (<100 mg/dL), very low-density lipoprotein (VLDL): 79 mg/dL (0–30 mg/dL), HDL: 65.5 mg/dL (40–60 mg/dL), total cholesterol: 153 mg/dL (<200 mg/dL), and non-HDL cholesterol: 87.5 mg/dL.

Abdominal ultrasound, carotid Doppler ultrasonography, and echocardiography revealed no abnormalities. The analysis of urine organic acids revealed significantly elevated levels of glycerol (hyperglyceroluria: 197.5 mmol/mol creatinine). Furthermore, the levels of lysosomal enzymes and chitotriosidase were found to be within the normal range.

Following the detection of hyperlipidemia, a series of lifestyle modifications were implemented, including the adoption of a low-fat diet, with daily cholesterol intake being restricted to no more than 300 mg, in accordance with the Cardiovascular Health Integrated Lifestyle Diet(CHILD-1 ) guidelines. Furthermore, omega-3 fatty acid supplementation was initiated. Following a period of three months during which no improvement was observed, fenofibrate was incorporated into the treatment regimen, and methylphenidate was discontinued in view of its capacity to adversely affect lipid metabolism. Despite the implementation of medical therapy and lifestyle interventions, a notable decrease in triglyceride levels was not observed over a three-year follow-up period (Figure 1).

Figure 1:

Figure showing changes in triglyceride levels over time with management strategies. No significant reduction in triglyceride levels was achieved over three years despite medical treatment and lifestyle changes.

In light of the patient’s non-response to lipid-lowering therapy and the absence of a lipemic appearance in centrifugation blood samples despite elevated triglyceride levels, pseudohypertriglyceridemia was suspected. Subsequent genetic testing revealed a hemizygous NM_001205019.2:c.213_214delAT (p.C72) mutation in the GK gene, thereby confirming a diagnosis of glycerol kinase deficiency. Consequently, the elevated triglyceride levels were attributed to disease-related hyperglycerolemia, and the therapeutic regimen aimed at reducing lipids was discontinued.

Case 3

A four-month-old male was admitted to the pediatric intensive care unit with pneumonia and recurrent infections, and was initially evaluated for chronic granulomatous disease. He was born at term and had a brief stay in the neonatal intensive care unit. On admission, he presented with hypotonia, abdominal distension, and a tracheostomy. His growth parameters were below −3 standard deviations.

Imaging studies revealed splenic abscesses, and serial laboratory evaluations during his 7-month ICU stay consistently demonstrated persistent hypertriglyceridemia, with triglyceride levels ranging from 600 to 1,500 mg/dL. Notably, HDL levels were initially low but later normalized, despite the absence of visible serum lipemia. Additionally, CK levels were markedly elevated, ranging from 596 to 15,489 U/L (1–190 U/L). Aspartate aminotransferase (AST), alanine aminotransferase (ALT), and C-reactive protein (CRP) levels were also elevated in the later stages, indicating ongoing systemic inflammation.

Following a 12-h fast, TG levels remained elevated, while total cholesterol and LDL levels were within normal limits. The patient was initiated on a specialized diet consisting of 60 % extremely low-fat formula (Basic F^®, Nutricia, Netherlands) and 40 % hydrolyzed protein and medium-chain triglyceride (MCT) oil-based formula (Pepti-Junior^®, Nutricia, Netherlands), along with omega-3 fatty acid supplementation. Despite dietary and pharmacologic intervention, TG levels remained persistently elevated. Notably, when centrifuged and stored blood samples were visually compared to those from patients with similarly high TG levels (approximately 1,400 mg/dL), no lipemic appearance was observed (Figure 2).

Figure 2:

Centrifuged and stored blood samples exhibited no lipemic appearance despite markedly elevated triglyceride levels.

Genetic analysis via microarray revealed a 6.9 Mb deletion spanning the Xp21.2–Xp11.4 region, which includes the GK gene as well as genes associated with Duchenne muscular dystrophy (DMD) and immunodeficiency. Based on the loss of multiple contiguous genes, the patient was diagnosed with immunodeficiency, DMD, and GKD, and was placed under multidisciplinary follow-up. The elevated triglyceride levels were attributed to pseudohypertriglyceridemia secondary to GKD, and both dietary and pharmacologic lipid-lowering treatments were discontinued.

The patient underwent haploidentical hematopoietic stem cell transplantation at 11 months of age, followed by a second transplantation at 16 months due to severe combined immunodeficiency. He remains on prophylactic antibiotic therapy as part of ongoing management.

Discussion

Glycerol kinase deficiency is classified as either isolated or complex and presents with a broad clinical spectrum, ranging from asymptomatic adult-onset cases to severe infantile-onset forms with multisystem involvement [1], 3]. In our series, two patients (Cases 1 and 2) were diagnosed with isolated GKD, presenting solely with asymptomatic, persistent hypertriglyceridemia. In contrast, the third patient (Case 3) exhibited a complex phenotype characterized by hypotonia, splenic abscesses, recurrent infections, and markedly elevated CK levels – findings consistent with an Xp21 contiguous gene deletion syndrome.

In this cohort, all three patients exhibited hypertriglyceridemia; however, none of the centrifuged blood samples demonstrated a lipemic appearance despite the elevated triglyceride levels. Among the two patients with isolated GKD, one had normal or mildly elevated HDL levels, while his sibling – harboring the same mutation – and the patient with the complex form displayed low HDL levels. The patient with the complex form also had markedly elevated CK levels. Additionally, both patients who underwent urinary organic acid analysis demonstrated significantly increased urinary glycerol excretion.

To date, more than 100 cases of GKD have been reported in the literature, with the majority exhibiting phenotypes similar to those observed in our patients. In a case series by Lamiquiz-Moneo et al. patients had moderately to severely elevated triglyceride levels (352–578 mg/dL), yet HDL cholesterol levels were normal or elevated [4]. Similarly, in our isolated GKD cases (Cases 1 and 2), triglyceride levels were elevated – 393 mg/dL and 403 mg/dL, respectively. One patient exhibited normal or slightly elevated HDL levels, while the other had mildly decreased HDL. Notably, neither of these patients presented with additional metabolic complications such as diabetes or pancreatitis, which have been reported in some prior studies [5].

In contrast, the patient with complex GKD (Case 3) showed markedly elevated triglyceride levels, ranging from 596 to 15,489 mg/dL, along with slightly reduced HDL levels. Elevated liver enzymes (AST, ALT) and increased inflammatory markers (CRP) were also observed, consistent with multisystem involvement and chronic inflammation. These findings are in line with previously reported features of complex contiguous gene deletion syndromes, as described by Pizza et al. [6]. Previous studies have documented similar phenotypic variability, reinforcing the clinical heterogeneity and diagnostic complexity of GKD [7].

Overall, the clinical presentations and laboratory findings in our cases align with existing literature and highlight key diagnostic clues – specifically, the presence of glyceroluria and the absence of serum lipemia despite elevated triglyceride levels. These features are critical in distinguishing GKD from primary hypertriglyceridemia and underscore the importance of considering GKD in the differential diagnosis of treatment-resistant hypertriglyceridemia [1], 8].

Glycerol kinase deficiency (GKD) results from pathogenic variants in the GK gene, located at Xp21.2. Isolated GKD is typically caused by small intragenic mutations, whereas complex GKD arises from large contiguous deletions that involve neighboring genes such as DMD, NR0B1, and IL1RAPL1 [9]. To date, 34 distinct pathogenic variants in the GK gene have been identified, including 26 classified as pathogenic and 8 as likely pathogenic [10]. In our series, genetic testing revealed a hemizygous c.213_214delAT (p.Cys72Ter) pathogenic variant in both siblings diagnosed with isolated GKD. In contrast, the patient with complex GKD was found to have a 6.9 Mb deletion spanning Xp21.2 to Xp11.4, encompassing the GK gene along with additional loci associated with chronic granulomatous disease and Duchenne muscular dystrophy (DMD). These findings are consistent with previously reported cases of Xp21 contiguous gene deletion syndromes [7], 11]. Our genetic results align with the current understanding of the molecular basis of both isolated and complex GKD, and emphasize the importance of comprehensive genetic testing – including both sequencing and copy number analysis – in suspected cases. The wide clinical heterogeneity observed in GKD poses challenges for establishing definitive genotype–phenotype correlations, highlighting the need for thorough clinical evaluation and individualized management strategies that account for the multifactorial nature of disease expression [2], 12].

No clinical or biochemical signs of GKD were observed in the mothers of our patients, consistent with the typically asymptomatic carrier status seen in heterozygous females. However, as previously reported, skewed X-chromosome inactivation may rarely lead to clinical manifestations such as adrenal insufficiency, hypogonadotropic hypogonadism, developmental delay, and myopathy in carrier females [13], 14].

Management of GKD is primarily supportive. In asymptomatic individuals with pseudohypertriglyceridemia, discontinuation of unnecessary lipid-lowering therapies is essential. Symptomatic infants may require targeted nutritional interventions, while patients with complex GKD should receive treatment tailored to their associated multisystemic conditions [3], 15]. In our cohort, cessation of lipid-lowering agents following diagnosis led to an uneventful follow-up, without any hyperlipidemia-related complications.

This case series contributes to the limited literature on GKD by presenting detailed clinical, biochemical, and genetic data from both isolated and complex forms. Notable strengths include genetic confirmation of the diagnosis and longitudinal follow-up, which provide valuable insights into disease progression and management.

Conclusions

Glycerol kinase deficiency (GKD) is an important yet frequently underrecognized cause of pseudohypertriglyceridemia. Persistent hypertriglyceridemia in the absence of a lipemic serum appearance should raise suspicion for GKD, particularly when refractory to lipid-lowering therapy. Our findings, in alignment with previous reports, underscore the clinical, biochemical, and genetic heterogeneity of the disorder. Early and accurate diagnosis through targeted biochemical and molecular testing is essential to prevent misdiagnosis, avoid unnecessary treatments, and guide appropriate clinical management. Further studies involving larger cohorts are warranted to enhance our understanding of the natural history, genotype–phenotype correlations, and long-term outcomes associated with both isolated and complex forms of GKD.

Learning points

1. GKD may be present in isolated or complex forms with varying severity.

2. Persistent hypertriglyceridemia without lipemic serum should prompt suspicion of GKD.

3. Hyperglyceroluria is a key diagnostic clue; genetic testing confirms the diagnosis.

4. Timely diagnosis prevents unnecessary lipid-lowering therapy.

What is new?

1. This case series illustrates phenotypic variability in siblings with the same GK mutation and reports a large Xp21 deletion in a complex case. It emphasizes the diagnostic value of glyceroluria and the absence of serum lipemia in pediatric patients with unexplained hypertriglyceridemia.