Kliniczne i molekularne aspekty genetycznej hipertriglicerydemiiu dzieci z północnych Indii: seria przypadków

Introduction

Hypertriglyceridaemia (HTG) is a relatively rare condition in children but can be associated with significant morbidity and mortality [1]. While the majority of aetiologies of HTG are mild, secondary, and transient, primary disorders caused by genetic defects in triglyceride (TG) synthesis and metabolism tend to be severe, permanent, and of significant concern. Monogenic HTG is associated with an increased risk of developing recurrent pancreatitis and premature cardiovascular disease (CVD). Therefore, conditions of marked HTG in children, although infrequent, need careful consideration and workup in the relevant context.

TG-rich lipoproteins (TRLs) are secreted from either the intestine (chylomicrons) or the liver (very low-density lipoproteins, VLDL). Chylomicrons are produced in enterocytes from dietary lipids, such as fatty acids and cholesterol, and typically disappear within 4 hours after the last meal. Chylomicronaemia syndrome refers to fasting chylomicronaemia, which results in severe HTG (TG levels > 1,000 mg/dl). Its causes can be monogenic, leading to rare, early-onset familial chylomicronaemia syndrome (FCS), or secondary to polygenic factors, resulting in the commoner, late-onset, multifactorial chylomicronaemia syndrome (MCS) [2].

TRLs are metabolised by lipoprotein lipase (LPL), which requires lipase maturation factor (LMF-1) and glycosylphosphatidylinositol high-density lipoprotein binding protein 1 (GPIHPB-1), an endothelial anchoring protein, for its function. The action of LPL is further regulated by other apolipoproteins, including APOC2 (activates LPL) and APOA5 (stabilises LPL-TRL complex) [3]. Therefore, FCS can be caused by loss of function mutations in genes encoding any of the above factors. Other rare monogenic disorders (lipodystrophy, glycogen storage disorders) can also cause marked HTG due to increased VLDL synthesis [4]. While early-stage HTG (TG level < 2000 mg/dl) is asymptomatic, progressive TG elevation can herald distinctive clinical manifestations, such as eruptive xanthomas, lipemia retinalis, and severe complications like pancreatitis and CVD [1, 5]. Inadequately managed HTG poses long-term consequences, which necessitate effective management during childhood with the central goal of preventing acute pancreatitis and reducing the CVD risk [5].

This article aims to present the diverse aetiological spectrum of paediatric HTG due to genetic causes through a series of cases.

Material and methods

We conducted a retrospective review of the medical records of all children (aged 1 month to 14 years) referred to our paediatric endocrinology service between January 2017 and December 2023 for evaluation and management of HTG. We collected data on demographic details, clinical symptoms and signs at the time of presentation, and results of diagnostic laboratory evaluation. Severe HTG, for the purpose of this study, was defined as having TG levels exceeding 500 mg/dl [6]. Blood samples for serum lipid analysis were collected after an 8-hour overnight fasting. The levels of total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and TG were measured enzymatically using an automatic analyser AU5800 (Beckmann Coulter, California). All children with HTG documented twice were considered for inclusion in the study. Subjects with diabetes, obesity, chronic kidney disease, nephrotic syndrome, congenital or acquired hypothyroidism, and drug intake (corticosteroid, antipsychotic, antidepressant, immunosuppressives, and L-asparaginase) were excluded. Genetic testing was performed using Whole Exome Sequencing (Illumina, San Diego, USA) after parental counselling. Sequencing of the protein-coding regions in genes was performed at a mean depth of 80-100X with the percentage of bases covered at 20X depth in the target region and 100-200X depth for the mitochondrial gene. The pathogenicity of the sequence variants was interpreted according to the American College of Medical Genetics (ACMG) guidelines. Persistent, severe HTG was managed as per protocol [6].

Results

During the study period, 18 children were diagnosed with severe HTG at our hospital. Seven subjects were excluded due to various causes (type 1 diabetes – 3, nephrotic syndrome – 2, and drug intake – 2). Those excluded (mean age 8.4 ±1.3 years, and baseline TG level 1052.8 ±187.7 mg/dl) due to secondary causes were treated only for their primary illnesses and all had normalization of TG levels at their 6-month follow-up visits.

A total of 11 children were included in the study. The median age at diagnosis was 0.9 years (0.45–2.4 years). Seven of the 11 patients were boys. The chief complaints at the time of presentation were incidental detection of lipemic serum (7, 63.3%), failure to thrive (4, 36.3%), loss of subcutaneous fat (2, 18.2%), and abdominal distension (2, 18.2%), either alone or in combination. Physical examination revealed growth failure in 4 patients (mean weight Z-score –1.36 ±1.26, and length/height Z-score –1.06 ±1.58) at baseline. Six (54.5%) patients had hepatosplenomegaly. Lipaemia retinalis was detected on ophthalmological examination in 4 patients (Table I).

The median TG, LDL-C, TC, and HDL-C levels at baseline were 3040 mg/dl (range, 1797–3543 mg/dl), 69.1 mg/dl (range: 36.7–176.1 mg/dl), 256 mg/dl (range: 169–451 mg/dl), and 51 mg/dl (range: 14.8–162 mg/dl), respectively. Ten patients had a confirmed genetic diagnosis; 6 (60%) had FCS, 2 (20%) had congenital lipodystrophy, one (10%) had glycogen storage disorder type 1a, and one (10%) had glycerol kinase deficiency (Table I). The details of the variants identified (4 were novel) are shown in Table II. All children with FCS presented significantly earlier compared to the non-FCS group [0.9 vs. 6.4 years, p = 0.042). However, TG levels at baseline could not be used to differentiate between the 2 groups (p = 0.083).

The medical management undertaken and TG levels on follow-up for all patients are summarised in Table III. At diagnosis, all patients were initiated on a low-fat diet and medium-chain triglyceride (MCT) oil substitution. The addition of lipid-lowering drugs was considered if TG levels were > 2,000 mg/dl at diagnosis or in follow-up. Fenofibrates alone and atorvastatin either alone or in combination with ezetemibe were used in 7 and 3 patients, respectively. Dietary management alone (n = 4, FCS = 3) reduced TG levels by 32.3% at the first follow-up visit while the addition of Fenofibrate in 2 of them (both with APOA5 mutation) further decreased TG levels by 29% at the next visit. None have reported any known side effects of drug therapy. The mean duration of follow-up was 1.75 ±1.0 years (2 children with FCS were lost to follow-up). The median TG levels for the FCS and non-FCS groups were 1240 mg/dl (610–1,685) and 412 mg/dl (247.5–993), respectively, at the last follow-up. Two patients (40%) with FCS had TG levels < 1,000 mg/dl, while all but one (75%) non-FCS subject had TG levels < 500 mg/dl at the last follow-up. One child has developed acute pancreatitis during the said duration.

Discussion

Our data supports previous studies stating that defects in LPL are the most common cause of FCS [7]. All children with FCS were incidentally detected during workup for febrile illnesses, wherein blood sampling revealed a milky white serum. The relatively innocuous characteristic of the disorder poses a considerable challenge to the early detection of this devastating and potentially fatal condition [8].

Unless detected incidentally, the median age at diagnosis of FCS is 9–24 years, usually with pancreatitis. Underdiagnosis and underreporting of FCS are thus common [6, 8], similarly to other genetic lipid disorders [9]. In our cohort, the median age at diagnosis was 0.9 years; most cases were incidentally detected. The lifetime risk of developing pancreatitis is approximately 6.5% in those with TG levels above 1,000 mg/dl but rises alarmingly to 20% in those with levels above 2,000 mg/dl. There is a 3% increased risk of pancreatitis for every 100 mg/dl increase in TG levels beyond 1,000 mg/dl. Notably, up to 50% of FCS patients experience recurrent abdominal pain, not amounting to pancreatitis [10, 11].

In non-emergent situations, dietary therapy remains the cornerstone of management along with pharmacotherapy if indicated. For neonates and infants, low-fat formulas are available, but they are costly. If these are unavailable or unaffordable, a combination of skimmed formula milk/skimmed breast milk, cornflour, MCT oil, or coconut oil can be tried [12]. MCT oil contains 8-12 chain fatty acids and is directly absorbed into the portal circulation without the need for chylomicron formation. Complementary feeding should be initiated early. While fat restriction (equally distributed through all meals) should be compensated by increasing protein intake, the intake of simple sugars (< 10% daily energy intake) should be reduced as well. Low-fat foods (egg whites, shrimps, beans, lentils, breast of poultry, vegetables) should be encouraged. Physicians treating these patients should also be aware of the risk of essential fatty acid deficiency (2–4% daily energy intake), which can lead to adverse neurodevelopmental outcomes. All our patients are currently on MCT oil therapy in addition to fat-restricted diets.

In several patients with FCS, dietary modifications alone fail to reduce TG levels to less than 1,000 mg/dl, thereby leaving the child at risk of pancreatitis in the long term. One of our patients with LPL deficiency showed an increase in TG levels (from 1,048 mg/dl to 3,467 mg/dl) after an initial response due to poor adherence to the suggested diet plan. In a multi-national study, most patients (> 90%) reported difficulties managing fat intake, and almost half continued to experience symptoms [13]. Such patients may benefit from pharmacotherapy, but there are no FDA-approved drugs for the treatment of FCS in children. Although long-term follow-up data are lacking, case reports generally suggest that fibrates are well tolerated in children [14]. Fenofibrates are preferred over gemfibrozil due to their once-daily dosing and lower incidence of side effects [10]. Fibrates decrease triacylglycerol substrate availability in the liver by stimulating fatty acid oxidation, thereby decreasing endogenous hepatic VLDL secretion. In those with low LPL activity, it further stimulates lipolysis of VLDL [15]. One of our FCS patients was started on fibrates because he was not tolerating low-fat formulae with consequent increasing TG levels (937 mg/dl to 1,927 mg/dl). Fibrate administration lowered the TG level to 1,237 mg/dl. Children with APOA5 variants responded well to fibrates when compared to those affected with LPL defects. Two-thirds of those with APOA5 achieved TG < 1,000 mg/dl on follow-up, while none with LPL defects achieved the same. The non-FCS HTG patients in our cohort have been effectively managed with dietary management, MCT oil, fibrates, and statins. Statins are the mainstay of CVD risk prevention in HTG patients. They have moderate TG-lowering action along with remnant particle clearance effects [10]. All our cases except the one with glycogen storage disease have TG levels below 500 mg/dl on follow-up.

Apart from true HTG, pseudo-HTG may be caused by genetic defects [16]. One child who was later diagnosed with glycerol kinase deficiency (absence of glycerol phosphorylation to glycerol-3-phosphate) presented with high TG levels non-responsive to multiple lipid-lowering drugs [17].

The limitations of our study include a small sample size and a relatively short duration of follow-up. A longer follow-up would allow for the assessment of disease-related complications and potential side effects of the administered drugs.

Conclusions

Paediatric HTG is usually incidentally detected, but it needs molecular testing to confirm the diagnosis. Early-onset HTG is usually monogenic and caused by mutations in the LPL gene. The treatment goal is to maintain TG levels below 1,000 mg/dl through dietary modifications and pharmacological interventions if required. To the best of our knowledge, this is the first report of a series of children with genetic HTG from Asia.