Hematologists Should Consider Sitosterolemia in Patients with Hemolytic Anemia, Thrombocytopenia, and High Cholesterol

The rare genetic condition sitosterolemia, characterized by increased plant sterol levels in the blood, may first present to a hematologist as hemolytic anemia or thrombocytopenia, sometimes without the additional clinical or laboratory manifestations that occur in some patients, such as xanthomas and high cholesterol.

Historically, sitosterolemia has been considered extremely rare. Some estimates place the prevalence between 1 in a million and 1 in 200,000 people. However, some experts believe that because of testing challenges and under-recognition, the true incidence may be considerably higher.^1,2

“Because it is rare, a hematologist may not be familiar with the condition and may not include it in the differential diagnosis of a patient with hemolytic anemia or thrombocytopenia,” said Eleftheria Hatzimichael, MD, PhD, an associate professor of hematology in the University of Ioannina’s School of Health Sciences in Ioannina, Greece.

A peripheral blood smear; the blue arrows show stomatocytes,
and the red arrows show platelets of varying sizes. Courtesy of
Eleftheria Hatzimichael, MD, PhD

Depending on presentation, some patients may be misdiagnosed with chronic immune thrombocytopenia (ITP), homozygous familial hypercholesterolemia (FH), or other conditions.^2,3 Early diagnosis is important, because correct treatment with dietary management and ezetimibe can significantly improve outcomes for people with sitosterolemia, both in terms of hematologic manifestations and important complications such as cardiovascular disease.⁴

ASH Clinical News spoke with both Dr. Hatzimichael and Gregory A. Graf, PhD, a professor of physiology and associate director of the Saha Cardiovascular Research Center at the University of Kentucky in Lexington, about the disease and its etiology, as well as considerations regarding clinical manifestations, diagnosis, and management.

Disease History and Pathophysiology

The sterol group of organic molecules performs diverse functions across eukaryotes; different organisms have evolved to preferentially use certain sterol classes. Higher animals evolutionarily prefer cholesterol, for example, as a substrate for cellular membranes and as a building block for steroid hormones, and cholesterol is the sterol obtained from most animal food products.⁵

In contrast, xenosterols refer to sterols found in non-animal foods, and a subgroup of these, the phytosterols, are predominantly found in plants. Phytosterols possess the same basic chemical structure as cholesterol, differing mostly in their “R” tail. For example, one type of phytosterol, sitosterol, has an extra ethyl group at the carbon 24 position.

In 1974, Ashim K. Bhattacharyya, PhD, and William E. Connor, MD, reported on two sisters who had tendon and tuberous xanthomas but normal cholesterol levels. Blood testing revealed that they had elevated levels of the phytosterols sitosterol, campesterol, and stigmasterol, and the pair absorbed radiolabeled β-sitosterol at 35 times the normal amount.⁶ The newly characterized disorder became known as sitosterolemia, although some researchers have since then suggested that “phytosterolemia” or “xenosterolemia” would more accurately reflect the physiology.

Twenty-six years later, scientists uncovered the genetic cause of the disease: mutations in the ABCG5/ABCG8 genes. These encode a sterol efflux heterodimer transporter expressed in hepatocytes, gallbladder epithelium, and enterocytes. The transporter is responsible for sterol excretion through the intestines and bile. This includes phytosterol excretion and, to a much lesser extent, cholesterol excretion.

People normally absorb less than 5% of the phytosterols taken in through their diet.⁷ Under normal circumstances, the concentration of phytosterols in the blood is extremely low, roughly 1,000-fold less than that of cholesterol.^5,8 It is currently believed that phytosterols do not fulfill a normal role in human metabolism; some research has suggested that increasing phytosterols in the diet might consequently lower LDL cholesterol levels and positively combat atherosclerosis in normal individuals, but this is controversial.⁹

However, the situation is quite different for people with defects in the ABCG5 or ABCG8 genes. Mutations lead to hyper-retention of plant sterols, with a 30- to 100-fold increased concentration compared to people without the disease, especially of sitosterol, campesterol, and stigmasterol.⁸

Dr. Graf explained that when present at these quantities, phytosterols may be incorporated instead of cholesterol, for example, into cells’ lipid bilayers. Differences in the R-group chains of the molecules may impact sterol packing and may have other physiologic impacts, not all of which are well understood. Phytosterols can accumulate in various parts of the body, such as the vascular wall, where they might play an additional role in atherosclerosis.⁴

Patient History and Clinical Findings

If sitosterolemia is already identified in the family, this brings the recessive autosomal condition into consideration. But this family history is not always known, and heterozygous carriers of the gene mutation are largely asymptomatic. While the disease has been identified across many different ethnic groups, it is often associated with the founder effect in populations of increased consanguinity, including the Amish and some populations in South Africa and Finland.

Sitosterolemia is somewhat heterogeneous in its severity and presentation. Classically, patients may first present with tendinous and cutaneous xanthomas, elevated cholesterol (although this is extremely variable), and premature cardiovascular disease. For unknown reasons, cholesterol may be extremely high in some infants. However, none of these findings are universally present, and some people may be asymptomatic. Other manifestations can also occur, such as splenomegaly, arthritis, adrenal dysfunction, elevated liver function tests, and cirrhosis.⁷

Blood changes might be absent; when present, they might manifest in different ways. “Some [patients] may present with fatigue, pallor, and weakness due to the underlying anemia,” Dr. Hatzimichael said. “Others may present with easy bruising or bleeding due to the underlying macrothrombocytopenia, or they may even be asymptomatic but thrombocytopenic.”

Although the exact mechanisms are unknown, the incorporation of plant sterols into red blood cell membranes may make them more fragile and prone to destruction, leading to hemolytic anemia. Splenomegaly from lipid accumulation and consequent splenic sequestration may contribute to both anemia and thrombocytopenia. Incorporation of plant sterols into platelet membranes may also contribute to platelet hyperactivation and premature removal.¹⁰

Blood changes are sometimes the only clinical or laboratory findings present, although it is unclear how common this is. Researchers initially described a poorly understood blood condition with stomatocytosis and macrothrombocytopenia in a population of Mediterranean immigrants to Australia, but further investigation revealed that the condition is simply another presentation of sitosterolemia. Such solely hematologic presentations may provide further evidence that the condition, although rare, might be more prevalent than once believed.¹¹

Some in the field have speculated that cultural differences in the dietary intake of specific phytosterols might contribute to differences in symptom manifestations, Dr. Graf said. For example, some diets are higher in certain phytosterols compared to others, and their different R-group chains might have different impacts, such as on cell membrane packing or phytosterol toxicity, which might influence disease presentation. At present, this remains unknown.

Diagnostic Challenges

For the hematologist, a careful examination of a peripheral blood smear often provides a key diagnostic clue. The smear may reveal the enlarged platelets often seen in the disease — sometimes almost as large as red blood cells — as well as stomatocytes (red blood cells with a slit-like central pallor). This combination of enlarged platelets and stomatocytes, although not present in every patient, seems to be specific to sitosterolemia.¹²

Dr. Hatzimichael noted that the index of suspicion for sitosterolemia should increase for any of these blood findings, especially if patients have proven refractory to their current treatment. Clinicians should also consider the diagnosis for unexplained hemolytic anemia (especially with a negative direct antiglobulin test), unexplained macrothrombocytopenia, unexplained dyslipidemia, premature atherosclerosis, or for any patients with a family history.

Some experts recommend that all patients with hypercholesterolemia be evaluated for platelet size, with enlarged platelets indicating that plant sterol testing might be needed.¹¹ Patients with sitosterolemia, especially pediatric ones, can be misdiagnosed as having homozygous FH, which manifests with xanthomas and very high cholesterol; studies indicate that sitosterolemia may be significantly underdiagnosed in such settings.²

Dr. Graf explained that the common clinical assays used to measure cholesterol cannot distinguish between cholesterol and non-cholesterol sterols such as phytosterols (which are present at very low levels in unaffected individuals) because of the high degree of similarity in their structures. Any elevation in phytosterols in patients with sitosterolemia registers as cholesterol. Thus, the cholesterol reading for these patients is falsely inflated, although their actual cholesterol levels may still be somewhat high. When elevated phytosterols are not flagged, this can lead to misdiagnosis.

Dr. Graf first began studying ABCG5/ABCG8 genes as part of a postdoctoral project analyzing some patients who were initially misdiagnosed with homozygous FH, but who actually had “pseudo FH,” that is, sitosterolemia. Although these patients had classic characteristics of homozygous FH, specialized testing revealed accumulation of non-cholesterol sterols. These plant sterols can be quantified via gas-liquid chromatography, gas chromatography/mass spectrometry, or high-pressure liquid chromatography. However, these gold-standard tests are expensive and not available in all clinical labs; patients may need to be referred to specialized academic medical centers for testing. Although not always necessary, genetic testing can help confirm the diagnosis in patients with corresponding clinical and lab findings.

Misdiagnosis is also a hazard for patients presenting predominantly with thrombocytopenia, an issue not uncommon in other types of inherited thrombocytopenia as well. Some are incorrectly diagnosed with ITP, ITP secondary to arthritis, or Evans syndrome. Consequently, some patients are then treated with inappropriate or even harmful therapies, such as corticosteroids or a splenectomy.³

Treatment

Dietary modifications are an important aspect of managing sitosterolemia. Patients should restrict intake of foods high in plant sterols such as vegetable oils (for example, corn or soybean oil), nuts, seeds, margarine, chocolate, and shellfish (which are high in an algae-derived phytosterol). “Instead, they should focus on consuming a diet rich in lower-sterol fruits and vegetables, lean proteins, and whole grains,” Dr. Hatzimichael said.

However, medication is usually needed as well. The current standard of care is ezetimibe, which has provided breakthrough treatment for the disease. First explored as a cholesterol-lowering agent, the drug inhibits absorption of both cholesterol and plant sterols from the intestine by blocking the NPC1L1 transporter, preventing the accumulation of plant sterols in the tissues.⁴

“Even in patients with normal or near-normal cholesterol levels, ezetimibe therapy can be beneficial in reducing plant sterol levels and helping alleviate associated problems such as premature atherosclerosis,” Dr. Hatzimichael said.

A bile acid sequestrant such as cholestyramine is another option for patients who are unable to take ezetimibe, or it can be used as adjunct therapy. Patients often respond well, but this treatment tends to be less well tolerated compared to ezetimibe.^3,4

Dr. Graf pointed out that even among the treated population, sterol levels remain five to 10 times higher than in unaffected individuals. Still, ezetimibe treatment usually improves many disease manifestations, including xanthomas, lipid profiles, and hematologic abnormalities.

However, Dr. Hatzimichael noted that the extent of improvement can vary, with some patients requiring ongoing management. Some may also need additional interventions, like antiplatelet therapy, to manage complications such as premature atherosclerosis or interventions to address cardiovascular risk factors.

While other patients may be good candidates for adjunctive statin therapy to further lower their LDL cholesterol, statins alone do not effectively reduce plant sterol levels. This contrasts with homozygous FH, in which statins are often an appropriate first-line treatment.

Dr. Hatzimichael recommends working with cardiology specialists as part of patient management to help minimize effects from premature atherosclerosis. Such specialists can help identify early signs of cardiovascular complications and provide recommendations on lifestyle modifications and pharmacologic therapies for reducing cardiovascular risk.

Although the prognosis can vary, sitosterolemia is a treatable condition, and early diagnosis and intervention are critical for preventing complications like heart disease. “By following a combination of dietary therapy, medication, and regular monitoring, patients can significantly reduce their blood plant sterol levels, prevent complications, and improve their hematologic parameters,” Dr. Hatzimichael said.

The key is keeping a high index of suspicion diagnostically so these patients don’t fall through the cracks, which includes taking an especially careful look at the peripheral blood smear. “Once you see a peripheral blood smear from a patient with sitosterolemia, you never forget it,” Dr. Hatzimichael said.

References

1. Tada H, Nohara A, Inazu A, et al. Sitosterolemia, hypercholesterolemia, and coronary artery disease. J Atheroscler Thromb. 2018;25(9):783-789.
2. Lee JH, Song DY, Jun SH, et al. High prevalence of increased sitosterol levels in hypercholesterolemic children suggest underestimation of sitosterolemia incidence. PLoS One. 2020;15(8):e0238079.
3. Bastida JM, Benito R, Janusz K, et al. Two novel variants of the ABCG5 gene cause xanthelasmas and macrothrombocytopenia: a brief review of hematologic abnormalities of sitosterolemia. J Thromb Haemost. 2017;15(9):1859-1866.
4. Tzavella E, Hatzimichael E, Kostara C, et al. Sitosterolemia: a multifaceted metabolic disorder with important clinical consequences. J Clin Lipidol. 2017;11(4):1095-1100.
5. Kanuri B, Fong V, Patel SB. Chapter 24: Role of xenosterols in health and disease. In: Ntambi JM, ed. Lipid Signaling and Metabolism. Elsevier; 2020:505-519.
6. Bhattacharyya AK, Connor WE. Beta-sitosterolemia and xanthomatosis: a newly described lipid storage disease in two sisters. J Clin Invest. 1974;53(4):1033-43.
7. Patel SB, Graf GA, Temel RE. ABCG5 and ABCG8: more than a defense against xenosterols. J Lipid Res. 2018;59(7):1103-1113.
8. Tada MT, Rocha VZ, Lima IR, et al. Screening of ABCG5 and ABCG8 genes for sitosterolemia in a familial hypercholesterolemia cascade screening program. Circ Genom Precis Med. 2022;15(3):e003390.
9. Makhmudova U, Schulze PC, Lütjohann D, et al. Phytosterols and cardiovascular disease. Curr Atheroscler Rep. 2021;23(11):68.
10. Kanaji T, Kanaji S, Montgomery RR, et al. Platelet hyperreactivity explains the bleeding abnormality and macrothrombocytopenia in a murine model of sitosterolemia. Blood. 2013;122(15):2732-2742.
11. Rees DC, Iolascon A, Carella M, et al. Stomatocytic haemolysis and macrothrombocytopenia (Mediterranean stomatocytosis/macrothrombocytopenia) is the haematological presentation of phytosterolaemia. Br J Haematol. 2005;130(2):297-309.
12. Zheng J, Ma J, Wu RH, et al. Unusual presentations of sitosterolemia limited to hematological abnormalities: a report of four cases presenting with stomatocytic anemia and thrombocytopenia with macrothrombocytes. Am J Hematol. 2019;94(5):E124-E127.