1. Disease characteristics

1.1 Name of the disease (synonyms)

Sitosterolaemia (phytosterolaemia; Mediterranean stomatocytosis/macrothrombocytopenia).

1.2 OMIM# of the disease

210250.

1.3 Name of the analysed genes or DNA/chromosome segments

ABCG5, ABCG8.

1.4 OMIM# of the gene(s)

605459, 605460.

1.5 Mutational spectrum

The sitosterolaemia genes ABCG5 (NM_022436.2) and ABCG8 (NM_022437.2) lie ‘head to head’ on chromosome 2.1, 2, 3 They each contain 13 exons and encode a half-transporter (sterolin-1 and sterolin-2, respectively), with the C-terminus only containing 6 of the usual 12 transmembrane domains of the other ABC transporters.3 Together they form a heterodimeric transporter responsible for sterol trafficking in the liver and intestine. Loss-of-function mutations associated with sitosterolaemia have been described throughout the ABCG5 and ABCG8 genes.

1.6 Analytical methods

Sequencing (Sanger or NGS). NGS approaches include using exome analysis or as part of a hypercholesterolaemia/cardiac gene panel.

1.7 Analytical validation

Where a mutation(s) can be identified using sequencing, the test is repeated from a fresh dilution of DNA for confirmation. When heterozygosity for two mutations is found, testing of the patient’s parents is recommended, to confirm that the two mutations are present in trans (that is, on opposite chromosomes).

1.8 Estimated frequency of the disease (incidence at birth (‘birth prevalence’) or population prevalence)

If known to be variable between ethnic groups, please report:

No published data are available on the prevalence of sitosterolaemia, an autosomal recessive disorder, although the condition appears to be mainly owing to ABCG8 mutations in Caucasians, whereas in Chinese, Japanese and Indian patients with sitosterolaemia (20% of known cases), it is mainly owing to ABCG5 mutations.4 On the basis of allele frequencies of loss-of-function variants (frameshift, nonsense and splicing only; not missense) in the ExAC database, sitosterolaemia has a global prevalence of at least 1 in 2.6 million for ABCG5 and 1 in 360 000 for ABCG8; the most common loss-of-function variant appears to be ABCG8 c.1083G>A (p.(Trp361Ter)) (Exome Aggregation Consortium; http://exac.broadinstitute.org/)

1.9 Diagnostic setting

Comment: Use of genetic testing is essentially limited to confirmatory diagnosis in a subject suspected to be affected, rather than other applications such as predictive testing or prenatal diagnosis.

2. Test characteristics

2.1 Analytical sensitivity

(proportion of positive tests if the genotype is present)

Approximately 100%.

2.2 Analytical specificity

(proportion of negative tests if the genotype is not present)

Approximately 100%.

2.3 Clinical sensitivity

(proportion of positive tests if the disease is present)

The clinical sensitivity can be dependent on variable factors such as age or family history. In such cases, a general statement should be given, even if a quantification can only be made case by case.

Sitosterolaemia is a phenotypically heterogeneous disorder that is clinically characterised by increased plasma concentrations of plant sterols, xanthomas, arthralgia and premature atherosclerotic cardiovascular disease.5, 6 In addition, some patients can present with haematological abnormalities including macrothrombocytopenia, stomatocytes, haemolytic anaemia and splenomegaly.7, 8 Very rare patients can present primarily with elevated plasma levels of low-density lipoprotein cholesterol and cutaneous xanthomas, expressing a phenotype that resembles heterozygous familial hypercholesterolaemia (FH),9 and in severe cases, resembling homozygous FH, with coronary disease in childhood and adolescence.10, 11 The condition should be considered even when consumption of plant sterols has not commenced, as phytosterols can be found in breast milk.12

2.4 Clinical specificity

(proportion of negative tests if the disease is not present)

The clinical specificity can be dependent on variable factors such as age or family history. In such cases, a general statement should be given, even if a quantification can only be made case by case.

Approximately 100%.

2.5 Positive clinical predictive value (life-time risk of developing the disease if the test is positive)

100%.

2.6 Negative clinical predictive value (probability of not developing the disease if the test is negative)

Assume an increased risk based on family history for a non-affected person. Allelic and locus heterogeneity may need to be considered.

Index case in that family had been tested:

100%.

Index case in that family had not been tested:

Patients with sitosterolaemia can be clinically differentiated from patients with other childhood xanthomatoses, such as FH, by the inheritance pattern, or cerebrotendinous xanthomatosis, by the absence of neurological involvement or cataracts.

3. Clinical utility

3.1 (Differential) diagnostics: The tested person is clinically affected

(To be answered if in 1.9 ‘A’ was marked).

Sitosterolaemia should be considered in the differential diagnosis of severe hypercholesterolaemia, including apparent homozygous FH.

3.1.1 Can a diagnosis be made other than through a genetic test?

3.1.2 Describe the burden of alternative diagnostic methods to the patient

Very low. Patients with sitosterolaemia exhibit generalised hyperabsorption of dietary sterols including cholesterol, shellfish sterols and plant sterols (sitosterol, stigmasterol and campesterol), which, combined with impaired biliary excretion, leads to markedly elevated plasma levels of these plant sterols; >30-fold, with sitosterol being the most abundant species.4, 13 High levels of plant sterols in plasma are considered pathognomonic for sitosterolaemia, although elevations in plasma plant sterols may also be seen in primary biliary cirrhosis.14 Cholesterol comprises only ~80% of the total plasma sterols in patients with sitosterolaemia. Some obligate heterozygotes have mildly increased plant sterol levels.15, 16 Mass spectrometry (GC and LC) plant sterol analysis of plasma is only available in specialist laboratories.

3.1.3 How is the cost-effectiveness of alternative diagnostic methods to be judged?

Not applicable.

3.1.4 Will disease management be influenced by the result of a genetic test?

3.2 Predictive Setting: The tested person is clinically unaffected, but carries an increased risk based on family history

(To be answered if in 1.9 ‘B’ was marked).

3.2.1 Will the result of a genetic test influence lifestyle and prevention?

If the test result is positive (please describe)Not applicable.

If the test result is negative (please describe)Not applicable.

3.2.2 Which options in view of lifestyle and prevention does a person at-risk have if no genetic test has been done (please describe)?

Not applicable.

3.3 Genetic risk assessment in family members of a diseased person

(To be answered if in 1.9 ‘C’ was marked).

3.3.1 Does the result of a genetic test resolve the genetic situation in that family?

Not applicable.

3.3.2 Can a genetic test in the index patient save genetic or other tests in family members?

Not applicable.

3.3.3 Does a positive genetic test result in the index patient enable a predictive test in a family member?

Not applicable.

3.4 Prenatal diagnosis

(To be answered if in 1.9 ‘D’ was marked).

3.4.1 Does a positive genetic test result in the index patient enable a prenatal diagnosis?

Not applicable.

4. If applicable, further consequences of testing

Please assume that the result of a genetic test has no immediate medical consequences. Is there any evidence that a genetic test is nevertheless useful for the patient or his/her relatives? (Please describe).