Diagnosis of Niemann-Pick disease type C

Marc C. Patterson, MD; Peter Clayton, MD; Paul Gissen, MD, PhD; Mathieu Anheim, MD, PhD; Peter Bauer, PhD; Olivier Bonnot, MD, PhD; Andrea Dardis, PhD; Carlo Dionisi-Vici, MD; Hans-Hermann Klünemann, MD; Philippe Latour, PharmD; Charles M. Lourenço, MD; Daniel S. Ory, MD; Alasdair Parker, MD, PhD; Miguel Pocoví, PhD; Michael Strupp, MD; Marie T. Vanier, MD, PhD; Mark Walterfang, MBBS, PhD; Thorsten Marquardt, MD, PhD.   Recommendations for the detection and diagnosis of Niemann-Pick disease type C An update.  Neurology Clinical Practice.  http://cp.neurology.org/content/early/2017/10/23/CPJ.0000000000000399.full.pdf+html

Diagnosis

Biomarkers

• Biomarker screening methods can now be considered a first-line step in the NP-C

diagnostic process.

• Compared with the filipin staining test, biomarkers have several advantages, including

noninvasiveness, rapidity, higher throughput, lower cost, and ease of use (regarding sample

preparation, stability, and shipment). Sensitivity and specificity of these methods have been

reported. 

Currently available biomarkers

Oxysterols

• Oxysterols (cholesterol oxidation products) are the most established, accessible, and

widely used biomarkers, with the largest evidence base to support their reliability and

sensitivity for NP-C.

• Cholestane-3b,5a,6b-triol (C-triol) and 7-ketocholesterol (7-KC) were shown to be

elevated in plasma from patients with NP-C.

• C-triol is the preferred oxysterol biomarker, with superior specificity and sensitivity for

NP-C compared with 7-KC.  C-triol values for the upper quartile of NP-C carriers

overlap with those of patients with NP-C.

• Elevated C-triol and 7-KC levels occur in other diseases, mainly NP-A and NP-B, and

acid lipase deficiency; therefore, elevated oxysterol levels should be interpreted with

caution.

• Prolonged storage of samples at room temperature can lead to autoxidation of cholesterol,

which may cause false-positive results.

Other currently validated biomarkers

• Newer biomarkers may one day eclipse oxysterols. Desirable qualities include increased

stability, more convenient sampling methods (e.g., dried blood spot [DBS] or urine), smaller

blood volumes needed (e.g., DBS with infants), more convenient handling, and characteristic

NP-C profiles facilitating differential diagnosis when used in multianalyte panels.

Lysosphingomyelin-509 and other lysosphingolipids

• Lysosphingomyelin-509 (Lyso-SM-509) is elevated in plasma from patients with NP-C

and NP-A/B compared with controls.

• The increase of Lyso-SM in patients with NP-C is very small compared with that of

patients with NP-A/B; therefore, measurement of combinations of biomarkers, such as

Lyso-SM and Lyso-SM-509, allows distinction between NP-C and sphingomyelinase

deficiencies (NP-A/B).   In the future, additional derivatives of Lyso-SM and multianalyte panels may be able to further differentiate between NP-A/B, NP-C, and  other related diseases. 

Bile acids

• Specific bile acids have been found to be elevated in patients with NP-C. The analytical

species of choice is 3b,5a,6b-trihydroxy-cholanoyl-glycine.  The assay is applicable

to DBS, plasma, and urine, and preanalytical auto-oxidation is not a concern.

• 3b,5a,6b-trihydroxy-cholanoyl-glycine appears to be more specific for NP-C than

C-triol; other than NP-C, it is only known to be elevated in NP-A/B, and it better

discriminates NP-C carriers from patients.

When and in whom to use biomarkers

Any/all biomarkers should be tested as early as possible in the following:

• Patients presenting with splenomegaly/hepatosplenomegaly, cholestatic jaundice in neonates

or young infants, or neurologic or psychiatric symptoms

• Patients with a high clinical suspicion of NP-C

• Patients in at-risk clinical groups

In addition:

• When the initially selected biomarker does not show a profile consistent with that of

NP-C, additional biomarkers should be considered.

• Oxysterols may not be discriminatory in the presence of neonatal cholestasis; bile

acid biomarkers may be more suitable for diagnostics in this population.  Although

newborn screening is technically feasible and being explored, there is currently insufficient

evidence to recommend implementation, and ethical considerations must

first be accounted for.

• Biomarkers alone provide a very high suspicion of NP-C but diagnoses must be confirmed

by genetic testing. 

Current place of the filipin staining test

• The filipin staining test, the historical gold standard assay for NP-C diagnosis, is no

longer favored as the initial laboratory diagnostic test.

• The filipin staining test is a tool to assess the functional significance of new NPC1 or

NPC2 genetic variants, and helpful for confirming a diagnosis in patients for whom

genetic testing has not allowed identification of 2 pathogenic alleles.

Molecular genetics and NP-C

• It is crucial to genetically confirm a diagnosis in patients with high clinical suspicion

and/or a biomarker profile consistent with NP-C.

• Almost 700 variants, 400 of which are designated pathogenic mutations, are known

for NPC1 and 23 have been described for NPC2. The highly polymorphic nature of

NPC1 can confound diagnostic conclusions and make interpretation of new mutations

a challenge.

• The estimated proportion of NP-C cases in which detection of mutations on both alleles

can be reached using routine sequencing methods (exons and boundaries) is currently

higher than 90%.

It is crucial to genetically confirm a diagnosis in patients with high clinical suspicion and/or a biomarker profile consistent with NP-C.