Diagnosis of Niemann-Pick Type A
NPA, like NPB, is diagnosed by measuring the level of activity of an enzyme called acid sphingomylinase (ASM) in white blood cells. The test can be performed after taking a small blood sample from individuals suspected of having the disease and is available at many commercial laboratories in the United States and elsewhere. While this test will identify persons with Type A (as well as Type B), it is not very reliable for detecting persons who are carriers (who have only one non-functional copy of the ASM gene). Further, the test will show decreased activity of ASM, but it cannot always predict whether the individual will have type A or Type B or an intermediate variant of the disease; that requires clinical evaluation of the individual.
The Mount Sinai Department of Human Genetics has identified certain populations where specific mutations account for a high percentage of cases of ASM Deficiency*. For NPA, the mutations R496L, fsP330 and L302P account for over 95% of disease-causing genetic changes in the Ashkenazi Jewish population. Direct testing of individuals in this population for these 3 changes is used for carrier identification. In other populations, the mutations must first be identified in the affected individual before DNA carrier testing can be performed for family members. More recently, comprehensive analysis of the entire ASM gene structure has been used for carrier testing for partners of known Type A carriers. This is available at several US laboratories, including GeneDx in Gaithersburg, MD, Ambry Genetics in Aliso Viejo, CA and Emory Molecular Genetics Laboratory in Atlanta, GA.
If you have any questions about diagnostic or molecular genetic testing for Niemann-Pick Disease, type B or need assistance in arranging testing, please contact the NNPDF .
*Information from "Identification and expression of a common missense mutation (L302P) in the acid sphingomyelinase gene of Ashkenazi Jewish type A Niemann-Pick disease patients" by Levran O, Desnick RJ, Schuchman EH. Blood, Oct 15, 1992.
[June 28, 2010 mem]