Study of Clinical Profile and Molecular Basis of Niemann Pick Diseases in Cases from Local Population

Abstract

Lysosomes are heterogeneous organelles, containing different hydrolytic enzymes, operating at acidic pH, used to catalysed different biological molecules. Deficiency in these enzymes lead to storage of molecules results in lysosomal storage disorders. There are many lipids storage disorders, of which Niemann Pick Disease (NPD) is one. Initially it is reported by Albert Niemann in 1914 and transmitted in autosomal recessive manner which means two homozygous alleles must be present to cause disease phenotype. It has been divided into two groups, Group 1 having subtypes, Niemann pick type A and B, is due to deficiency of enzyme acid sphingomyelinase (ASM), caused by mutation in gene sphingomyeline phosphodiesterase 1 (SMPD1) whereas Group 2 having subtypes, C and D, caused due to mutation in gene NPC Intracellular Cholesterol Transporter 1 (NPC1) (95%) and NPC Intracellular Cholesterol Transporter 2 (NPC2) (5%). NPC1 gene encodes a trafficking protein, mutation in which lead to defective trafficking of acid sphingomyeline. Niemann Pick Disease type A (NPD-A) and Niemann Pick Disease type C (NPD-C) has neuropathic involvement whereas Niemann Pick disease type B (NPD-B) is non-neuropathic. NPD-A is severe form as it does not respond to any treatment whereas NPD-B can be treatable to some extent through enzyme replacement therapy. NPD-C is also difficult to diagnose and treat. The subtypes of NPD largely overlap in clinical manifestations. In this study three NPD affected families were enrolled. Family history and clinical reports and blood sample were collected after informed consent. Clinical symptoms of patients include hepatosplenomegaly, developmental delay, respiratory infection, liver problems, short height, anaemic and jaundice. Clinical profile of patients, lab reports, ultrasound, X-rays and enzymes analysis reports were collected. Enzymes analysis shows normal levels of ASM in patients of NPD families. This suggested the presence of mutation in gene responsible for NPD type C or type D. As NPC1 gene is responsible for Type C NPD in 95% of cases therefore it is selected. DNA was extracted. Primers were designed for mutational hot spot exon 21 of NPC1 gene and polymerase chain reaction (PCR) and Sanger Sequencing were performed. Sequenced data was analysed and pathogenicity of the variant was checked through various In-Silico tools. Sequencing results of NPC1 selected exon revealed a novel homozygous disease-causing missense variant c.3230G>C causing arginine into proline (p.Arg1077Pro) in one patient of NP005 family. The NPC1 mutational profile for Pakistani population is not well recognised. As NPD has autosomal recessive mode of inheritance. High frequency of consanguineous marriages (60%) Study of Clinical Profile and Molecular Basis of Niemann Pick Disease in Cases from Local Population xii ABSTRACT caused increased incidence of autosomal recessive disorder. Our result necessities the analysis of other exons of NPC1 gene as well as NPC2 gene for other two affected families to identify disease which will pave the way to targeted treatment options. Further comprehensive molecular genetic studies are required to be conducted to get data for genotype-phenotype correlation, new born screening program and novel treatment strategies in future.