Incidence and carrier frequency of Sandhoff disease in Saskatchewan determined using a novel substrate with detection by tandem mass spectrometry and molecular genetic analysis

Abstract

Sandhoff disease is a rare progressive neurodegenerative genetic disorder with a high incidence among certain isolated communities and ethnic groups around the world. Previous reports have shown a high occurrence of Sandhoff disease in northern Saskatchewan. Newborn screening cards from northern Saskatchewan were retrospectively screened in order to investigate the incidence and determine the carrier frequency of Sandhoff disease in these communities. PCR-based screening was conducted for the c.115delG (p.(Val39fs)) variant in the HEXB gene that was previously found in 4 Sandhoff disease patients from this area. The carrier frequency for this allele was estimated to be ~1:27. MS/MS-based screening of hexosaminidase activity along with genetic sequencing allowed for the identification of additional variants based on low total hexosaminidase activity and high % hexosaminidase A activity relative to c.115delG carriers. In total 4 pathogenic variants were discovered in the population (c.115delG, c.619A>G, c.1601G>T, and c.1652G>A) of which two are previously unreported (c.1601G>T and c.1652G>A). The combined carrier frequency of these alleles in the study area was estimated at ~1:15. Based on the number of cases of Sandhoff disease from this area we estimate the incidence to be ~1:390 corresponding to a child being born with the disease every 1–2 years on average. The results from our study were then compared with variants in the HEXB gene from the genomes available from the 1000 Genomes project. A total of 19 HEXB variants were found in the 1092 genomes of which 5 are suspected of having a deleterious effect on hexosaminidase activity. The estimated carrier frequency of Sandhoff disease in Saskatchewan at 1:15 is more than 3 times higher than the carrier frequency in the global sample provided by the 1000 Genomes project at 1:57.

Infantile onset Sandhoff disease: clinical manifestation and a novel common mutation in Thai patients

“…HEXB gene is the only gene known to cause SD with approximately 107 variants described with the vast majority being missense and nonsense variants [ 29 ]. Most of the HEXB pathologic variants are family-specific but common HEXB variant have been observed in certain ethnic groups such as c.850C>T (p.Arg284*) in Indian population [ 24 ], c.171delG (p.Trp57CysfsX6) in Spanish descendants [ 30 ], and c.115delG (p.Val39fs) in northern Saskatchewan population of Canada [ 27 ]. Possible genotype-phenotype correlation has been suggested, for example c.626C>T (p.Thr209Ile) and c.1404delT (p.P468PfsX62) are likely linked to the infantile form of SD [ 31 ].…”

“…Data on incidence/prevalence and carrier frequency of SD and TSD in Asian populations is limited, with only six Chinese [ 28 , 31 ] and four Japanese cases [ 32 ] with infantile onset SD described. The carrier frequency of SD has been well demonstrated in some populations, such as 1 in 310 in Australian [ 33 ], 1 in 276 in non-Jewish American [ 34 ], and 1 in 15–27 in Saskatchewan [ 27 ]. Based on the total of 8,751,131 live births in Thailand during the study period ( http://bps.moph.go.th/new_bps/healthdata ), the prevalence of infantile SD is estimated at least at 1 in 1,458,521 and carrier frequency at 1 in 604 as ascertained by using Hardy-Weinberg equation.…”

Newborn Screening for Lysosomal Storage Diseases

“…An MS/MS variation of this assay has been recently developed that is done in a 2-plex fashion with a pair of differentially mass-encoded substrates (78). …”

Diagnosing Lysosomal Storage Disorders: The GM2 Gangliosidoses

“…TSD has long been associated with the Ashkenazi Jewish population, but also has increased prevalence among Louisiana Cajuns and French Canadians. An apparent increase in incidence in other ethnic groups could be attributed to carrier screening programs (Bley et al, 2011), and SD is common in Argentina, Cyprus, and northern Saskatchewan (Fitterer et al, 2014). Treatment for all forms of GM2 gangliosidosis is supportive.…”

“…The identification of affected individuals is straightforward; however, carrier screening requires simultaneous comparison of the total hexosaminidase enzyme activity (Total Hex) and the proportion of Hex A activity (%Hex A). Biochemical assays in other sample types are possible, including serum (O'Brien et al, 1970) and dried blood spots, (Chamoles et al, 2002;Fitterer et al, 2014), but determination in leukocytes is the most robust.…”