Genetic and Environmental Factors for Type 1 Diabetes: Data from the province of Oristano, Sardinia, Italy

doi:10.2337/diacare.24.10.1846

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Letters: Observations
Letter

Genetic and Environmental Factors for Type 1 Diabetes

Data from the province of Oristano, Sardinia, Italy

Onorato Frongia, MD¹, Cristiana Pascutto, PHD², Giuseppe M. Sechi, MD³, Miriam Soro, MD⁴ and Rosa M. Angioi, PHARM D⁵

¹ Epidemiology Unit, Oristano, Italy
² University of Pavia, Pavia, Italy
³ Regional Department of Epidemiology, Cagliari, Italy
⁴ Pediatrics Department, S. Martino Hospital, Oristano, Italy
⁵ Pharmaceutical Service, Oristano, Italy

The exact role of genetic susceptibility and environmental factorsin determining type 1 diabetes has not been clarified. The islandof Sardinia has one of the highest rates of type 1 diabetesin the world (1). The contribution of genetic susceptibilityto the etiology of type 1 diabetes has been investigated inseveral studies of migrants of Sardinian origin who live invarious regions of Italy (2,3,4). However, few data are availableon the risk of type 1 diabetes among immigrants of mainlandorigin who live in Sardinia (5).

To further examine the contribution of genetic and environmentalfactors to type 1 diabetes in Sardinia, we enrolled all incidentcases of type 1 diabetes diagnosed in the province of Oristanoaged 0–29 years between 1993 and 2000 (5). Data sourcesincluded the local pharmaceutical service, which provides patientswith insulin syringes and other material needed for daily therapy(primary source), a register of individuals with chronic conditionsthat are exempt from health service fees, and the pediatricdepartment of Oristano’s provincial hospital. Finally,our data were checked with the Eurodiab Ace Sardinian Registry.We used the capture-recapture method to assess the completenessof our sources, and we used the population data supplied annuallyto the Public Health Service by the 78 municipalities of theprovince to calculate rates.

The primary source completeness was 100%. As of 31 December2000, 303 patients aged 0–29 years who were residentsof the Oristano province had been identified, including 10 withsecondary diabetes who were excluded from further analysis.The point prevalence was 5.4 per 1,000 (95% CI 4.8–6.0).The incidence rate, standardized by age (direct method; referencepopulation with a uniform age distribution) from 1993 to 2000,was 52.7 per 100,000 person-years (95% CI 42.4–63.0) inthe 0- to 14-year age-group, 21.5 (16.0–26.9) in the 15-to 29-year age-group, and 37.1 (31.2–42.7) in the overall0- to 29-year age-group.

Incidence was not stable in the 8-year period of the study.In particular, we observed a peak in the 0- to 14-year age-groupin 1998, with 24 cases observed (incidence rate 105.2/100,000,95% CI 70.5–157.0), as compared with 13 cases expected,resulting in a standardized incidence ratio (SIR) of 1.92 (95%CI 1.23–3.01, P = 0.004; Wald test). In 1994, five caseswere observed (incidence rate 19.6/100,000, 8.2–47.1),far lower than the 14.5 cases that were expected, resultingin a SIR of 0.35 (0.14–0.88, P = 0.03).

When the overall 0–29 year age-group was considered, annualfluctuations were less dramatic, with SIRs of 1.56 (95% CI 1.06–2.30,P = 0.02) in 1998 and 0.44 (0.26–0.74, P = 0.002) in 1994.These data suggest that there is an interaction between type1 diabetes and age.

When we examined geographic variations, no cases were reportedin the municipality of Arborea among the population aged 0–29years (1,571 inhabitants, 8.5 cases expected, P = 0.0005; Fisher’sexact test). According to previous observations (5), this islikely explained by the fact that $~$ 65% of the Arborea populationis of non-Sardinian origin; most were originally from the VenetoRegion of Italy. Also, data on the incidence of type 1 diabetesin the previous 12 years (our data plus data for 1989–1992,published by the Regional Department of Epidemiology) (6) supportour findings of a lower-than-expected rate in Arborea (0 casesof 20,896 person-years in the 0- to 29-year age-group vs. 234cases of 718,739 person-years in the remaining province, P =0.002; Fisher’s exact test).

In conclusion, our data further support the findings of othersthat type 1 diabetes is determined by genetic and environmentalfactors. The rapid fluctuations in incidence during the studyperiod suggest a role of environmental factors, whereas theabsence of observed cases among a population mainly of non-Sardinianorigin suggests the role of genetic risk factors.

Footnotes

Address correspondence to Dr. Onorato Frongia, OsservatorioEpidemiologico ASL 5, Via Carducci 35, 09170 Oristano, Italy.E-mail: onoratof{at}tin.it.

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