High Risk of Medium Chain Acyl-Coenzyme A Dehydrogenase Deficiency among Gypsies

Abstract

Medium chain acyl-CoA dehydrogenase (MCAD) deficiency is recognized as the most common hereditary defect of fatty acid oxidation in humans. Death is the outcome of the first attack in about 25% of cases. A point mutation (Ala to Gly at position 985) of the MCAD gene represents more than 90% of alleles causing MCAD deficiency. The frequency of this allelic variant exhibits considerable geographical variations. In Spain, where the few diagnosed patients are mostly of Gypsy origin, the frequency is low as occurs in other Southern European countries (1 heterozygote among 200 individuals). Here we have analyzed the frequency of the G985 allele among Spanish gypsies. Heterozygotes were detected at a frequency of 1/17, with a 95% confidence interval ranging from 1/11 to 1/39. This represents the highest G985 rate described so far and calls for preventive measures, such as selective screening in this population.

Main

MCAD deficiency is the most common genetic defect of fatty acid oxidation in humans⁽¹⁾. The disorder is clinically characterized by episodic hypoglycemia, encephalopathy, apnea, and sudden death. Unidentified patients have a significant risk of morbidity and/or mortality that can be prevented by a simple and effective treatment⁽²⁾. An Ala⁹⁸⁵ → Gly transition is the single prevalent mutation representing approximately 90% of the disease-causing alleles^(3–6).

Many studies have demonstrated a significant geographical variation in the frequency of that mutation in Europe^(7,8), ranging from high heterozygote frequencies (above 1/100) in northwestern and central European countries to lower frequencies in France and Italy (below 1/250). In Spain, the prevalence of MCAD deficiency and the frequency of the mutant G985 allele is similar to that found in France or in Italy⁽⁸⁾. Interestingly, most of the diagnosed Spanish patients are of Gypsy origin. This observation prompted us to initiate a screening for the G985 allele in this population.

METHODS

The study of the carrier frequency for the G985 mutation in the MCAD gene in Spain was focused on two population groups: general and Gypsy population. Blood spots or peripheral blood from a total of 2003 individuals were used to study the frequency of the G985 mutation in the general Spanish population. Samples were collected from neonatal screening blood cards and from volunteer individuals from the Sevilla bone marrow unrelated donor program. To screen people of Gypsy origin, a total of 220 samples coming from different areas of Spain were analyzed: Aragon(n = 66), Madrid (n = 90), Andalusia (n = 42), and Catalonia (n = 22). To avoid consanguinity in the group under study, only one individual from each Gypsy family was included. Informed consent was obtained for each individual screened. DNA was isolated from peripheral blood or eluted from filter paper by standard protocols. To study the G985 common mutation, samples were analyzed using amplification refractory mutation system⁽⁹⁾, and positive results were confirmed by polymerase chain reaction/NcoI digestion (Pharmacia Biotech, Uppsala, Sweden) as described previously⁽¹⁰⁾.

RESULTS AND DISCUSSION

In agreement with previous data⁽⁸⁾, the carrier frequency for the G985 MCAD mutation in the general Spanish population was 1/200 (Table 1), and the predicted homozygous frequency averages 1/160 480. This frequency is very low compared with that of northwestern European countries (1/55 in Netherlands, 1/77 in Belgium) but similar to that found in other southern European countries^(7,8). Conversely, the rate of heterozygotes for the MCAD allelic variant among Spanish Gypsies is extremely high: a global frequency of 1 in 17 individuals (Table 1) was found, being the predictable homozygous frequency as high as 1 in 1146 individuals. These data are in agreement with the high proportion of Gypsies among the few MCAD-deficient Spanish patients diagnosed so far in our laboratories, 7 out of 10, all but one homozygous for the G985 mutation.

Table 1 Survey for G985 carriers in Gypsy and native Spanish Populations

Two individuals, one from the general population group and a second from the Gypsy group were found to be homozygous for the allele. The former came from an anonymous sample; the later was from an asymptomatic child whose MCAD deficiency was confirmed by further metabolite studies carried out after informed consent.

Taking into account the endogamic behavior of the Gypsy society, we analyzed in this study individuals coming from different areas of Spain(Aragon, Madrid, Andalusia, and Catalonia), and from each family, just one individual was included. This allowed us to obviate the possible bias due to the analysis of individuals belonging to the same large family. Detailed results are shown in Table 1. The carrier frequency for each geographic group is strikingly high and does not differ significantly between them. From these results we can conclude that the frequency of the G985 allele among Gypsies is the highest described so far. In this regard, it is interesting to point out the high frequency of the G985 allele among Bulgarian unselected newborns (1/91)⁽⁸⁾, where at least 10% of inhabitants are of Gypsy origin. On the contrary, only 1.5% of the Spanish population (600 000 individuals approximately) is composed of individuals of Gypsy extraction(data obtained from Union Romani, Barcelona).

Although these data need to be further expanded, two main conclusions can be inferred: 1) the apparent high incidence of the G985 allele among Gypsies is consistent with the hypothesis that the mutation occurred out of Europe and was brought into Europe by Indo-European-speaking people⁽⁸⁾ or is the result of a strong founder-effect from the Gypsy population which moved into Spain; and 2) the Gypsy population should be considered as a group at risk for MCAD deficiency. Data presented here justify a pilot study to assess the usefulness of a newborn screening program for detecting the G985 mutation among Gypsies.