ABL is a rare autosomal recessive metabolic disorder characterized by complete absence of plasma apo B-containing lipoproteins that results from mutations in MTTP gene. Patients with ABL may develop severe neurological manifestations if they are not treated [2, 17, 18].
Abetalipoproteinemia is characterized, in term of clinical manifestations, with homozygous Familial Hypobetalipoproteinmia (FHBL; OMIM 107730) and Chylomicron Retention disease (CMRD; OMIM 246700), two other monogenic disorders of lipid metabolism by manifestations such as steatorrhea, retarded growth and chronic diarrhea associated with hypocholesterolemia. Under these conditions the use of the lipid profile of proband’s parents is essential to lead and facilitate the molecular diagnosis of these metabolic disorders: familial hypobetalipoproteinemia is a rare autosomal codominant disorder of lipoprotein metabolism, it results that both parents are expected to have plasma levels of TC, LDL-C and apoB lower than those found in normal subjects [3, 19]. In ABL and CMRD with an autosomal recessive transmission, obligate heterozygotes have almost normal plasma lipoprotein profiles, with some exception and usually normal level of TG in patients homozygous for CMRD [2, 20].
In this paper, we characterize two new Tunisian families with a severe form of hypobetalipoproteinemia. The common clinical features shared by patients were diarrhea, failure to thrive, acanthocytosis and lipid accumulation in enterocytes. The diagnosis of ABL was confirmed in these families and patients were found to be homozygotes for two mutations; one of them, which involves the exon 18, is novel on MTTP gene.
In the case of families E and D, probands presented with a similar lipid profile and very low plasma levels of TC, TG and undetectable LDL-C and apoB. In family E, the lipid values of proband’s parents, were within the normal range (Table. 1). The MTTP gene sequencing showed that proband of family E was homozygous for a novel mutation on MTTP gene (c.2611delC), predicted to result in a protein of 898 amino acids. It is well known that C-terminal domain (residues 604–894) mediate the lipid binding and the transfer catalytic activity of MTP [8, 27]. The abnormal protein is expected to be non functional, since the sequence of the C-terminal domain was changed. It was demonstrated that C-terminal domain is preferentially conserved amongst vertebrates suggesting that this region might be critical for the triacylglycerol transfer activity associated with MTP . The truncated MTP protein resulting from the mutation (p.G865X), involving the exon 18, was suggested to interfere with the association between the 97 kD subunit of MTP and PDI [24, 29]. Other MTP truncations devoid of the C-terminal domain [25, 30] or missense mutations located in this domain [12, 13] have been described in patients with ABL highlighting the functional role of the C-terminal domain.
In family D, we found the third Tunisian patient with (c.923 G > A) mutation in homozygous state. This mutation was found for the first time in two Tunisian affected brothers (unrelated to the patient in the present study) and it leads to a truncated MTP protein of 307 amino acids (p.W308X) devoid of function . The proband’s parents were heterozygous and his brother was normal. Proband’s mother (subject I.1 in Table 2) had low plasma lipid levels suggestive of hypobetalipoproteinemia, even though, the lipid plasma values within the lower limit or low values of TC and apoB were observed in heterozygous subjects for the same mutation (c.923 G > A) in a previous study . This is not surprising as heterozygous ABL (specifically one of the parents) were reported with low plasma TC, LDL-C and apoB in other single families [21–23], and the possibility that human MTP deficiency is inherited in a codominant manner had been also suggested in heterozygous of MTP knockout mice with reduced plasma apoB levels .
The proband’s brother (subject II.1 in Table 2), with normal genotype, had low levels of TC and apoB although he was healthy and had no history of malabsorption or failure to thrive. The biochimical phenotype of family D strongly confirms the existence of a Tunisian specificity associated with monogenic dyslipidemias, since we recently showed a low level of plasma lipids in heterozygous for CMRD  and normal or moderate levels of CT and LDL-C in heterozygous familial hypercholestetolemia (FH) . The lipid profile of proband’s brother (subject II.1 in Table 2) can be attributed either to the low content of fats in the mediterranean diet which characterize Tunisian population  or to the coexistence of other cholesterol-lowering gene(s) associated with hypobetalipoproteinemia in this family. In addition, it has been shown that some polymorphisms in genes related to the metabolism of lipoproteins, have been associated with lipid levels in Tunisian patients affected by other diseases like cardiovascular pathologies . This reflects, in part, the genetic background heterogeneity of the Tunisian population demonstrated in other genetic disorders .
Many mutations in MTTP gene described in literature as cause of ABL were private and related to single families. There was an exception with (p.G865X) mutation which was found in ABL patients from USA and Canada and was also reported in the Ashkenazi Jewish population with a high prevalence and a founder effect [12, 25]. The (419insA) mutation was also associated with ABL in patients of European ancestry [13, 26]. In Tunisian population, four mutations in MTTP gene (c.923 G > A, c.619-3 T > G, c.1236 + 2 T > G and c.2342 + 1 G > A) have been reported as cause of ABL in four single families [10, 36]. In this study, the diagnosis of the mutation (c.923 G > A) in a new Tunisian family is suggestive of a high prevalence of the mutant allele in heterozygote state.
Although, the clinical data available were rather incomplete (especially hepatic exploration), our patients presented the typical hematologic and gastrointestinal features of ABL such as acanthocytosis, diarrhea during infancy and malabsorption with mild anemia (proband II.2 of family D in this paper) which was reported in some cases of ABL [10, 21]. Usually neurological manifestations begin later in life in ABL patients, especially those who have not been given a supplementation of vitamin E . Later supplementation of fat soluble vitamins in patient II.2 (family D) allowed the osteotendinous reflexes reduction. Previous studies showed that early therapy with vitamin E, before the age of 16 months prevents neurologic dysfunction [32, 33]. Elevated Transaminases serum of the proband II.1 (family E) is suggestive of hepatic steatosis which is needed to be confirmed by liver biopsy in this patient. It was reported that hepatic manifestations in ABL cases included hepatomegaly due to hepatic steatosis in association with elevated transaminases [34, 41]. The absence of neurological and opthalmological manifestations in patient II.1 (family E) is consistant with the observation that the diagnosis of ABL should be made in children with malabsorption, acanthocytosis and hypocholesterolemia. Indeed a low-fat diet and fat-soluble vitamins supplementation can prevent later complications .