From the research on AFLP, we know that a mutation in the mitochondrial enzyme responsible for processing an important mitochondrial transporter evokes some, but not all of the cases of this disease process. Notably, in some women with hyperemesis, the fetus carries the mutation and evokes the vomiting, while mom is simply a heterozygous carrier.
The mutation (L-3-hydroxyacyl-CoA dehydrogenase deficiency – LCHAD) involves an enzyme (carnitine palmitoyltransferase I – CPT I) responsible for synthesizing the protein that acts as key transporter for fatty acids across the mitochondrial membrane. The protein involved is called carnitine.
When a fetus carries the CPT I mutation, the fetus’ inability to metabolize fatty acids and the associated bi-products are kicked back into maternal circulation effectively overriding the mom’s capacity to process these compounds. The increased load on the mom’s liver induces the vomiting, leading, in some cases, to the compensatory reaction of fat deposits within the liver cells – AFLP. Since AFLP is relatively rare, developing in only 7-10 per every 100,000 pregnancies, is not present in all hyperemesis cases (50% of women with severe vomiting show some liver damage), and the fetal mutation is even rarer, we can deduce that neither AFLP nor the mutations that impair fetal fatty acid metabolism account for the totality of hyperemesis cases or even the morning sickness of early pregnancy.
Nevertheless, this research provides several important clues about hyperemesis. First, given the right set of circumstances, e.g. pregnancy or another high intensity stressor, carriers of a particular mutation may become symptomatic. We often view heterozygous carriers as being asymptomatic or less symptomatic than their homozygous counterparts. This may not be true. We may be simply viewing the symptom status incorrectly. Secondly, mitochondrial fatty acid metabolism is likely impaired and in some manner related to carnitine. Thirdly, maternal hyperemesis may not be a primary mitochondrial disorder in the classical sense (those definitions are changing, however). Even though there are a number of possible genetic mutations involved with the carnitine pathway, most are either severe enough to be identified during infancy (save except CPT II, which may remain latent until adolescence or early adulthood) and/or present differently (with muscular weakness and cardiomyopathy), and therefore preclude them from our differential. For all intents and purposes, hyperemesis presents during pregnancy, mostly in women with no known fatty acid oxidation or carnitine-related mutations, suggesting non-genetic mechanisms at play. In other words, I think we’re looking for functional mitochondrial disturbances in fatty acid metabolism related to carnitine.