The results of studies in this large series of 16 families with dominant congenital hyperinsulinism caused by ABCC8 or KCNJ11 mutations in the β cell KATP channel demonstrate a consistent biochemical and clinical phenotype. All of the mutations were conservative single–amino acid changes, allowing for normal channel formation at the plasma membrane. In the case of ABCC8 mutations, the expressed mutant channels had impaired responsiveness to channel agonists such as diazoxide and MgADP. In the KCNJ11 mutations, the expressed channels were low conducting or nonconducting. Some individuals with dominant KATP hyperinsulinism presented early in life with severe symptoms of hypoglycemia that cause seizures or brain injury. However, many other individuals with the mutation showed little or no evidence of hypoglycemia even with provocative testing and escaped recognition until childhood or later. Dominant KATP hyperinsulinism is associated with abnormal responses to insulin secretagogues. These include a pattern of positive acute insulin responses to calcium, impaired responses to glucose and tolbutamide, as well as sensitivity to protein-induced hypoglycemia. Diazoxide is effective in controlling hypoglycemia in nearly all cases. Among the large number of adult patients described in this report, the suggestion that the risk of diabetes is increased by dominant inactivating KATP channel mutations was not confirmed.
The hypoglycemia phenotype of dominant KATP
hyperinsulinism mutations is much milder than that of recessive KATP
hyperinsulinism mutations. Of the 39 cases with recessive KATP
hyperinsulinism we previously reported (15
), 100% presented within the first week of life. We have seen only 2 cases with recessive disease that presented after 6 months of age. By contrast, only 37% of the probands with dominant KATP
mutations presented before a week of age (P
< 0.0001) and, as illustrated in the Case study
, many went unrecognized until much later in life. Nevertheless, large-for-gestational-age birth weights as evidence of increased in utero insulin exposure was similar in both the recessive and dominant groups (87% vs. 75%, respectively). It is important to note that the “milder” hypoglycemia designation in patients with dominant defects is not meant to imply that it is benign, since, as illustrated in the Case study
and previous reports of Family 3 and Family 7, some of the affected individuals suffered severe symptoms of hypoglycemia, including seizures and permanent brain damage (10
One of the most important differences in clinical phenotype between dominant and recessive KATP hyperinsulinism is the responsiveness to diazoxide therapy. In general, diazoxide is not effective in controlling hypoglycemia in patients with recessive disease, although partial improvements in glucose requirements are sometimes noted. In contrast, essentially all of the dominant KATP cases in the present series achieved complete resolution of both fasting and protein-induced hypoglycemia on moderate doses of diazoxide. Although exceptions to this rule cannot be excluded as future patients are identified, complete control of hyperinsulinism by diazoxide may be a useful phenotypic marker for the dominant form of KATP channel mutations. The difference in clinical response to diazoxide between recessive and dominant hyperinsulinism indicates that KATP channel function is partially preserved in the dominant form. This is consistent with the observation that responses to tolbutamide and glucose appear to be impaired to a lesser degree in dominant compared with recessive KATP hyperinsulinism patients.
Sensitivity to oral protein is an important clinical feature shared by patients with both dominant and recessive KATP
hyperinsulinism mutations. As noted previously in studies of recessive KATP
hyperinsulinism, and unlike patients with hyperinsulinism due to activating mutations of glutamate dehydrogenase, this protein sensitivity does not seem to be due to hyperresponsiveness to leucine-stimulated insulin release (11
). In vitro studies of isolated islets from SUR1-null mice confirm that the KATP
channel defect blocks insulin response to leucine stimulation but induces a dramatic insulin response to glutamine stimulation, possibly by a direct action of this amino acid on downstream steps in insulin release when cytosolic calcium is elevated (17
An important controversy concerning the autosomal dominant form of KATP
channel hyperinsulinism is whether the defect is a risk factor for development of adult onset diabetes. This possibility was suggested by Huopio et al. (8
) in the Finnish family with dominant hyperinsulinism due to the same ABCC8
E1507K mutation found in our Family 10 (see Case study
). Their proposal arose from efforts to identify susceptibility genes for type 2 diabetes and predated the recent discovery of KATP
activating mutations in permanent neonatal diabetes (18
). It was based partly on observations of β cell apoptosis in mice expressing a KCNJ11
dominant-negative transgene and on reports of β cell apoptosis in pancreas from children with congenital hyperinsulinism (19
). In their family with the ABCC8
E1507K mutations, Huopio et al. noted that 6 of 8 mothers with the mutation had been diagnosed with impaired glucose tolerance during pregnancy. They interpreted this as evidence of progressive β cell failure, possibly due to apoptosis secondary to persistent elevation of cytosolic calcium, and thus proposed that dominant KATP
hyperinsulinism mutations were a risk factor for development of adult onset diabetes. The results in our larger group of families with dominant KATP
hyperinsulinism mutations do not support this suggestion that the defects predispose to development of diabetes. Only 13.8% of the carrier adults had been diagnosed with NIDDM and only 3 of 13 women had been diagnosed with gestational diabetes during pregnancy. The prevalence of diabetes in these adult mutation carriers is comparable with recent estimates of the prevalence of type 2 diabetes in adults in the United States, which range from 9.6% to 21% (21
). A possible explanation for the high rate of glucose intolerance emphasized by Huopio et al. is that patients with dominant KATP
hyperinsulinism mutations have somewhat impaired insulin responses to glucose because of inadequate functioning KATP
channels (see Table ). As noted by Remedi et al. (23
), there appears to be an “inverse U” curve of islet glucose responsiveness to membrane excitation, with increased insulin responses in Sur1+/–
mice but with decreased insulin responses to glucose in homozygous mice with complete ablation of KATP
channels. Similar impairment of insulin response to glucose stimulation has been reported by other investigators in similar knockout mouse models (24
). Similarly, insulin release in response to acute or ramp glucose stimulation is diminished in patients with recessive KATP
). Of note, unlike mice with dominant-negative SUR1 or Kir6.2 transgenes in which transgene expression may have induced toxicity, mice with complete ablation of either SUR1 or Kir6.2 do not show evidence of apoptosis and do not develop diabetes, indicating that KATP
channel–independent pathways of insulin regulation are capable of preventing elevations in basal glucose levels. Thus, in the case of patients with dominant or recessive KATP
inactivating mutations, impaired glucose tolerance is a direct consequence of defective KATP
activity and cannot be interpreted as a sign of impending diabetes.
The biochemical phenotype of dominant inactivating mutations of the KATP
channel also appears to be distinct from the recessive defects, which have been studied in great detail previously (27
). The recessive mutations tend to have defects in channel biogenesis or trafficking of mature functional channels to the plasma membrane and, as a result, tend to have near absence of KATP
channel activity. Heterozygous carriers of these recessive mutations have no evidence of abnormalities in β cell function, indicating that the mutations do not interfere with the normal allele, which is sufficient to confer normal β cell function (26
). In contrast, the dominant mutations demonstrate normal assembly with their respective WT partner and normal trafficking of assembled channels to the plasma membrane when expressed in vitro. These mutant channels when expressed as homozygous mutants in COS cells are either nonconducting or low conducting, in the case of KCNJ11
mutations, or in the case of ABCC8
mutations, are incapable of opening in response to their physiological agonist MgADP and pharmacological agonist diazoxide.
In heterozygous patients with these dominant KATP mutations, the islet β cells presumably contain an array of channel heterooctamers with 0 to 4 SUR1 or Kir6.2 mutant subunits. In affected patients, the resulting impairment in channel activity is sufficient to activate voltage-gated calcium channels and cause abnormal insulin release, but not constitutively as occurs in patients with 2 recessive mutations. In studies simulating heterozygous expression of these dominant KATP mutations (e.g., SUR1-S1386P shown in Figure ), the overall channel response to MgADP or diazoxide was intermediate between that in WT and that in homomeric mutant channels. It is possible that the mutant allele has a dominant-negative effect over the WT allele and that evidence of residual functioning channels in heterozygous patients or in expression studies simulating patients represents solely the small percentage of channels that contain no mutant subunits. In the SUR1-S1386P example shown in Figure , the MgADP or diazoxide response was reduced by more than 50%. This would suggest that the S1386P mutation has a dominant-negative effect over the WT allele. However, it is unlikely that the dominant-negative effect is complete. In a complete dominant-negative scenario, the MgADP or diazoxide response is expected to be much less than what we observed, since only 1/16 of the total channel population is expected to be pure WT channels and able to respond to MgADP and diazoxide.
Channel activities may be graded based on the number of mutant subunits present and the extent to which the mutant subunit exerts its adverse effect on channel activity in a complex containing both mutant and WT subunits. The resulting channel activity is difficult to predict and may vary depending upon the different mutations. For example, while all SUR1 mutations reduce or abolish channel response to MgADP and diazoxide without affecting channel activity in the absence of nucleotides, the 3 Kir6.2 mutations reduce channel function by preventing the pore from opening. Depending on how many mutant SUR1 or Kir6.2 subunits are involved in the functional stoichiometry of a channel, a mutant subunit may weigh in differently on the function of a channel. Future detailed characterization of these heteromeric mutant channels will be necessary to gain a full understanding of their functional impact and may shed light on the structural mechanisms regulating channel function.
In summary, these 16 families with dominant inactivating mutations of the KATP channel genes demonstrate a distinct clinical and biochemical phenotype of congenital hyperinsulinism. The mutations associated with dominant KATP hyperinsulinism permit normal trafficking of channel subunits to the plasma membrane, but either impair responsiveness to channel agonists such as MgADP and diazoxide or result in complete loss of channel activity. Important clinical differences from the more commonly reported recessive KATP hyperinsulinism disorder include a milder hypoglycemia presentation and a good response to medical treatment with diazoxide so that surgical pancreatectomy is not required. Since individuals carrying dominant KATP hyperinsulinism mutations can easily escape recognition, careful genetic and biochemical evaluation of relatives of children with medically controllable hyperinsulinism should be considered.