Determining pathogenicity of a variant is now the biggest challenge in the interpretation of exome-based sequencing data and of huge importance in the clinical context, where incorrect interpretation can have serious clinical consequences [
47]. Assigning pathogenicity is a particular issue in genes associated with monogenic diseases such as MODY, where distinguishing between variants that are clearly disease-causing versus those that impair protein function or are neutral, is problematic [
48]. This issue is further complicated by the fact that many of the variants found in the common MODY genes are novel or arise de novo, making it impossible to gain insights from other affected individuals [
48,
49].
A variety of guidelines, both national and international, have attempted to standardise the approach to investigating variant pathogenicity using in silico modelling, database searches and other parameters to assist with classification and to also standardise notation [
47,
50,
51]. Variants are graded from 1 to 5; 1, benign; 2, likely benign; 3, variant of unknown significance; 4, likely pathogenic; and 5, pathogenic; and guidelines stipulate that variants classed between 3 and 5, should be reported to requestors (see Table
2).
Table 2
Strategies to establish pathogenicity of a variant in diabetes genes
In silico data | Genome database searches | Search of genome/exome databases e.g. GnomAD to establish mean allele frequency and also assess presence of other variants at affected nucleotide position |
Sequence variant databases | For example, dbSNP, Exome Variant Server or 1000 genomes NGRL |
Mutation database searches | Human Mutation Genetic Database search, imports published data on genetic mutations. Limitations, as mutations published may not necessarily prove to be pathogenic |
Amino acid change | Examines the effect of the amino acid substitution on charge and polarity. A significant change in polarity or charge from the amino acid substitution might be more likely to impair protein function |
Species conservation | Conservation can be scored using tools such as ConSurf, through which multiple sequence alignments can be undertaken to compare orthologs across species. Essential sites for protein function are likely to be invariant across species (highly conserved) |
Software prediction | Software prediction models: SIFT, PolyPhen2 and AlignGVGD Grantham Distance assessed the physico-chemical difference between amino acid properties |
Cryptic splice site | Online software that can predict whether the variant creates a cryptic splice site or alters an existing one |
In vitro data | Cellular studies | For HNF1A, GCK and KCNJ11, published studies exist that demonstrate effects of variants on protein function. In turn, these can be fed back to affected individuals to establish best treatment options or management approaches. Lack of accessibility limits widespread use |
Clinical data | Biomarkers/systemic features | Examining for features associated with the genetic mutation may help decipher whether a variant is disease-causing or benign. For example, high-sensitivity C-reactive protein levels are known to be lower in people with HNF1A MODY, so demonstrating undetectable levels in a person with a variant of unknown significance may be a helpful indicator to prove functional effects Low magnesium levels or evidence of pancreatic exocrine failure in people with HNF1B variants, is another example A history of neonatal hypoglycaemia and/or fetal macrosomia in HNF4A variants might be helpful In GCK variants, demonstrating only fasting hyperglycaemia or a history of fetal macrosomia may assist |
| Treatment response | Demonstrating sensitivity to sulphonylurea therapy is a compelling piece of evidence favouring pathogenicity in HNF1A and HNF4A variants Other monogenic genes are not so easily readily identified in this way |
| Co-segregation studies | Proving the variant segregates with diabetes in a family is compelling evidence in favour of pathogenicity. To achieve this robustly, in an affected kindred, people with diabetes should have the variant and those without the variant an absence of diabetes. Conclusive proof comes from co-segregation in a different kindred to the proband |
Database Searches
One such database is the Genome Aggregation Database (GnomAD), which contains data from over 100,000 exomes and 15,000 whole-genomes. GnomAD reports minor allele frequency (MAF) depending on ancestry. These databases can be used to assess whether a variant found in a dominant gene might be disease-causing. For example, the presence of a variant in more individuals than would be predicted from the population prevalence of MODY (50–100 per million [
9]), assuming complete penetrance, would indicate that the variant is not acting as an autosomal dominant pathogenic mutation, since it would be present in people without disease.
However, knowledge of MAF is not a magic bullet. Each exome in GnomAD contains an average of 7.6 rare variants (MAF < 0.1%) in Mendelian disease genes, which suggests tolerance to genetic variation is higher than previously assumed. Such data have been used to downgrade pathogenicity status in variants that have previously been thought to be disease-causing [
52].
The spectrum of low-frequency variation (MAF < 1%) in the seven commonest genes associated with MODY was examined in 4003 individuals from population studies [
53]. Although 1.5% of those studied harboured genetic variants that had been previously reported to cause MODY, the majority were still euglycaemic through to middle age. This study was an early indication that careful consideration in assigning pathogenicity to variants in dominant genes, is needed.
In Silico Predictions
Using web-based applications, missense variants can be classified based on the predicted effects of the variant on protein function. The prediction obtained from a single method should not be taken in isolation. Broadly speaking, four main approaches exist: those based on sequence conservation methods; those assessing impact on protein sequences; protein structure; and physicochemical properties and splicing-predictions [
51,
58].
Software programs such as SIFT (J. Craig Venter Institute, La Jolla CA) [
59,
60], Align-GVGD (International Agency for Research on Cancer hosted by the World Health Organization [
61,
62]) and PolyPhen-2 [
63,
64] can combine a variety of assessments to predict pathogenicity.
Despite these aligned approaches, it is still often difficult to confidently assign pathogenicity. In these cases, further work may be necessary to shed light on variant functionality. These may be clinical studies in the individual or family members, or by use of reliable in vitro functional or biochemical assays that assess the effect of the variant in a cellular system model [
48]. Even then, confident assignment of pathogenicity may not always be possible due to the limitations of these techniques.
In Vitro Functional Studies
A series of assays have also been described to investigate transcription factor variants, such as those in
HNF1A, based on published studies [
65‐
67]. For the
HNF1A gene, which has been studied extensively, these include an assessment of transactivation potential (the transcriptional activity of HNF1A), protein production, nuclear-cytosolic localisation and DNA-binding in cellular systems.
In vitro functional studies have also been successfully deployed in cases of neonatal diabetes to support precision-based treatment. In individuals with
KCNJ11 mutations causing neonatal diabetes, the success of sulphonylurea treatment is determined by the specific mutation, as not all affected channels are sensitive to blockade. Knowledge of specific mutant effects in vitro can therefore help clinicians decipher whether to try higher doses to achieve full blockade of the channel, or whether insulin is needed [
68,
69].
Guidelines recommend collaboration with molecular laboratories in cases where functional effects of variants using established techniques prove inconclusive, but where there is real clinical impact in assigning status. However, this may be challenging for clinicians to access readily as most MODY gene variants have not been studied in this way and accessible pathways to support these efforts routinely may need to be developed.
In practice, for people with monogenic diabetes, treatments can be trialled empirically with or without confirmed mutation status, but the frustration and anxiety in failing to receive a conclusive diagnosis in those with variants of unknown significance is high and the psychological burden remains unexplored.
Clinical studies
In many cases, the clues towards pathogenicity come from studying the proband and their family members in more detail. For novel variants, familial co-segregation studies that demonstrate the variant in relatives with diabetes, but absence in those without diabetes, is compelling evidence in favour of pathogenicity. Similarly, demonstrating sulphonylurea sensitivity in those with uncertain variants in HNF1A or HNF4A can also be very helpful.
Is the Gene or Variant Really Causing Diabetes?
Aside from the common MODY genes described, a number of other genes have been implicated as causing diabetes. These include the beta-cell transcription factors IPF1 and NEUROD1, and PAX4, KLF11 and BLK. The genetic evidence for these genes causing penetrant autosomal dominant monogenic diabetes is weak and initial findings have not been borne out from large-scale sequencing panels of genes.
Extremely rare mutations in other genes have also been identified. For example, deletions shortening variable number tandem repeats in the
CEL gene appear to cause diabetes and pancreatic exocrine dysfunction [
70] and specific mutations in
POLD1 can result in severe insulin resistance syndromes [
71]. Loss of function variants in
APPL1 have also been identified in two families from an exome sequencing study. Co-segregation studies are promising, but it will remain to be seen whether
APPL1 variants will be reported in other MODY [
72].
Understanding the phenotypic spectrum of variants from benign to disease-causing in dominant genes is also key. Common variants in
KCNJ11 and
GCK can lead to small changes in glucose homeostasis, whilst less common variants leading to a more severe functional impact cause MODY and neonatal diabetes. Another example is the low-frequency E508K variant in
HNF1A, which in the Latino population appears to increase risk of type 2 diabetes [
73] rather than causing monogenic diabetes. A similar finding has been observed in the Wolfram syndrome gene
WFS1, which causes a MODY-like phenotype in a small number of families [
74], but common variation in the gene is also associated with risk of type 2 diabetes.
As new data are gathered, revisiting existing genes can also be helpful. Protein truncating variants in the beta-cell transcription factor
RFX6 have now been reported to cause MODY with low penetrance [
75]. Previously, however, only homozygous mutations in
RFX6 had been implicated in neonatal diabetes with gut and gallbladder anomalies and the role as a MODY-gene was questionable due to variable penetrance [
76]. Protein truncating variants robustly co-segregate with diabetes, but with an increased age of onset compared to HNF1A-MODY, as only 27% had developed diabetes by age 25, compared to 55% of HNF1A-MODY cases [
75].
Management of Monogenic Diabetes
The management of monogenic diabetes is based predominantly on observational data. The main application of personalised medicine is in the use of sulphonylurea (SU) agents in
HNF1A/HNF4A–MODY and in neonatal diabetes caused by mutations in K-ATP channel components. This is supported by a small randomised controlled trial of gliclazide versus metformin in
HNF1A-MODY [
77] and in observational data collected in cases of neonatal diabetes. Therefore, people with mutations in
HNF1A or
HNF4A causing MODY can be managed with low-dose sulphonylurea therapy, and those misdiagnosed may therefore potentially stop insulin injections or have tablet regimens rationalised. These precision-based treatments not only achieve good glycaemic control, but there is compelling evidence demonstrating that they are superior to conventional approaches [
8]. There are case reports of decades of successful treatment on SU therapy in
HNF1A-MODY, although clinicians also, not infrequently, see cases who do not respond well despite possessing an
HNF1A variant that is clearly diabetes-causing (personal observation). The reason for this variation in treatment response remains unclear. However, a recent study has suggested that higher HbA1c, higher BMI and longer duration of diabetes at the time of transfer to SU therapy from insulin, may predict failure of SU monotherapy, making the case for early genetic diagnosis [
78].
A study of liraglutide use in
HNF1A- and
GCK-MODY examined both clinical response and mechanistic aspects [
79]. This study showed no significant difference in glucose lowering effect between liraglutide and SU treatment, but more hypoglycaemia with SU use, suggesting that the SU dose used was too high. The additional expense and inconvenience of injection therapy does not appear to justify routine GLP1 therapy in
HNF1A-MODY, and it is unknown how effective these agents would be in the context of long duration MODY with secondary SU failure. Most clinicians use metformin or a dipeptidyl peptidase-4 inhibitor as second-line treatment although there is no particular evidence base. The effect of sodium-glucose transporter-2 inhibitor agents in HNF1A-MODY, who already have decreased expression of
SGLT2 and low renal threshold, was shown in a single dose study to induce greater glycosuria than in type 2 diabetes [
80] and it is unknown whether this would lead to greater adverse effects or increased efficacy of the agents.
Permanent neonatal diabetes (PND) caused by mutations in
KCNJ11 or
ABCC8 can be managed with high-dose sulphonylurea therapy in > 90% of cases, negating the need for insulin therapy and associated complications in affected neonates [
68]. For neonatal diabetes, a recent case series of 10-year follow-up of individuals with
KCNJ11 or
ABCC8 variants treated with SU agents reported that 93% of cases remained on SU therapy alone, achieving glycaemic targets and with a good safety profile [
81].
GCK MODY, characterised by lifelong, non-progressive fasting hyperglycaemia, requires no pharmacological therapy and, following diagnosis, affected individuals may be able to stop all treatment. In the largest longitudinal study to date of people with
GCK mutations, there is no higher burden of clinically significant microvascular or macrovascular complications, compared to normoglycemic individuals [
82].
Most other forms of MODY, neonatal diabetes and mitochondrial diabetes are characterised by insulin deficiency and therefore, in most cases, individuals progress to insulin therapy. Those with mitochondrial diabetes tend to have increased lactate levels, which has raised concerns over metformin use, although there is no evidence that there is an increased prevalence of lactic acidosis, since metformin only appears to cause lactic acidosis in the setting of severe renal dysfunction [
83]. The pragmatic consensus is that other oral agents should be used in preference to metformin, particularly in those with neurological features.
Further Research
The drive to deliver precision-based approaches in diabetes undoubtedly underpin many of the recent advances in monogenic diabetes, although there is still much to achieve. A critical step, as discussed, is to develop and validate strategies for finding cases of monogenic diabetes that have transethnic applicability.
Supporting clinical practitioners to understand and deliver often complex genetic information to people with diabetes, is also a key area that lacks firm guidance. As accessibility to sequencing technologies expands, particularly in understudied populations, there is a potential harm from poor interpretation of variant pathogenicity that needs to be addressed. Close collaboration between clinical practitioners and centralised laboratories that undertake these activities may be helpful in this context.
Moving beyond monogenic diabetes and understanding the impact of common variants on type 2 diabetes risk, phenotype and treatment effects, is likely to be the next clinically meaningful direction for the lessons learnt from monogenic diabetes. Efforts to address this unmet need are underway and are very likely to change the approaches to management of common diabetes types [
84].