Heterozygote advantage for the phenylketonuria allele

Population and evolutionary genetics of the PAH locus to uncover overdominance and adaptive mechanisms in phenylketonuria: Results from a multiethnic study

BACKGROUND

Phenylketonuria (PKU) is the most common inborn error of amino acid metabolism in Europe. The reasons underlying the high prevalence of heterozygous carriers are not clearly understood. We aimed to look for pathogenic PAH variant enrichment according to geographical areas and patients' ethnicity using a multiethnic nationwide cohort of patients with PKU in France. We subsequently appraised the population differentiation, balancing selection and the molecular evolutionary history of the PAH locus.

METHODS

The French nationwide PKU study included patients who have been referred at the national level to the University Hospital of Nancy, and for whom a molecular diagnosis of phenylketonuria was made by Sanger sequencing. We performed enrichment analyses by comparing alternative allele frequencies using Fisher's exact test with Bonferroni adjustment. We estimated the amount of genetic differentiation among populations using Wright's fixation index (Fst). To estimate the molecular evolutionary history of the PAH gene, we performed phylogenetic and evolutionary analyses using whole-genome and exome-sequencing data from healthy individuals and non-PKU patients, respectively. Finally, we used exome-wide association study to decipher potential genetic loci associated with population divergence on PAH.

FINDINGS

The study included 696 patients and revealed 132 pathogenic PAH variants. Three geographical areas showed significant enrichment for a pathogenic PAH variant: North of France (p.Arg243Leu), North-West of France (p.Leu348Val), and Mediterranean coast (p.Ala403Val). One PAH variant (p.Glu280Gln) was significantly enriched among North-Africans (OR = 23·23; 95% CI: 9·75-55·38). PAH variants exhibiting a strong genetic differentiation were significantly enriched in the 'Biopterin_H' domain (OR = 6·45; 95% CI: 1·99-20·84), suggesting a balancing selection pressure on the biopterin function of PAH. Phylogenetic and timetree analyses were consistent with population differentiation events on European-, African-, and Asian-ancestry populations. The five PAH variants most strongly associated with a high selection pressure were phylogenetically close and were located within the biopterin domain coding region of PAH or in its vicinity. Among the non-PAH loci potentially associated with population divergence, two reached exome-wide significance: SSPO (SCO-spondin) and DBH (dopamine beta-hydroxylase), involved in neuroprotection and metabolic adaptation, respectively.

INTERPRETATION

Our data provide evidence on the combination of evolutionary and adaptive events in populations with distinct ancestries, which may explain the overdominance of some genetic variants on PAH.

FUNDING

French National Institute of Health and Medical Research (INSERM) UMR_S 1256.

The evolution of altruism between siblings: Hamilton's rule revisited

This paper explores the validity of Hamilton's rule in the case of other-only altruism in which the benefits are shared by other members of the sibling group excluding the donor. It presents a model of competition between two alleles which code for different kinds of altruism. It derives a simple replicator equation for allele frequencies under conditions of strong selection. This equation does not depend on the size of the sibling group. In mathematical form, the equation is similar to Hamilton's original rule in the case of inbreeding, although the causal mechanism is different. The paper derives a simple criterion to determine whether there will be a polymorphism in which both alleles coexist permanently. Such an event is rare and victory will normally go to the allele with the higher value of 1/2b-c, where b is the total benefit which an offspring confers on its siblings and c is the cost to the donor. The paper also considers how an offspring will behave in particular circumstances. Using a specialized version of the basic model, it shows how, in the absence of polymorphism, natural selection should take the system towards the point of 50% marginal altruism. With this type of altruism, an offspring will perform any act for which the expected cost to the donor is at most half the expected benefit to its siblings. Acts which do not satisfy this criterion are not performed. This accords with Haldane's quip that he would sacrifice his own life for two of his brothers, but not for less. Numerical simulation is used to explore these issues in greater depth. The paper also examines briefly the implications of heterozygote advantage for Hamilton's rule. It concludes with a brief discussion of the connection between other-only altruism and whole-group altruism, in which the donor gains some benefit from its actions.

From genetics to epigenetics

In the post human-genome area, the challenge is to derive details of heritable variation in relation to how human variation reflects adaptation to the different environments. Heterozygote advantage represents a superior genetic adaptation presumably explaining the presence of the allele at frequencies above those to be expected from a simple replacement of a homozygous lethal allele by mutation alone (Saugstad 1977a, 1975b, 1972). Mean birthweight of unaffected offspring of parents heterozygous for the phenylketonuria (PKU) allele averaged significantly above mean weight of all Norwegian births, rendering unaffected offspring more viable at birth and thus improving the chance for survival of the allele. A successful adaptation requires natural selection acting on that part of the body that makes a difference in survival. Skin colour variation is such a successful adaptation, for the North as opposed to the dark skins of the equator. Human Evolution in Africa and subsequent adaptations have enabled human survival all over the world with highly different light intensity (Jablonski & Chaplin 2000). That continuous variables, height, pubertal age and brain development, are multifactorially inherited and affected by epigenetic factors, was nicely demonstrated in the increase in height in Norway 1860-1960 with at the same time a reduction in pubertal age by 4yrs which may have affected the final stage in brain development. This created an increased need for brain food, N-3, to secure optimal brain function. Body growth is not brain growth. Given that the consumption of brain food (N-3) has declined to 20% only of the level 100yrs ago, what disorders are to be expected with an N-3 dietary deficit: in pregnancy, infancy and later in life? In this paper I discuss the significance of prepubertal selective pruning of excitatory synapses compared to delayed pruning and suggest relationships with brain disorders.

A role for overdominant selection in phenylketonuria? Evidence from molecular data

To date, the reason is unknown for the high prevalence of phenylalanine hydroxylase (PAH) mutations causing phenylketonuria (PKU) in extant European populations. However, molecular genetic data generated over the last decade suggest that the concomitant excess of (unaffected) PKU carriers is at least in part the result of overdominant selection ("heterozygous advantage"). Such selection would have acted upon several different mutations in different historical populations. The exact nature of the underlying selective mechanism is unknown; its elucidation requires further investigation.

Manic depressive psychosis and schizophrenia are neurological disorders at the extremes of CNS maturation and nutritional disorders associated with a deficit in marine fat

The maturational theory of brain development comprises manic depressive psychosis and schizophrenia. It holds that the disorders are part of human diversity in growth and maturation, which explains their ubiquity, shared susceptibility genes and multifactorial inheritance. Rate of maturation and age at puberty are the genotype; the disorders are localized at the extremes with normality in between. This is based on the association between onset of puberty and the final regressive event, with pruning of 40% of excitatory synapses leaving the inhibitory ones fairly unchanged. This makes excitability, a fundamental property of nervous tissue, a distinguishing factor: the earlier puberty, the greater excitability--the later puberty, the greater deficit. Biological treatment supports deviation from the norm: neuroleptics are convulsant; antidepressives are anti-epiletogenic. There is an association between onset of puberty and body-build: early maturers are pyknic broad-built, late ones linearly leptosomic. This discrepancy is similar to that in the two disorders, supporting the theory that body-build is the phenotype. Standard of living is the environmental factor, which affects pubertal age and shifts the panorama of mental illness accordingly. Unnatural death has increased with antipsychotics. Other treatment is needed. PUFA deficit has been observed in RBC in both disorders and striking improvements with addition of minor amounts of PUFA. This supports that dietary deficit might cause psychotic development and that prevention is possible. Other neurological disorders also profit from PUFA, underlining a general deficit in the diet.

Optimality of the birth population reduces learning and behaviour disorders and sudden infant death after the first month

The weight distribution pattern of all births can be divided into a "skewing to the left" to lower weights and high neonatal mortality, a "skewing to the right" to higher weights (>3500g) and minimum neonatal and postneonatal mortality, and a "symmetrical distribution" with mortality in between. This study was initiated with the hypothesis that a deficit in newborns of more than 3500 g would adversely affect postneonatal death. Higher and rising postneonatal mortality solely attributable to sudden infant death of unknown cause (sudden infant death syndrome; SIDS) was observed in the Nordic countries with a lower proportion of heavy newborns. Minor environmental intervention almost eliminated excess mortality from this cause, supporting raised susceptibility with a depressed birthweight in postneonatal SIDS. This contrasts with classical neonatal low birthweight SIDS, which is stable despite numerous attempts at reduction, supporting a multi-factorial aetiology: low maternal age, low education, low socioeconomic status, maternal smoking, infection, etc. The postneonatal SIDS epidemic associated with a deficit in heavy newborns is thought to be a result of changing behaviour in pregnancy: moderate iatrogenic dietary restriction and young women favouring a low-calorie, low-fat diet, especially in the third trimester when the foetus is most vulnerable, which delays myelination and somatic growth and renders the infant susceptible to minor morbidity and irregularity. The timing of death and neuropathological findings suggestive of repeated hypoxic episodes in more than 80% of cases of SIDS prior to death support this theory. The similar weight distribution patterns in SIDS and all births in Denmark, the UK and the USA suggest a substantial proportion of the neonates in these countries could be growth-retarded and at risk of hypoxic episodes in infancy. A few cases, particularly males (sex-ratio = 1.7), suffer SIDS, the majority survive. Many, mostly males, present minor CNS signs and learning and behaviour problems. The male predominance accords with males more than 500 g higher optimal birthweight than females and susceptibility to a depressed weight at birth. In order to prevent postneonatal dying, SIDS and reduce learning/behaviour disorders it is necessary to raise the proportion of heavy newborns by promoting foetal growth rate equal to the maternal intrinsic rate by eating to one's appetite a balanced diet, favouring a diet high in marine fat, especially in third trimester, in order to ensure maturation of the CNS and prolong gestation, thereby increasing birthweight. Although the increased survival of some very low birthweight neonates confounds the issue, a division between SIDS in neonatal and postneonatal death is recommended in order to assess the proportion of "avoidable infant death" as opposed to persistent classical neonatal SIDS.

Optimal foetal growth in the reduction of learning and behaviour disorder and prevention of sudden infant death (SIDS) after the first month

A theory is presented that a diet low in polyunsaturated fatty acids (PUFA) in the third trimester of pregnancy may delay myelination and brain maturation. This may underpin learning and behaviour disorders and sudden infant death (SIDS) after the first month, conditions that are associated with lower than average birthweight. Epidemiological evidence is reviewed showing an inverse relation between the proportion of heavy newborns (> 3500 g) and infant mortality rate. Some countries with a lower proportion of heavy newborns despite equally high standards of living and medical care have higher post-neonatal death rates. The higher rates are solely due to SIDS which has a peak mortality within 80-100 days. It is hypothesised that as this is a time when myelination peaks, SIDS may be due to maturational delay. Evidence of subtle CNS changes in brainstem structures and in the neuromuscular system supports an instability in brainstem control systems. Moderate iatrogenic dietary restriction predominates today, but a rising number of women favour a low-caloric low-fat diet especially in the third trimester when the foetus is most susceptible. This may lead to a depressed birthweight, delayed somatic growth and neuronal maturation, such as is observed in SIDS victims. The majority exposed to suboptimal conditions survive, but a few suffer SIDS; confirming post-neonatal susceptibility. Many, especially males, present minor CNS signs and learning/behaviour disorders that could be the sequelae of repeated hypoxic episodes, such as recorded in more than 80% of SIDS victims. To reduce learning/behaviour disorders and prevent death from SIDS after the first month, it is necessary to ensure optimal development by promoting foetal growth. It is advised to avoid unnecessary dieting and to favour a diet high in PUFAs, thus prolonging pregnancy and so increasing birthweight.

Birth weight and pathogenesis in phenylketonuria

The birthweights of an ethnically homogeneous sample of infants with phenylketonuria, their unaffected siblings, and control infants were compared after adjusting for the effects of: mother's age, mother's date of birth, mother's height and obstetric history, the length of gestation, the infant's sex, the place and date of birth. There were no significant differences between the infants with phenylketonuria and their unaffected siblings either in adjusted or unadjusted birthweights. Control infants had slightly, but statistically significant, greater adjusted and unadjusted birthweights than the combined phenylketonuria and unaffected sibling groups. This effect of the phenylketonuria gene is a previously unreported finding but unlikely to be related to the pathogenesis of phenylketonuria. Our results do not provide support for the "justification" hypothesis that the mental and neurological defects in phenylketonuria result from prenatal tyrosine deprivation which would be reflected in lower birthweights.

Dermatoglyphic variation and weight and length at birth

Relationships between dermatoglyphic variables, including finger ridge counts and finger, palmar, and plantar pattern intensities, and weight and length at birth, were tested in a sample of 184 boys and 202 girls from Warsaw schools. No convincing evidence for such relationships has been obtained from the results of correlations and one-way analysis of variance, although there are indications that some palmar traits may be related to length at birth in females. The data agree with the common belief that birth weight and birth length are mainly determined by influences operating in later stages of pregnancy, that is, after the 20th gestational week.

Cold stress and congenital adrenal hyperplasia heterozygotes

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency is common in certain regions of the world characterised by cold winters. The persistence of this potentially lethal recessively inherited disease suggests that an evolutionary advantage is conferred upon the partially affected heterozygotes. Profound hypothermia following acute cold exposure in normal subjects carries a considerable mortality especially from cardiac arrhythmias and dehydration. A hypothesis is proposed to suggest that the incomplete block at the 21-hydroxylase step of steroid biosynthesis decreases stress-induced steroid responses, especially in the younger heterozygote and clinically non-salt-losing homozygote; glucocorticoid and mineralocorticoid insufficiency reduces the severity of cold-induced water diuresis; prevention of severe dehydration contributes towards the ability to survive profound hypothermia. Studies into the salt and water metabolism of the congenital adrenal hyperplasia heterozygote at various ages as well as examination of antidiuretic hormone and steroid hormone interactions upon the renal tubule in cold-exposed normal individuals are merited.